'use strict';

// Security note: xlsx (SheetJS Community Edition) has known prototype-pollution
// and ReDoS CVEs (no OSS fix available as of 2026). Mitigations applied here:
//   1. cellFormula: false  — never parse/execute formulas
//   2. cellHTML: false     — never parse HTML markup
//   3. 10 MB file-size cap (enforced by caller via express.raw limit)
//   4. XLSX magic-bytes check before parsing
//   5. Endpoint requires authenticated session; no anonymous uploads
//   6. All cells treated as plain string data; no formula result access
//   7. Cell content validation - reject non-string values where unexpected
//   8. Content-type validation via express.raw type whitelist

const xlsx    = require('xlsx');
const crypto  = require('crypto');
const { getDb, ensureUserDefaultCategories } = require('../db/database');
const { computeBalanceDelta, applyBalanceDelta } = require('./billsService');
const { toCents, fromCents } = require('../utils/money.mts');

// ─── Constants ────────────────────────────────────────────────────────────────

const SESSION_TTL_MS = 24 * 60 * 60 * 1000; // 24 h
const MAX_ROWS       = 5_000;

const LABEL_PATTERNS = {
  autopay:    /\bauto(?:pay)?\b/i,
  past_due:   /\bpast\s*due\b/i,
  double_pay: /\bdouble\s*pay\b/i,
  skipped:    /\b(?:skip(?:ped)?|n\/?a|not\s*applicable)\b/i,
};

const HEADER_PATTERNS = {
  bill_name: /^(?:bill|name|bill\s*name|description|payee|vendor|service)$/i,
  amount:    /^(?:amount|amt|expected|expected\s*amount|cost|price|payment|value)$/i,
  due_date:  /^(?:due\s*date|due|due\s*day)$/i,
  paid_date: /^(?:paid\s*date|date\s*paid|payment\s*date)$/i,
  date:      /^(?:date|due\s*date|due|paid\s*date|when|day)$/i,
  category:  /^(?:category|cat|type|group)$/i,
  notes:     /^(?:notes?|comment|label|status|memo|remark)$/i,
};

const CATEGORY_KEYWORDS = [
  { words: ['electric', 'power', 'utility', 'utilities', 'water', 'sewer', 'gas'], categories: ['Utilities'] },
  { words: ['netflix', 'hulu', 'disney', 'spotify', 'subscription', 'streaming'], categories: ['Streaming', 'Entertainment', 'Subscriptions'] },
  { words: ['capital one', 'credit card', 'discover', 'visa', 'mastercard', 'amex'], categories: ['Credit Cards', 'Credit Card', 'Cards'] },
  { words: ['rent', 'mortgage'], categories: ['Housing'] },
  { words: ['insurance'], categories: ['Insurance'] },
  { words: ['loan', 'car payment', 'auto loan'], categories: ['Loans'] },
];

// Sheet names that clearly are not month/bill-data tabs
const NON_MONTH_SHEET_RE = /^(?:summary|totals?|dashboard|info|notes?|categories|settings?|overview|index|readme|instructions?|help|template|data|master|all|annual|yearly|archive|backup|charts?|graphs?|sheet\d+|.*tax(?:es)?|.*debt.*to.*als?|home\s*ownership\s*expenses)$/i;

// Full month-name lookup (abbrev and full forms)
const MONTH_LOOKUP = {
  jan: 1, january: 1, januaru: 1,
  feb: 2, february: 2, febuary: 2,
  mar: 3, march: 3,
  apr: 4, april: 4,
  may: 5,
  jun: 6, june: 6,
  jul: 7, july: 7,
  aug: 8, august: 8,
  sep: 9, sept: 9, september: 9,
  oct: 10, october: 10,
  nov: 11, november: 11, novevmber: 11,
  dec: 12, december: 12,
};

const MONTH_KEYS_BY_LENGTH = Object.keys(MONTH_LOOKUP).sort((a, b) => b.length - a.length);

// ─── Sheet Name Parsing ───────────────────────────────────────────────────────

/**
 * Detect year and month from a worksheet tab name.
 * Returns { year, month, is_non_month_sheet }.
 * year and month may be null if not determinable.
 * Exported for testing.
 */
function parseSheetName(name) {
  const clean = (name || '').trim();

  // Known non-data sheet names → skip
  if (NON_MONTH_SHEET_RE.test(clean)) {
    return { year: null, month: null, is_non_month_sheet: true };
  }

  // YYYY-MM or YYYY/MM  (e.g. "2026-01", "2026/5")
  let m = clean.match(/^(\d{4})[\/\-](\d{1,2})$/);
  if (m) {
    const yr = parseInt(m[1], 10), mo = parseInt(m[2], 10);
    if (yr >= 2000 && yr <= 2100 && mo >= 1 && mo <= 12)
      return { year: yr, month: mo, is_non_month_sheet: false };
  }

  // MM-YYYY or MM/YYYY  (e.g. "01-2026", "1/2026")
  m = clean.match(/^(\d{1,2})[\/\-](\d{4})$/);
  if (m) {
    const mo = parseInt(m[1], 10), yr = parseInt(m[2], 10);
    if (yr >= 2000 && yr <= 2100 && mo >= 1 && mo <= 12)
      return { year: yr, month: mo, is_non_month_sheet: false };
  }

  // Extract 4-digit year if present anywhere in the name. Some real workbook
  // tabs use names like "July2017", so this cannot rely on word boundaries.
  const yearM = clean.match(/(20\d{2})/);
  const year  = yearM ? parseInt(yearM[1], 10) : null;

  // Extract a month name (full, abbreviated, common typos, or compact
  // MonthYYYY / YYYYMonth forms from legacy spreadsheets).
  const lower = clean.toLowerCase();
  const compact = lower.replace(/[^a-z0-9]/g, '');
  for (const key of MONTH_KEYS_BY_LENGTH) {
    const wordMatch = new RegExp(`(^|[^a-z])${key}([^a-z]|$)`, 'i').test(lower);
    const compactMonthYear = year && (
      compact.includes(`${key}${year}`) ||
      compact.includes(`${year}${key}`)
    );
    if (wordMatch || compactMonthYear) {
      return { year, month: MONTH_LOOKUP[key], is_non_month_sheet: false };
    }
  }

  // No month found — year might still be present (ambiguous)
  return { year, month: null, is_non_month_sheet: false };
}

// ─── XLSX Parsing ─────────────────────────────────────────────────────────────

function isXlsxBuffer(buffer) {
  // XLSX = ZIP file: magic bytes PK\x03\x04
  return (
    Buffer.isBuffer(buffer) &&
    buffer.length >= 4 &&
    buffer[0] === 0x50 && buffer[1] === 0x4b &&
    buffer[2] === 0x03 && buffer[3] === 0x04
  );
}

function parseXlsxBuffer(buffer) {
  // Additional input sanitization
  if (!Buffer.isBuffer(buffer) || buffer.length === 0) {
    const err = new Error('Invalid file format. Empty or missing file data.');
    err.status = 400;
    throw err;
  }

  if (buffer.length > 10 * 1024 * 1024) {
    const err = new Error('File too large. Maximum 10MB allowed.');
    err.status = 413;
    throw err;
  }

  if (!isXlsxBuffer(buffer)) {
    const err = new Error('Invalid file format. Only XLSX files are supported.');
    err.status = 400;
    throw err;
  }

  let workbook;
  try {
    workbook = xlsx.read(buffer, {
      type:        'buffer',
      cellFormula: false,
      cellHTML:    false,
      cellNF:      false,
      cellStyles:  false,
      dense:       false,
    });
  } catch {
    const err = new Error('Could not read XLSX file. The file may be corrupted or in an unsupported format.');
    err.status = 400;
    throw err;
  }

  if (!workbook.SheetNames.length) {
    const err = new Error('XLSX file contains no sheets.');
    err.status = 400;
    throw err;
  }

  // Content-type validation: verify sheet names and cell content types
  for (const sheetName of workbook.SheetNames) {
    const sheet = workbook.Sheets[sheetName];
    if (!sheet) continue;
    
    // Validate sheet name - reject names with potential injection attempts
    const safeSheetName = String(sheetName || '').trim();
    if (safeSheetName.length === 0 || safeSheetName.length > 31) {
      const err = new Error(`Invalid sheet name length: ${sheetName || 'empty'}`);
      err.status = 400;
      throw err;
    }
    if (!/^\w[\w\s\-\.]*$/.test(safeSheetName)) {
      const err = new Error(`Invalid sheet name format: ${safeSheetName}`);
      err.status = 400;
      throw err;
    }
    
    // Validate cell content types - reject non-expected content
    const range = xlsx.utils.decode_range(sheet['!ref'] || 'A1');
    for (let R = range.s.r; R <= range.e.r; ++R) {
      for (let C = range.s.c; C <= range.e.c; ++C) {
        const cellAddress = { c: C, r: R };
        const cellRef = xlsx.utils.encode_cell(cellAddress);
        const cell = sheet[cellRef];
        
        if (!cell) continue;
        
        // Strict cell type validation
        // Only allow n (number), t (text/string), b (boolean), d (date), s (shared formula)
        // Reject array (a), error (e), formula (f)
        if (cell.t && !['n', 't', 'b', 'd', 's'].includes(cell.t)) {
          const err = new Error(`Invalid cell type '${cell.t}' found in ${cellRef}. Only numbers and text are supported.`);
          err.status = 400;
          throw err;
        }
        
        // String content validation - reject long strings that could indicate abuse
        if (cell.t === 't' && cell.v && typeof cell.v === 'string') {
          const strLen = String(cell.v).length;
          if (strLen > 10000) {
            const err = new Error(`Cell content too long in ${cellRef} (${strLen} chars). Maximum 10000 characters.`);
            err.status = 400;
            throw err;
          }
        }
      }
    }
  }

  return workbook;
}

function getSheetRows(workbook, sheetName) {
  const sheet = workbook.Sheets[sheetName];
  if (!sheet) return [];
  try {
    // raw:false → formatted string values; no formula results can leak through
    return xlsx.utils.sheet_to_json(sheet, { header: 1, defval: null, raw: false });
  } catch (err) {
    console.error(`[import] sheet="${sheetName}" failed to parse rows — skipping:`, err.message);
    return [];
  }
}

// ─── Header Detection ─────────────────────────────────────────────────────────

function detectHeaders(firstRow) {
  if (!Array.isArray(firstRow)) return {};
  const map = {};
  firstRow.forEach((cell, idx) => {
    if (cell == null) return;
    const val = String(cell).trim();
    for (const [field, pattern] of Object.entries(HEADER_PATTERNS)) {
      if (pattern.test(val) && !(field in map)) map[field] = idx;
    }
  });
  return map;
}

// ─── Dual-Column Header Detection ──────────────────────────────────────────────
/**
 * Detect all header sets in a row, handling dual-column layouts.
 * When a single row contains TWO sets of bill headers (e.g., columns A-E and G-K),
 * this function returns an array of header groups, each with its own column range.
 * 
 * Each group has: startCol, endCol, map, defaultDueDay (1 or 15)
 */
function detectAllHeaderSets(firstRow) {
  if (!Array.isArray(firstRow)) return [];
  
  // First, detect header cells and their column indices
  const headerCells = [];
  firstRow.forEach((cell, idx) => {
    if (cell == null) return;
    const val = String(cell).trim();
    if (!val) return;
    for (const field of Object.keys(HEADER_PATTERNS)) {
      if (HEADER_PATTERNS[field].test(val)) {
        headerCells.push({ idx, field, val });
        break;
      }
    }
  });
  
  if (headerCells.length === 0) return [];
  
  // Group header cells into sets by detecting when a field repeats.
  // When we see the same field name again (e.g., second "Bill", second "Amount"),
  // that indicates the start of a new header group (dual-column layout).
  // Null columns between fields within a group are just empty columns — they
  // don't split the group (left half has: Due date | Bill | Amount | null | Date Cleared).
  const seenFields = new Set();
  const groups = [];
  let currentGroup = { cells: [headerCells[0]] };
  seenFields.add(headerCells[0].field);
  
  for (let i = 1; i < headerCells.length; i++) {
    const cell = headerCells[i];
    
    // Start a new group if this field was already seen (repeat = new column set)
    // or if there's a large column gap (>3 empty columns) between this and previous
    const prevCell = headerCells[i - 1];
    const colGap = cell.idx - prevCell.idx;
    const isRepeatField = seenFields.has(cell.field);
    const isLargeGap = colGap > 3;
    
    if (isRepeatField || isLargeGap) {
      groups.push(currentGroup);
      currentGroup = { cells: [cell] };
      seenFields.clear();
      seenFields.add(cell.field);
    } else {
      currentGroup.cells.push(cell);
      seenFields.add(cell.field);
    }
  }
  groups.push(currentGroup);
  
  // Convert groups to return format with header maps and default due days
  const result = [];
  for (const group of groups) {
    const map = {};
    group.cells.forEach(h => map[h.field] = h.idx);
    
    const startCol = group.cells[0].idx;
    const endCol = group.cells[group.cells.length - 1].idx;
    const defaultDueDay = startCol < 5 ? 1 : 15;
    
    // Require at least 2 header fields (bill_name + amount, or similar) to count as a real header set.
    // This filters out spurious rows like "Left Over | $3,204.20 | Paid" where
    // "Paid" alone matches the amount pattern but isn't a real column header.
    if (Object.keys(map).length >= 2) {
      result.push({ startCol, endCol, map, defaultDueDay });
    }
  }
  
  return result;
}


// ─── Row Classification ───────────────────────────────────────────────────────

function isBlankRow(cells) {
  return cells.every((c) => c == null || String(c).trim() === '');
}

/**
 * Check if a row is blank for a specific header set's columns.
 * For dual-column layouts, a row may be blank on the left but have data on the right.
 * Uses absolute column indices from the header set map.
 */
function isBlankRowForHeaderSet(cells, headerSet) {
  const { map } = headerSet;
  
  // Check the bill_name column and amount column for this header set
  const billNameIdx = map.bill_name;
  const amountIdx = map.amount;
  
  // If we can't find bill_name or amount columns, fall back to full-row blank check
  if (billNameIdx === undefined && amountIdx === undefined) {
    return isBlankRow(cells);
  }
  
  const billNameCell = billNameIdx !== undefined ? cells[billNameIdx] : undefined;
  const amountCell = amountIdx !== undefined ? cells[amountIdx] : undefined;
  
  const billNameBlank = billNameCell == null || String(billNameCell).trim() === '';
  const amountBlank = amountCell == null || String(amountCell).trim() === '' || parseAmount(amountCell) === null;
  
  // If both bill name and amount are blank, this row is empty for this set
  return billNameBlank && amountBlank;
}

function isLikelyHeaderRow(cells) {
  const nonEmpty = cells.filter((c) => c != null && String(c).trim() !== '');
  if (nonEmpty.length === 0) return false;
  let matches = 0;
  for (const cell of nonEmpty) {
    for (const pattern of Object.values(HEADER_PATTERNS)) {
      if (pattern.test(String(cell).trim())) { matches++; break; }
    }
  }
  return matches >= Math.ceil(nonEmpty.length * 0.5);
}

function isLikelyTotalRow(cells) {
  return cells.some(
    (c) => c != null && /^(?:total|subtotal|sum|grand\s*total|.*total\s*-+>|auto\s+total)/i.test(String(c).trim()),
  );
}

/**
 * Detect rows that are financial summaries, not bill entries.
 * Catches "Paycheck", "Left Over", "Enter how much...", etc.
 */
function isLikelySummaryRow(cells) {
  return cells.some(
    (c) => c != null && /^(?:paycheck|left\s*over|enter\s+how\s+much|starting\s+balance|ending\s+balance|carry\s*over|carried\s*over|balance\s+(?:forward|carried)|bank\s+balance)/i.test(String(c).trim()),
  );
}

// ─── Cell-Value Parsers ───────────────────────────────────────────────────────

function parseAmount(raw) {
  if (raw == null) return null;
  const str = String(raw).trim();
  if (!str) return null;
  const cleaned = str.replace(/[$,\s]/g, '').replace(/^\((.+)\)$/, '-$1');
  const val = parseFloat(cleaned);
  return Number.isFinite(val) ? val : null;
}

function parseDate(raw) {
  if (raw == null) return null;
  const str = String(raw).trim();
  if (!str) return null;

  // MM/DD/YYYY or M/D/YYYY
  let m = str.match(/^(\d{1,2})\/(\d{1,2})\/(\d{4})$/);
  if (m) {
    const [, mon, day, yr] = m;
    return {
      year:  parseInt(yr, 10),
      month: parseInt(mon, 10),
      day:   parseInt(day, 10),
      iso:   `${yr}-${mon.padStart(2, '0')}-${day.padStart(2, '0')}`,
    };
  }
  // YYYY-MM-DD
  m = str.match(/^(\d{4})-(\d{2})-(\d{2})$/);
  if (m) {
    const [, yr, mon, day] = m;
    return { year: parseInt(yr, 10), month: parseInt(mon, 10), day: parseInt(day, 10), iso: str };
  }
  // MM/DD (no year)
  m = str.match(/^(\d{1,2})\/(\d{1,2})$/);
  if (m) {
    return { year: null, month: parseInt(m[1], 10), day: parseInt(m[2], 10), iso: null };
  }
  // Month name: "May 2026" or "May"
  m = str.match(/^(Jan(?:uary)?|Feb(?:ruary)?|Mar(?:ch)?|Apr(?:il)?|May|Jun(?:e)?|Jul(?:y)?|Aug(?:ust)?|Sep(?:t(?:ember)?)?|Oct(?:ober)?|Nov(?:ember)?|Dec(?:ember)?)\s*(\d{4})?$/i);
  if (m) {
    const key   = m[1].toLowerCase();
    const month = MONTH_LOOKUP[key] ?? MONTH_LOOKUP[key.slice(0, 3)];
    if (month) return { year: m[2] ? parseInt(m[2], 10) : null, month, day: null, iso: null };
  }

  return null;
}

function resolveDateIso(parsedDate, fallbackYear) {
  if (!parsedDate?.month || !parsedDate?.day) return null;
  const year = parsedDate.year ?? fallbackYear ?? null;
  if (!year || year < 2000 || year > 2100) return null;
  if (parsedDate.month < 1 || parsedDate.month > 12 || parsedDate.day < 1 || parsedDate.day > 31) return null;
  return `${year}-${String(parsedDate.month).padStart(2, '0')}-${String(parsedDate.day).padStart(2, '0')}`;
}

function detectLabels(text) {
  if (!text) return [];
  return Object.entries(LABEL_PATTERNS)
    .filter(([, pattern]) => pattern.test(text))
    .map(([label]) => label);
}

// ─── Name Normalization ───────────────────────────────────────────────────────

function normalizeName(name) {
  return String(name)
    .trim()
    .toLowerCase()
    .replace(/\s+/g, ' ')
    .replace(/[.,!?;:'"()]/g, '')
    .trim();
}

// ─── Bill Matching ────────────────────────────────────────────────────────────

function canonicalizeName(name) {
  const normalized = normalizeName(name)
    .replace(/\bcap\b/g, 'capital')
    .replace(/\bcc\b/g, 'credit card')
    .replace(/\bco\b/g, 'company')
    .replace(/\bsvc\b/g, 'service')
    .replace(/\bservices\b/g, 'service');

  return normalized
    .split(' ')
    .filter((t) => t && !['the', 'bill', 'payment', 'pay', 'paid'].includes(t))
    .join(' ')
    .trim();
}

function nameTokens(name) {
  return canonicalizeName(name)
    .split(' ')
    .filter((t) => t.length > 1 && !['card', 'credit'].includes(t));
}

function matchScore(detectedName, bill) {
  const target = canonicalizeName(detectedName);
  const candidate = canonicalizeName(bill.name);
  if (!target || !candidate) return null;

  if (candidate === target) {
    return { match_confidence: 'high', match_reason: 'exact_normalized_match', score: 100 };
  }

  const tTokens = nameTokens(target);
  const bTokens = nameTokens(candidate);
  if (!tTokens.length || !bTokens.length) return null;

  const tSet = new Set(tTokens);
  const bSet = new Set(bTokens);
  const overlap = tTokens.filter((t) => bSet.has(t));
  const overlapRatio = overlap.length / Math.max(tSet.size, bSet.size);
  const sameTokens = overlap.length === tSet.size && overlap.length === bSet.size;

  if (sameTokens) {
    return { match_confidence: 'medium', match_reason: 'token_reorder_match', score: 86 };
  }

  if (candidate.includes(target) || target.includes(candidate)) {
    const targetIsShortSubset = tTokens.length === 1 && bTokens.length > 1;
    const extraTokens = bTokens.filter((t) => !tSet.has(t));
    const isKnownStrongSubset = tTokens[0] === 'capital' && extraTokens.includes('one');
    return {
      match_confidence: targetIsShortSubset && !isKnownStrongSubset ? 'low' : 'medium',
      match_reason: isKnownStrongSubset ? 'contains_match: Capital -> Capital One' : 'contains_match',
      score: isKnownStrongSubset ? 82 : (targetIsShortSubset ? 45 : 72),
    };
  }

  if (overlap.length > 0 && overlapRatio >= 0.5) {
    return { match_confidence: 'medium', match_reason: 'token_overlap_match', score: Math.round(60 + overlapRatio * 20) };
  }

  if (overlap.length > 0) {
    return { match_confidence: 'low', match_reason: 'weak_token_overlap', score: 35 };
  }

  return null;
}

function findBillMatches(detectedName, userBills) {
  if (!detectedName || !userBills.length) return [];

  const matches = [];

  for (const bill of userBills) {
    const scored = matchScore(detectedName, bill);
    if (!scored) continue;
    matches.push({
      bill_id: bill.id,
      bill_name: bill.name,
      category_id: bill.category_id ?? null,
      category: bill.category_name || null,
      expected_amount: fromCents(bill.expected_amount),
      due_day: bill.due_day ?? null,
      match_confidence: scored.match_confidence,
      match_reason: scored.match_reason,
      score: scored.score,
    });
  }

  const ORDER = { high: 0, medium: 1, low: 2 };
  matches.sort((a, b) => ORDER[a.match_confidence] - ORDER[b.match_confidence] || b.score - a.score);
  return matches;
}

// ─── Recommendation Helpers ──────────────────────────────────────────────────

function normalizeCategoryName(name) {
  return normalizeName(name).replace(/&/g, 'and');
}

function findCategoryByName(name, categories) {
  if (!name) return null;
  const target = normalizeCategoryName(name);
  return categories.find((c) => normalizeCategoryName(c.name) === target) || null;
}

function inferCategory({ billName, detectedCategory, notesText, topMatch }, categories) {
  if (!categories.length) return null;

  const explicit = findCategoryByName(detectedCategory, categories);
  if (explicit) return { category: explicit, reason: 'category_column_match' };

  if (topMatch?.category_id && topMatch?.category) {
    return {
      category: { id: topMatch.category_id, name: topMatch.category },
      reason: `matched_bill_category: ${topMatch.bill_name}`,
    };
  }

  const text = canonicalizeName([billName, detectedCategory, notesText].filter(Boolean).join(' '));
  for (const rule of CATEGORY_KEYWORDS) {
    if (!rule.words.some((word) => text.includes(canonicalizeName(word)))) continue;
    for (const catName of rule.categories) {
      const category = findCategoryByName(catName, categories);
      if (category) return { category, reason: `keyword_category_match: ${catName}` };
    }
  }

  return null;
}

function isPaymentDateHeader(header) {
  return /\b(?:paid|payment|pay)\b/i.test(String(header || ''));
}

function isDueDateHeader(header) {
  return /\bdue\b/i.test(String(header || ''));
}

function amountsDiffer(a, b) {
  if (a == null || b == null) return false;
  return Math.abs(Number(a) - Number(b)) >= 0.01;
}

function buildRecommendation({
  billName,
  detectedAmount,
  parsedDate,
  parsedPaidDate,
  dateHeader,
  detectedCategory,
  notesText,
  possibleMatches,
  categories,
  warnings,
  errors,
  paymentDateIso,
  defaultDueDay = null,
}) {
  const recWarnings = [...warnings];
  const topMatch = possibleMatches[0] || null;
  const highMatches = possibleMatches.filter((m) => m.match_confidence === 'high');
  const mediumMatches = possibleMatches.filter((m) => m.match_confidence === 'medium');

  const dateDay = parsedDate?.day;
  let dueDay = Number.isInteger(dateDay) && dateDay >= 1 && dateDay <= 31 ? dateDay : null;
  // Fall back to the paid-date column's day (e.g. column D), then to defaultDueDay
  if (dueDay === null) {
    const paidDay = parsedPaidDate?.day;
    if (Number.isInteger(paidDay) && paidDay >= 1 && paidDay <= 31) dueDay = paidDay;
  }
  if (dueDay === null && defaultDueDay !== null) {
    dueDay = defaultDueDay;
  }
  const paymentDate = isPaymentDateHeader(dateHeader);
  if (dueDay && paymentDate && !isDueDateHeader(dateHeader)) {
    recWarnings.push('Date appears to be a payment date, not a due date');
  }

  let action = 'create_new_bill';
  let confidence = 'low';
  let reason = 'no_bill_match';
  let selectedMatch = null;

  if (errors.includes('No bill name detected')) {
    action = 'skip_row';
    confidence = 'high';
    reason = 'blank_or_summary_row';
  } else if (highMatches.length === 1) {
    action = 'match_existing_bill';
    confidence = 'high';
    reason = highMatches[0].match_reason;
    selectedMatch = highMatches[0];
  } else if (highMatches.length > 1) {
    action = 'ambiguous';
    confidence = 'low';
    reason = 'multiple_exact_matches';
    recWarnings.push('Multiple exact matches — choose one');
  } else if (mediumMatches.length === 1) {
    action = 'match_existing_bill';
    confidence = paymentDate ? 'low' : 'medium';
    reason = mediumMatches[0].match_reason;
    selectedMatch = mediumMatches[0];
    recWarnings.push('Suggested match should be reviewed before apply');
  } else if (possibleMatches.length > 0) {
    action = 'ambiguous';
    confidence = 'low';
    reason = possibleMatches.length > 1 ? 'multiple_possible_matches' : possibleMatches[0].match_reason;
    recWarnings.push('Possible weak match found — choose a bill, create a new bill, or skip');
  }

  const categoryGuess = inferCategory({
    billName,
    detectedCategory,
    notesText,
    topMatch: selectedMatch || (possibleMatches.length === 1 ? possibleMatches[0] : null),
  }, categories);

  if (action === 'create_new_bill' && !categoryGuess) {
    recWarnings.push('Category uncertain — no existing category matched clearly');
  }

  if (selectedMatch && dueDay && selectedMatch.due_day && dueDay !== selectedMatch.due_day) {
    recWarnings.push('Spreadsheet due day differs from current bill due day');
  }

  if (selectedMatch && amountsDiffer(detectedAmount, selectedMatch.expected_amount)) {
    recWarnings.push('Spreadsheet amount differs from current bill expected amount');
  }

  return {
    action,
    bill_id: selectedMatch?.bill_id ?? null,
    bill_name: selectedMatch?.bill_name ?? (action === 'create_new_bill' ? billName : null),
    category_id: categoryGuess?.category?.id ?? null,
    category_name: categoryGuess?.category?.name ?? null,
    due_day: dueDay,
    expected_amount: action === 'create_new_bill' && detectedAmount != null ? detectedAmount : null,
    actual_amount: detectedAmount,
    payment_amount: paymentDateIso && detectedAmount != null ? detectedAmount : null,
    payment_date: paymentDateIso,
    confidence,
    reason: categoryGuess?.reason && action === 'create_new_bill' ? `${reason}; ${categoryGuess.reason}` : reason,
    warnings: [...new Set(recWarnings)],
  };
}

// ─── Year/Month Resolution ────────────────────────────────────────────────────

/**
 * Determine the final year/month for a row and where they came from.
 * Priority: full row date > partial row date (sheet/default year) > sheet name > default > ambiguous.
 * Never falls back to today's date.
 * Exported for testing.
 */
function resolveYearMonth(parsedDate, sheetYear, sheetMonth, defaultYear, defaultMonth) {
  // Full date with year in the row cell
  if (parsedDate?.year && parsedDate?.month) {
    return { year: parsedDate.year, month: parsedDate.month, source: 'row_date', warnings: [] };
  }

  // Partial date (month/day, no year) in row — supplement year from sheet or default
  if (parsedDate?.month && !parsedDate?.year) {
    const year   = sheetYear ?? defaultYear ?? null;
    const source = year ? (sheetYear ? 'sheet_name' : 'default') : null;
    const w      = year ? [] : ['Year unknown — row date has no year; provide default_year'];
    return { year, month: parsedDate.month, source: source || 'ambiguous', warnings: w };
  }

  // No date in row — use sheet tab name values
  if (sheetMonth) {
    const year    = sheetYear ?? defaultYear ?? null;
    const hasYear = year !== null;
    return {
      year,
      month:   sheetMonth,
      source:  hasYear ? 'sheet_name' : 'ambiguous',
      warnings: hasYear ? [] : ['Year unknown — sheet name has no year; provide default_year'],
    };
  }

  // No month from row or sheet — last resort: request defaults
  if (defaultMonth) {
    const year = defaultYear ?? null;
    return {
      year,
      month:   defaultMonth,
      source:  'default',
      warnings: year ? [] : ['Year unknown — provide default_year'],
    };
  }

  // Nothing available
  return { year: null, month: null, source: 'ambiguous', warnings: ['No year or month could be determined'] };
}

// ─── Row Analysis Helpers ─────────────────────────────────────────────────────

function findFirstAmountCell(cells, skipIndices) {
  for (let i = 0; i < cells.length; i++) {
    if (skipIndices.has(i) || cells[i] == null) continue;
    const v = parseAmount(cells[i]);
    if (v !== null && v > 0) return cells[i];
  }
  return null;
}

function collectNotesCells(cells, headerMap, billName, allHeaderColumns = null) {
  const skipIndices = new Set(Object.values(headerMap));
  if (allHeaderColumns) {
    for (const idx of allHeaderColumns) skipIndices.add(idx);
  }
  const parts = [];
  for (let i = 0; i < cells.length; i++) {
    if (skipIndices.has(i) || cells[i] == null) continue;
    const val = String(cells[i]).trim();
    if (!val) continue;
    if (parseAmount(val) !== null) continue;
    if (billName && normalizeName(val) === normalizeName(billName)) continue;
    const parsed = parseDate(val);
    if (parsed && (parsed.year || parsed.month)) continue;
    parts.push(val);
  }
  return parts.join(' ').trim() || null;
}

// ─── Single-Row Analyzer ──────────────────────────────────────────────────────

function analyzeRow(rowIndex, cells, headerMap, headerLabels, userBills, categories, sheetName, sheetYear, sheetMonth, defaultYear, defaultMonth, rowIdPrefix, defaultDueDay = null, headerSetIndex = null, allHeaderColumns = null) {
  const get = (field) => {
    const idx = headerMap[field];
    return idx !== undefined ? cells[idx] : undefined;
  };

  const rawBillName  = get('bill_name') ?? cells[0];
  const billName     = rawBillName ? String(rawBillName).trim() || null : null;

  // Skip indices: own header columns + all other header sets' columns (for dual-column layouts)
  // This prevents fallback lookups from picking up values from the other column group.
  const skipIndices    = new Set(Object.values(headerMap));
  if (allHeaderColumns) {
    for (const idx of allHeaderColumns) skipIndices.add(idx);
  }
  const rawAmount      = get('amount') ?? findFirstAmountCell(cells, skipIndices);
  const detectedAmount = parseAmount(rawAmount);

  const parsedDueDate = parseDate(get('due_date'));
  const parsedGenericDate = parseDate(get('date'));
  const parsedDate = parsedDueDate ?? parsedGenericDate;
  const dateHeader = headerMap.due_date !== undefined
    ? headerLabels[headerMap.due_date]
    : (headerMap.date !== undefined ? headerLabels[headerMap.date] : null);
  const {
    year:     detectedYear,
    month:    detectedMonth,
    source:   yearMonthSource,
    warnings: ymWarnings,
  } = resolveYearMonth(parsedDate, sheetYear, sheetMonth, defaultYear, defaultMonth);

  const detectedDate     = parsedDate?.iso ?? resolveDateIso(parsedDate, sheetYear ?? defaultYear ?? null);
  const parsedPaidDate   = parseDate(get('paid_date'));
  const paidDateYear     = parsedPaidDate?.year ?? sheetYear ?? detectedYear ?? defaultYear ?? null;
  const detectedPaidDate = resolveDateIso(parsedPaidDate, paidDateYear);
  const rawCategory      = get('category');
  const detectedCategory = rawCategory ? String(rawCategory).trim() || null : null;
  const notesText        = collectNotesCells(cells, headerMap, billName, allHeaderColumns);
  const allText          = cells.filter((c) => c != null && typeof c === 'string').map((c) => c.trim()).join(' ');
  const detectedLabels   = detectLabels(allText);
  const rawValues        = cells.map((c) => (c != null ? String(c) : null));

  const warnings = [...ymWarnings];
  const errors   = [];
  if (!billName)           errors.push('No bill name detected');
  if (detectedAmount === null) warnings.push('No amount detected');

  // ── Diagnostic logging for auto-detected patterns ──────────────────────────
  const _rawDue  = get('due_date')  != null ? String(get('due_date')).trim()  : '';
  const _rawPaid = get('paid_date') != null ? String(get('paid_date')).trim() : '';
  const _loc     = `sheet="${sheetName}" row=${rowIndex + 1}${billName ? ` bill="${billName}"` : ''}`;

  if (detectedLabels.includes('autopay') && billName) {
    if (_rawDue && /auto/i.test(_rawDue) && /\d/.test(_rawDue)) {
      console.log(`[import] ${_loc} autopay+date in due col: "${_rawDue}" (date portion not extracted)`);
    } else {
      console.log(`[import] ${_loc} autopay detected`);
    }
  }
  if (detectedLabels.includes('past_due')) {
    console.log(`[import] ${_loc} PAST DUE detected`);
  }
  if (_rawPaid && !parsedPaidDate) {
    console.log(`[import] ${_loc} unparseable paid date: "${_rawPaid}"`);
  }
  // ───────────────────────────────────────────────────────────────────────────

  const possibleMatches = billName ? findBillMatches(billName, userBills) : [];
  const recommendation = buildRecommendation({
    billName,
    detectedAmount,
    parsedDate,
    parsedPaidDate,
    dateHeader,
    detectedCategory,
    notesText,
    possibleMatches,
    categories,
    warnings,
    errors,
    paymentDateIso: detectedPaidDate,
    defaultDueDay,
  });

  const proposedAction = recommendation.action === 'ambiguous' ? 'mark_ambiguous' : recommendation.action;
  const confidence = recommendation.confidence;
  const requiresUserDecision = recommendation.action === 'ambiguous';

  const rowId = rowIdPrefix ? `${rowIdPrefix}_r${rowIndex + 1}` : `row_${rowIndex + 1}`;

  return {
    row_id:                rowId,
    source_row_number:     rowIndex + 1,
    sheet_name:            sheetName,
    year_month_source:     yearMonthSource,
    raw_values:            rawValues,
    detected_bill_name:    billName,
    detected_category:     detectedCategory,
    detected_amount:       detectedAmount,
    detected_date:         detectedDate,
    detected_paid_date:    detectedPaidDate,
    detected_payment_amount: detectedPaidDate && detectedAmount != null ? detectedAmount : null,
    detected_year:         detectedYear,
    detected_month:        detectedMonth,
    detected_notes:        notesText,
    detected_labels:       detectedLabels,
    proposed_action:       proposedAction,
    confidence,
    warnings:              recommendation.warnings,
    errors,
    possible_bill_matches: possibleMatches,
    requires_user_decision: requiresUserDecision,
    due_day:               recommendation.due_day,
    header_set_index:      headerSetIndex,
    recommendation,
  };
}

// ─── Sheet Row Parser ─────────────────────────────────────────────────────────

/**
 * Parse all data rows from one sheet's rawRows array.
 * sheetYear/sheetMonth come from parseSheetName(); rowIdPrefix is null for single-sheet mode.
 */
function parseSheetRows({ name, rawRows, year: sheetYear, month: sheetMonth, rowIdPrefix }, userBills, categories, defaultYear, defaultMonth) {
  if (!rawRows.length) return { rows: [], headerRow: null };

  // Detect all header sets in each row to handle dual-column layouts
  let headerRowIndex = 0;
  let headerLabels = rawRows[0]?.map((c) => (c != null ? String(c).trim() : null)) || [];
  
  // First try to detect headers in row 0
  let allHeaderSets = detectAllHeaderSets(rawRows[0]);
  
  // If no headers in row 0, scan up to 5 rows
  for (let scanIdx = 1; scanIdx < Math.min(5, rawRows.length); scanIdx++) {
    const candidateSets = detectAllHeaderSets(rawRows[scanIdx]);
    if (candidateSets.length > 0) {
      headerRowIndex = scanIdx;
      headerLabels = rawRows[scanIdx].map((c) => (c != null ? String(c).trim() : null));
      allHeaderSets = candidateSets;
      // Check if this set has all required fields
      let hasAllRequired = false;
      for (const set of allHeaderSets) {
        if (set.map.due_date !== undefined && set.map.bill_name !== undefined && set.map.amount !== undefined) {
          hasAllRequired = true;
          break;
        }
      }
      if (hasAllRequired) {
        break;
      }
    }
  }
  
  // Check if we have valid headers (must have due_date, bill_name, amount)
  let hasValidHeaders = false;
  for (const set of allHeaderSets) {
    if (set.map.due_date !== undefined && set.map.bill_name !== undefined && set.map.amount !== undefined) {
      hasValidHeaders = true;
      break;
    }
  }
  
  const hasHeaders = hasValidHeaders;
  const startRow = hasHeaders ? headerRowIndex + 1 : 0;

  // Log detected layout for this sheet
  const _colLetter = (i) => String.fromCharCode(65 + i);
  if (!hasHeaders) {
    console.log(`[import] sheet="${name}" no valid headers detected — sheet will be skipped`);
  } else {
    for (const [si, set] of allHeaderSets.entries()) {
      const mapped = Object.entries(set.map).map(([f, i]) => `${f}:${_colLetter(i)}`).join(', ');
      console.log(`[import] sheet="${name}" group=${si} defaultDueDay=${set.defaultDueDay} columns={${mapped}}`);
    }
  }

  // For dual-column layouts, collect ALL column indices across all header sets
  // so that fallback lookups (findFirstAmountCell, collectNotesCells) don't
  // accidentally pick up values from the other column set.
  // This includes the full range [startCol..endCol] for each set, not just
  // the mapped columns, because gap columns within a set also belong to that side.
  const allColumnsIndices = new Set();
  for (const set of allHeaderSets) {
    for (const idx of Object.values(set.map)) {
      allColumnsIndices.add(idx);
    }
    for (let i = set.startCol; i <= set.endCol; i++) {
      allColumnsIndices.add(i);
    }
  }

  const rows = [];
  
  // Process each header set independently
  for (let setIdx = 0; setIdx < allHeaderSets.length; setIdx++) {
    const headerSet = allHeaderSets[setIdx];
    const headerMap = headerSet.map;
    const defaultDueDay = headerSet.defaultDueDay;
    
    for (let i = startRow; i < rawRows.length; i++) {
      const cells = rawRows[i] || [];
      
      // For dual-column: skip rows blank in this header set's columns only
      // For single-column: fall back to regular isBlankRow
      if (allHeaderSets.length > 1 ? isBlankRowForHeaderSet(cells, headerSet) : isBlankRow(cells)) continue;
      
      // Skip duplicate header rows (but only if we found headers)
      if (hasHeaders && isLikelyHeaderRow(cells) && i > headerRowIndex) continue;
      
      // Skip total rows
      if (isLikelyTotalRow(cells)) continue;
      
      // Skip financial summary rows (Paycheck, Left Over, etc.)
      if (isLikelySummaryRow(cells)) continue;
      
      // Skip leftover calculation rows: null/blank bill name with negative amount, or dash separators
      const getBillName = (field) => {
        const idx = headerMap[field];
        return idx !== undefined ? cells[idx] : undefined;
      };
      const get = (field) => {
        const idx = headerMap[field];
        return idx !== undefined ? cells[idx] : undefined;
      };
      const rawBillName = getBillName('bill_name') ?? cells[0];
      const billName = rawBillName ? String(rawBillName).trim() || null : null;
      const rawAmount = get('amount') ?? findFirstAmountCell(cells, new Set(Object.values(headerMap)));
      const amount = rawAmount !== null ? parseAmount(rawAmount) : null;
      
      // Check if bill name is a dash separator (--- or ---->)
      const isDashSeparator = billName && (billName.match(/^-+>/) || billName.match(/^--+$/));
      
      // Check if this is a leftover calculation row (null/blank bill name + negative amount)
      // Skip if bill name is null AND amount is negative
      const isLeftoverCalcRow = !billName && amount !== null && amount < 0;
      
      if (isDashSeparator || isLeftoverCalcRow) continue;

      try {
        rows.push(analyzeRow(
          i, cells, headerMap, headerLabels, userBills, categories,
          name, sheetYear, sheetMonth,
          defaultYear, defaultMonth, rowIdPrefix,
          defaultDueDay, setIdx, allColumnsIndices,
        ));
      } catch (err) {
        console.error(`[import] sheet="${name}" row=${i + 1} failed to analyze — skipping:`, err.message);
      }
    }
  }

  return {
    rows,
    headerRow: hasHeaders ? headerLabels : null,
  };
}

// ─── Import Session Management ────────────────────────────────────────────────

function makeSessionId() {
  return crypto.randomBytes(16).toString('hex');
}

function saveImportSession(db, userId, sessionData) {
  const id        = makeSessionId();
  const now       = new Date().toISOString();
  const expiresAt = new Date(Date.now() + SESSION_TTL_MS).toISOString();

  db.prepare(`
    INSERT INTO import_sessions (id, user_id, created_at, expires_at, preview_json)
    VALUES (?, ?, ?, ?, ?)
  `).run(id, userId, now, expiresAt, JSON.stringify(sessionData));

  return id;
}

function loadImportSession(db, sessionId, userId) {
  const row = db.prepare(`
    SELECT preview_json FROM import_sessions
    WHERE id = ? AND user_id = ? AND expires_at > datetime('now')
  `).get(sessionId, userId);

  if (!row) {
    const err = new Error('Import session not found or expired. Please re-upload your file.');
    err.status = 404;
    throw err;
  }

  return JSON.parse(row.preview_json);
}

function deleteImportSession(db, sessionId) {
  db.prepare('DELETE FROM import_sessions WHERE id = ?').run(sessionId);
}

function pruneExpiredSessions(db) {
  db.prepare("DELETE FROM import_sessions WHERE expires_at <= datetime('now')").run();
}

// ─── Preview ──────────────────────────────────────────────────────────────────

async function previewSpreadsheet(userId, buffer, options = {}) {
  const db = getDb();
  try { pruneExpiredSessions(db); } catch (err) {
    console.error('[import] failed to prune expired sessions (non-fatal):', err.message);
  }
  ensureUserDefaultCategories(userId);

  const workbook     = parseXlsxBuffer(buffer);
  const sheetNames   = workbook.SheetNames;
  const parseAll     = options.parse_all_sheets === true;
  const defaultYear  = options.default_year  ?? null;
  const defaultMonth = options.default_month ?? null;

  // Load user's active bills once for matching across all sheets
  const userBills = db.prepare(`
    SELECT b.id, b.name, b.category_id, b.due_day, b.expected_amount, b.autopay_enabled,
           c.name AS category_name
    FROM   bills b
    LEFT   JOIN categories c ON c.id = b.category_id
    WHERE  b.active = 1 AND b.user_id = ?
    ORDER  BY b.name
  `).all(userId);

  const categories = db.prepare('SELECT id, name FROM categories WHERE user_id = ? ORDER BY name').all(userId);

  // ── Multi-sheet mode ─────────────────────────────────────────────────────────
  if (parseAll) {
    const sheetsMeta  = [];
    let   allRows     = [];
    let   totalParsed = 0;

    for (let si = 0; si < sheetNames.length; si++) {
      const name = sheetNames[si];
      const { year, month, is_non_month_sheet } = parseSheetName(name);

      if (is_non_month_sheet) {
        sheetsMeta.push({
          name, detected_year: null, detected_month: null,
          is_non_month_sheet: true, status: 'skipped', row_count: 0,
        });
        continue;
      }

      const rawRows    = getSheetRows(workbook, name);
      const rowIdPrefix = `s${si}`;

      const { rows, headerRow } = parseSheetRows(
        { name, rawRows, year, month, rowIdPrefix },
        userBills, categories, defaultYear, defaultMonth,
      );

      totalParsed += rows.length;
      if (totalParsed > MAX_ROWS) {
        const err = new Error(`Spreadsheet too large. Combined rows across all sheets exceed ${MAX_ROWS}.`);
        err.status = 400;
        throw err;
      }

      const resolvedYear = year ?? (month ? defaultYear : null);
      let status;
      if (!month) {
        status = 'ambiguous';
      } else if (resolvedYear) {
        status = 'parsed';
      } else {
        status = 'parsed_month_only';
      }

      sheetsMeta.push({
        name, detected_year: resolvedYear, detected_month: month,
        is_non_month_sheet: false, status,
        row_count: rows.length, header_row: headerRow,
      });

      allRows = allRows.concat(rows);
    }

    const sessionData = {
      user_id:           userId,
      original_filename: options.original_filename || null,
      sheet_name:        'multiple',
      rows:              allRows,
      default_year:      defaultYear,
      default_month:     defaultMonth,
      created_at:        new Date().toISOString(),
    };

    const importSessionId = saveImportSession(db, userId, sessionData);

    return {
      import_session_id: importSessionId,
      workbook: {
        sheet_names:     sheetNames,
        parse_mode:      'all_sheets',
        sheets:          sheetsMeta,
        total_row_count: allRows.length,
      },
      rows: allRows,
    };
  }

  // ── Single-sheet mode (existing behaviour) ────────────────────────────────
  const selectedSheet =
    options.sheet_name && sheetNames.includes(options.sheet_name)
      ? options.sheet_name
      : sheetNames[0];

  const rawRows = getSheetRows(workbook, selectedSheet);

  if (rawRows.length === 0) {
    const err = new Error('Selected sheet is empty.');
    err.status = 400;
    throw err;
  }
  if (rawRows.length > MAX_ROWS + 1) {
    const err = new Error(`Spreadsheet too large. Maximum ${MAX_ROWS} data rows supported.`);
    err.status = 400;
    throw err;
  }

  const { year: sheetYear, month: sheetMonth } = parseSheetName(selectedSheet);

  const { rows: previewRows, headerRow } = parseSheetRows(
    { name: selectedSheet, rawRows, year: sheetYear, month: sheetMonth, rowIdPrefix: null },
    userBills, categories, defaultYear, defaultMonth,
  );

  const sessionData = {
    user_id:           userId,
    original_filename: options.original_filename || null,
    sheet_name:        selectedSheet,
    rows:              previewRows,
    default_year:      defaultYear,
    default_month:     defaultMonth,
    created_at:        new Date().toISOString(),
  };

  const importSessionId = saveImportSession(db, userId, sessionData);

  return {
    import_session_id: importSessionId,
    workbook: {
      sheet_names:    sheetNames,
      selected_sheet: selectedSheet,
      parse_mode:     'single_sheet',
      row_count:      previewRows.length,
      header_row:     headerRow,
    },
    rows: previewRows,
  };
}

// ─── Apply ────────────────────────────────────────────────────────────────────

const VALID_ACTIONS = new Set([
  'match_existing_bill',
  'create_new_bill',
  'skip_row',
  'create_payment',
  'update_monthly_state',
  'add_monthly_note',
]);

function importValidationError(message, detail = {}) {
  const err = new Error(message);
  err.status = 400;
  err.code = 'IMPORT_VALIDATION_ERROR';
  err.details = [detail].filter(Boolean);
  return err;
}

function decisionValidationError(idx, d, field, message) {
  return importValidationError(`Decision [${idx}]: ${message}`, {
    row_id: d?.row_id ?? null,
    field,
    message,
  });
}

function coerceOptionalNumber(d, idx, field, { min = null, max = null } = {}) {
  if (d[field] === undefined || d[field] === null || d[field] === '') return null;
  const value = Number(d[field]);
  if (!Number.isFinite(value)) {
    throw decisionValidationError(idx, d, field, `${field} must be a finite number`);
  }
  if (min !== null && value < min) {
    throw decisionValidationError(idx, d, field, `${field} must be at least ${min}`);
  }
  if (max !== null && value > max) {
    throw decisionValidationError(idx, d, field, `${field} must be no greater than ${max}`);
  }
  d[field] = value;
  return value;
}

function coerceOptionalInteger(d, idx, field, { min = null, max = null } = {}) {
  if (d[field] === undefined || d[field] === null || d[field] === '') return null;
  const value = Number(d[field]);
  if (!Number.isInteger(value)) {
    throw decisionValidationError(idx, d, field, `${field} must be an integer`);
  }
  if (min !== null && value < min) {
    throw decisionValidationError(idx, d, field, `${field} must be at least ${min}`);
  }
  if (max !== null && value > max) {
    throw decisionValidationError(idx, d, field, `${field} must be no greater than ${max}`);
  }
  d[field] = value;
  return value;
}

function validateOptionalIsoDate(d, idx, field) {
  if (d[field] === undefined || d[field] === null || d[field] === '') return null;
  const value = String(d[field]).trim();
  if (!/^\d{4}-\d{2}-\d{2}$/.test(value)) {
    throw decisionValidationError(idx, d, field, `${field} must use YYYY-MM-DD format`);
  }
  d[field] = value;
  return value;
}

function validateDecision(d, idx) {
  if (!d.row_id)
    throw decisionValidationError(idx, d, 'row_id', 'row_id is required');
  if (!VALID_ACTIONS.has(d.action))
    throw decisionValidationError(idx, d, 'action', `unsupported action '${d.action}'`);
  if (d.action === 'create_new_bill' && !d.bill_name)
    throw decisionValidationError(idx, d, 'bill_name', 'bill_name is required for create_new_bill');

  if (['match_existing_bill', 'update_monthly_state', 'add_monthly_note', 'create_payment'].includes(d.action)) {
    if (d.bill_id === undefined || d.bill_id === null || d.bill_id === '')
      throw decisionValidationError(idx, d, 'bill_id', `bill_id is required for ${d.action}`);
    coerceOptionalInteger(d, idx, 'bill_id', { min: 1 });
  }

  coerceOptionalInteger(d, idx, 'due_day', { min: 1, max: 31 });
  coerceOptionalInteger(d, idx, 'year', { min: 2000, max: 2100 });
  coerceOptionalInteger(d, idx, 'month', { min: 1, max: 12 });
  coerceOptionalInteger(d, idx, 'category_id', { min: 1 });
  coerceOptionalNumber(d, idx, 'actual_amount', { min: 0 });
  coerceOptionalNumber(d, idx, 'expected_amount', { min: 0 });
  coerceOptionalNumber(d, idx, 'payment_amount', { min: 0 });
  validateOptionalIsoDate(d, idx, 'payment_date');

  if (d.action === 'create_payment' && !d.payment_amount && !d.actual_amount) {
    throw decisionValidationError(idx, d, 'payment_amount', 'payment_amount or actual_amount is required for create_payment');
  }
}

function resolveYear(decision, previewRow, sessionData) {
  return decision.year ?? previewRow?.detected_year ?? sessionData.default_year ?? null;
}

function resolveMonth(decision, previewRow, sessionData) {
  return decision.month ?? previewRow?.detected_month ?? sessionData.default_month ?? null;
}

function nullableString(value) {
  if (value == null) return null;
  const text = String(value).trim();
  return text === '' ? null : text;
}

function nullableNumber(value) {
  if (value == null) return null;
  const num = Number(value);
  return Number.isFinite(num) ? num : null;
}

function amountsEqual(a, b) {
  const left = nullableNumber(a);
  const right = nullableNumber(b);
  if (left == null || right == null) return left === right;
  return Math.abs(left - right) < 0.005;
}

function upsertMonthlyState(db, billId, year, month, amount, notes, isSkipped, allowOverwrite) {
  amount = toCents(amount); // incoming amount is dollars (decision/spreadsheet); column is cents
  const existing = db.prepare(`
    SELECT id, actual_amount, notes, is_skipped
    FROM   monthly_bill_state
    WHERE  bill_id = ? AND year = ? AND month = ?
  `).get(billId, year, month);

  if (!existing) {
    db.prepare(`
      INSERT INTO monthly_bill_state (bill_id, year, month, actual_amount, notes, is_skipped)
      VALUES (?, ?, ?, ?, ?, ?)
    `).run(billId, year, month, amount, notes, isSkipped ?? 0);
    return { result: 'created' };
  }

  const amountConflict = (amount != null && existing.actual_amount !== null && !amountsEqual(existing.actual_amount, amount));
  const notesConflict  = (notes  != null && existing.notes          !== null && nullableString(existing.notes) !== nullableString(notes));

  if ((amountConflict || notesConflict) && !allowOverwrite) {
    return { result: 'skipped_conflict', note: 'Monthly state already exists with different values — use overwrite:true to replace' };
  }

  const newAmount  = allowOverwrite ? amount  : (existing.actual_amount !== null ? existing.actual_amount : amount);
  const newNotes   = allowOverwrite ? notes   : (existing.notes          !== null ? existing.notes          : notes);
  const newSkipped = isSkipped != null ? isSkipped : existing.is_skipped;

  const noChange =
    amountsEqual(existing.actual_amount, newAmount)
    && nullableString(existing.notes) === nullableString(newNotes)
    && Number(existing.is_skipped ?? 0) === Number(newSkipped ?? 0);

  if (noChange && !allowOverwrite) {
    return { result: 'skipped_duplicate', note: 'Monthly state already exists with the same values' };
  }

  db.prepare(`
    UPDATE monthly_bill_state
    SET    actual_amount = ?, notes = ?, is_skipped = ?, updated_at = datetime('now')
    WHERE  bill_id = ? AND year = ? AND month = ?
  `).run(newAmount, newNotes, newSkipped, billId, year, month);

  return { result: allowOverwrite ? 'overwritten' : 'updated' };
}

function createPaymentFromImport(db, billId, amount, paidDate, notes, allowOverwrite) {
  if (!paidDate || amount == null || amount <= 0) return null;
  amount = toCents(amount); // incoming amount is dollars (decision/spreadsheet); column is cents

  const dup = db.prepare(`
    SELECT id, created_at FROM payments
    WHERE bill_id = ? AND paid_date = ? AND amount = ? AND deleted_at IS NULL
  `).get(billId, paidDate, amount);

  if (dup && !allowOverwrite) return { result: 'skipped_duplicate', existing_created_at: dup.created_at ?? null };

  // Read the bill fresh so sequential imports for the same bill chain correctly
  // (each payment reduces current_balance before the next one is computed).
  const bill = db.prepare(
    'SELECT current_balance, interest_rate, interest_accrued_month FROM bills WHERE id = ? AND deleted_at IS NULL'
  ).get(billId);

  const balCalc = bill ? computeBalanceDelta(bill, amount) : null;

  db.prepare(`
    INSERT INTO payments (bill_id, amount, paid_date, method, notes, balance_delta, interest_delta, payment_source)
    VALUES (?, ?, ?, ?, ?, ?, ?, 'file_import')
  `).run(billId, amount, paidDate, null, notes, balCalc?.balance_delta ?? null, balCalc?.interest_delta ?? null);

  applyBalanceDelta(db, billId, balCalc);

  return { result: 'created', existing_created_at: null };
}

function resolvePaymentInfo(decision, previewRow, amount) {
  const paymentDate = decision.payment_date ?? previewRow?.detected_paid_date ?? null;
  const paymentAmount = decision.payment_amount
    ?? previewRow?.detected_payment_amount
    ?? (paymentDate ? amount : null);
  return { paymentDate, paymentAmount };
}

function importRelatedPaidMonthsForNewBill(db, newBillId, billName, sourceRowId, sessionData, appliedRowIds, allowOverwrite, summary) {
  const targetName = normalizeName(billName);
  if (!targetName) return 0;

  let imported = 0;
  for (const row of sessionData.rows || []) {
    if (row.row_id === sourceRowId || appliedRowIds.has(row.row_id)) continue;
    if (normalizeName(row.detected_bill_name || '') !== targetName) continue;
    if (!row.detected_year || !row.detected_month) continue;

    const amount = row.detected_amount ?? null;
    const notes = row.detected_notes ?? null;
    const paymentDate = row.detected_paid_date ?? null;
    const paymentAmount = row.detected_payment_amount ?? (paymentDate ? amount : null);

    if (amount == null && !paymentDate) continue;

    const detail = { row_id: row.row_id, action: 'create_new_bill_related_month', result: 'imported', bill_id: newBillId };
    if (amount != null) {
      const st = upsertMonthlyState(db, newBillId, row.detected_year, row.detected_month, amount, notes, null, allowOverwrite);
      detail.monthly_state = st.result;
    }

    const paymentResult = createPaymentFromImport(db, newBillId, paymentAmount, paymentDate, notes, allowOverwrite);
    if (paymentResult) {
      detail.payment = paymentResult.result;
      detail.paid_date = paymentDate;
      detail.payment_amount = paymentAmount;
    }

    summary.created++;
    if (summary.skipped > 0) summary.skipped--;
    summary.details.push(detail);
    imported++;
  }

  return imported;
}

function applyOneDecision(db, userId, decision, previewRow, sessionData, allowOverwrite, summary, appliedRowIds) {
  const { row_id, action } = decision;

  if (action === 'skip_row') {
    summary.skipped++;
    summary.details.push({ row_id, action, result: 'skipped' });
    return;
  }

  try {
    const year   = resolveYear(decision, previewRow, sessionData);
    const month  = resolveMonth(decision, previewRow, sessionData);
    const amount = decision.actual_amount ?? previewRow?.detected_amount ?? null;
    const notes  = decision.notes ?? previewRow?.detected_notes ?? null;

    if (action === 'create_new_bill') {
      const name = (decision.bill_name || previewRow?.detected_bill_name || '').trim();
      if (!name) throw new Error('Bill name required');

      const existing = db.prepare(
        "SELECT id FROM bills WHERE name = ? COLLATE NOCASE AND active = 1 AND user_id = ?",
      ).get(name, userId);

      if (existing && !allowOverwrite) {
        summary.skipped++;
        summary.duplicates++;
        summary.details.push({ row_id, action, result: 'skipped_duplicate', note: `Bill "${name}" already exists (id=${existing.id})` });
        return;
      }

      let categoryId = decision.category_id ?? null;
      if (categoryId) {
        const cat = db.prepare('SELECT id FROM categories WHERE id = ? AND user_id = ?').get(categoryId, userId);
        categoryId = cat?.id ?? null;
      }

      const catName  = categoryId ? null : (decision.category || previewRow?.detected_category);
      if (!categoryId && catName) {
        const cat = db.prepare('SELECT id FROM categories WHERE name = ? COLLATE NOCASE AND user_id = ?').get(catName, userId);
        categoryId = cat?.id ?? null;
      }

      const dueDay = decision.due_day ?? 1;
      const expectedAmount = decision.expected_amount ?? amount ?? 0;
      const autopay = decision.autopay_enabled ?? (previewRow?.detected_labels?.includes('autopay') ? 1 : 0);

      const ins = db.prepare(`
        INSERT INTO bills (user_id, name, category_id, due_day, bucket, expected_amount, billing_cycle, cycle_type, cycle_day, autopay_enabled, active)
        VALUES (?, ?, ?, ?, ?, ?, 'monthly', 'monthly', ?, ?, 1)
      `).run(userId, name, categoryId, dueDay, dueDay <= 14 ? '1st' : '15th', toCents(expectedAmount), String(dueDay), autopay);

      const newBillId = ins.lastInsertRowid;
      summary.created++;
      const detail = { row_id, action, result: 'created', bill_id: newBillId, bill_name: name };

      if (year && month) {
        const st = upsertMonthlyState(db, newBillId, year, month, amount, notes, null, allowOverwrite);
        detail.monthly_state = st.result;
      }
      const { paymentDate, paymentAmount } = resolvePaymentInfo(decision, previewRow, amount);
      const paymentResult = createPaymentFromImport(
        db,
        newBillId,
        paymentAmount,
        paymentDate,
        decision.payment_notes ?? notes,
        allowOverwrite,
      );
      if (paymentResult) {
        detail.payment = paymentResult.result;
        detail.paid_date = paymentDate;
        detail.payment_amount = paymentAmount;
      }
      const relatedImported = importRelatedPaidMonthsForNewBill(
        db,
        newBillId,
        name,
        row_id,
        sessionData,
        appliedRowIds,
        allowOverwrite,
        summary,
      );
      if (relatedImported > 0) detail.related_months_imported = relatedImported;
      summary.details.push(detail);

    } else if (['match_existing_bill', 'update_monthly_state', 'add_monthly_note'].includes(action)) {
      const billId = decision.bill_id;
      const bill   = db.prepare('SELECT id, name, autopay_enabled FROM bills WHERE id = ? AND active = 1 AND user_id = ?').get(billId, userId);
      if (!bill) throw new Error(`Bill id=${billId} not found or inactive`);

      if (!bill.autopay_enabled && previewRow?.detected_labels?.includes('autopay')) {
        db.prepare(`UPDATE bills SET autopay_enabled = 1, updated_at = datetime('now') WHERE id = ?`).run(billId);
        console.log(`[import] bill id=${billId} "${bill.name}" autopay_enabled upgraded to 1`);
      }

      if (!year || !month) {
        summary.ambiguous++;
        summary.details.push({ row_id, action, result: 'ambiguous', error: 'year and month required for monthly state' });
        return;
      }

      const noteToStore   = action === 'add_monthly_note' ? (decision.notes ?? previewRow?.detected_notes ?? null) : notes;
      const amountToStore = action === 'add_monthly_note' ? null : amount;
      const isSkipped     = decision.is_skipped ?? null;

      const st = upsertMonthlyState(db, billId, year, month, amountToStore, noteToStore, isSkipped, allowOverwrite);

      if (st.result === 'skipped_conflict') {
        summary.skipped++;
      } else if (st.result === 'skipped_duplicate') {
        summary.skipped++;
        summary.duplicates++;
      } else if (st.result === 'created') {
        summary.created++;
      } else {
        summary.updated++;
      }

      const detail = { row_id, action, result: st.result, bill_id: billId };
      if (st.note) detail.note = st.note;

      const { paymentDate, paymentAmount } = resolvePaymentInfo(decision, previewRow, amount);
      if (paymentDate && paymentAmount != null && paymentAmount > 0) {
        const paymentResult = createPaymentFromImport(
          db,
          billId,
          paymentAmount,
          paymentDate,
          decision.payment_notes ?? noteToStore,
          allowOverwrite,
        );
        if (paymentResult) {
          detail.payment = paymentResult.result;
          detail.paid_date = paymentDate;
          detail.payment_amount = paymentAmount;
          if (paymentResult.result === 'skipped_duplicate') {
            summary.duplicates++;
            detail.existing_created_at = paymentResult.existing_created_at;
          }
        }
      }
      summary.details.push(detail);

    } else if (action === 'create_payment') {
      const billId = decision.bill_id;
      const bill   = db.prepare('SELECT id, current_balance, interest_rate, interest_accrued_month FROM bills WHERE id = ? AND active = 1 AND user_id = ?').get(billId, userId);
      if (!bill) throw new Error(`Bill id=${billId} not found or inactive`);

      const payAmount = decision.payment_amount ?? amount;
      const payDate   = decision.payment_date
        ?? (year && month ? `${year}-${String(month).padStart(2, '0')}-01` : null);

      if (!payAmount || !payDate) {
        summary.ambiguous++;
        summary.details.push({ row_id, action, result: 'ambiguous', error: 'payment_amount and payment_date required' });
        return;
      }

      // payAmount is dollars (decision/spreadsheet); payments columns are cents
      const payAmountCents = toCents(payAmount);

      const dup = db.prepare(`
        SELECT id, created_at, paid_date, amount FROM payments
        WHERE  bill_id = ? AND paid_date = ? AND amount = ? AND deleted_at IS NULL
      `).get(billId, payDate, payAmountCents);

      if (dup && !allowOverwrite) {
        summary.skipped++;
        summary.duplicates++;
        summary.details.push({
          row_id,
          action,
          result:              'skipped_duplicate',
          note:                'Identical payment already exists',
          existing_created_at: dup.created_at ?? null,
          existing_paid_date:  dup.paid_date  ?? null,
          existing_amount:     fromCents(dup.amount),
        });
        return;
      }

      const balCalcCp = computeBalanceDelta(bill, payAmountCents);

      db.prepare(`
        INSERT INTO payments (bill_id, amount, paid_date, method, notes, balance_delta, interest_delta, payment_source)
        VALUES (?, ?, ?, ?, ?, ?, ?, 'file_import')
      `).run(billId, payAmountCents, payDate, decision.payment_method ?? null, decision.payment_notes ?? null, balCalcCp?.balance_delta ?? null, balCalcCp?.interest_delta ?? null);

      applyBalanceDelta(db, billId, balCalcCp);

      summary.created++;
      summary.details.push({ row_id, action, result: 'created', bill_id: billId, paid_date: payDate, amount: payAmount });
    }
  } catch (err) {
    summary.errored++;
    summary.details.push({ row_id, action, result: 'error', error: err.message });
  }
}

async function applyImportDecisions(userId, importSessionId, decisions, opts = {}) {
  const db = getDb();
  ensureUserDefaultCategories(userId);

  const sessionData = loadImportSession(db, importSessionId, userId);
  const rowLookup   = Object.fromEntries(sessionData.rows.map((r) => [r.row_id, r]));
  const appliedRowIds = new Set(decisions.map((d) => d.row_id));

  for (let i = 0; i < decisions.length; i++) {
    validateDecision(decisions[i], i);
    if (!rowLookup[decisions[i].row_id]) {
      throw decisionValidationError(i, decisions[i], 'row_id', `row_id '${decisions[i].row_id}' was not found in this import session`);
    }
  }

  const allowOverwrite = opts.overwrite === true;
  const reviewedSkippedCount = Number.isInteger(Number(opts.reviewed_skipped_count))
    ? Math.max(0, Number(opts.reviewed_skipped_count))
    : 0;
  const summary = { created: 0, updated: 0, skipped: reviewedSkippedCount, duplicates: 0, errored: 0, ambiguous: 0, details: [] };

  const applyAll = db.transaction(() => {
    for (const decision of decisions) {
      applyOneDecision(db, userId, decision, rowLookup[decision.row_id], sessionData, allowOverwrite, summary, appliedRowIds);
    }
  });
  applyAll();

  try {
    db.prepare(`
      INSERT INTO import_history (
        user_id, imported_at, source_filename, file_type, sheet_name,
        rows_parsed, rows_created, rows_updated, rows_skipped, rows_ambiguous,
        rows_errored, options_json, summary_json
      ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
    `).run(
      userId,
      new Date().toISOString(),
      sessionData.original_filename,
      'xlsx',
      sessionData.sheet_name,
      sessionData.rows.length,
      summary.created,
      summary.updated,
      summary.skipped,
      summary.ambiguous,
      summary.errored,
      JSON.stringify(opts),
      JSON.stringify(summary.details),
    );
  } catch (histErr) {
    console.error('[import] history write failed:', histErr.message);
  }

  // Session is intentionally kept alive until its 24-hour TTL expires.
  // Deleting it here would prevent the user from importing additional bills
  // from the same file in a second apply call (Bills tab workflow).
  // pruneExpiredSessions() handles cleanup on the next preview call.

  return {
    success:           true,
    rows_parsed:       sessionData.rows.length,
    rows_created:      summary.created,
    rows_updated:      summary.updated,
    rows_skipped:      summary.skipped,
    rows_ambiguous:    summary.ambiguous,
    rows_errored:      summary.errored,
    rows_duplicates:   summary.duplicates ?? 0,
    details:           summary.details,
  };
}

// ─── Import History ───────────────────────────────────────────────────────────

function getImportHistory(userId) {
  const db = getDb();
  return db.prepare(`
    SELECT id, imported_at, source_filename, file_type, sheet_name,
           rows_parsed, rows_created, rows_updated, rows_skipped,
           rows_ambiguous, rows_errored
    FROM   import_history
    WHERE  user_id = ?
    ORDER  BY imported_at DESC
    LIMIT  100
  `).all(userId);
}

module.exports = {
  detectAllHeaderSets,
  previewSpreadsheet,
  applyImportDecisions,
  getImportHistory,
  // Exported for testing
  parseSheetName,
  resolveYearMonth,
  parseAmount,
  parseDate,
  normalizeName,
  findBillMatches,
  buildRecommendation,
};
