/*
 * Fix OVER-ESCAPED LaTeX backslashes in textbook HTML.
 *
 * BUG: some formulas in JS string literals have too many backslashes, e.g.
 *      "$V=\\\\dfrac{1}{3}S_{осн}\\\\cdot h$"   (4 backslashes)
 * After JS unescaping KaTeX receives  \\dfrac  -> it renders "\\" as a LINE
 * BREAK and prints "dfrac"/"cdot" as plain text (exactly the screenshot).
 * The correct literal is 2 backslashes ("\\dfrac" -> value \dfrac).
 *
 * PARITY RULE (critical — protects legitimate row separators):
 *   literal-run length  value backslashes  meaning
 *      2                  1   \cmd                         OK   keep
 *      4                  2   \\  + "cmd"(text)            BUG  -> 2
 *      6                  3   \\ + \cmd (rowbreak+cmd)     OK   keep
 *      8                  4   \\\\ + "cmd"(text)           BUG  -> 2
 *   => collapse ONLY runs whose length is a multiple of 4, AND only when the
 *      run is immediately followed by a known LaTeX command. Runs before "x",
 *      digits, etc. (real \\ row separators inside cases/array) are untouched.
 *
 * Usage:  node backend/scripts/fix_overescaped_latex.js          (dry run)
 *         node backend/scripts/fix_overescaped_latex.js --apply  (write)
 */
'use strict';
const fs = require('fs');
const path = require('path');

const APPLY = process.argv.includes('--apply');

// Known LaTeX commands observed at 4/8 backslashes (exact-match whitelist).
const CMDSET = new Set([
  'dfrac','tfrac','frac','sqrt','cdot','pi','log','ln','lg','alpha','beta','gamma',
  'delta','Delta','theta','lambda','mu','sigma','phi','varphi','omega','infty',
  'iff','in','notin','ne','neq','ge','geq','le','leq','mathbb','mathrm',
  'leftrightarrow','rightarrow','leftarrow','times','div','vec','overline',
  'perp','parallel','cos','sin','tan','cot','ldots','cdots','pm','mp','angle','triangle',
]);

let katex = null;
try { katex = require('katex'); } catch { /* validation optional */ }
function mathRegions(t) {
  const out = []; let i = 0;
  while (i < t.length) {
    const a = t.indexOf('$', i); if (a < 0) break;
    const dbl = t[a + 1] === '$'; const s = a + (dbl ? 2 : 1);
    let b = dbl ? t.indexOf('$$', s) : t.indexOf('$', s);
    if (b < 0 && dbl) b = t.indexOf('$', s);
    if (b < 0) break;
    out.push(t.slice(s, b));
    i = b + (dbl && t.slice(b, b + 2) === '$$' ? 2 : 1);
  }
  return out;
}
// These math strings live in JS literals; KaTeX sees them AFTER one level of JS
// unescaping. Emulate that so validation reflects what the browser renders.
function jsUnescape(s) {
  return s.replace(/\\\\/g, '\\');
}
function katexErrors(t) {
  if (!katex) return null;
  let bad = 0;
  for (const inner of mathRegions(t)) {
    const expr = jsUnescape(inner);
    try { katex.renderToString(expr, { throwOnError: true }); }
    catch { bad++; }
  }
  return bad;
}

const dir = path.join(__dirname, '..', '..', 'frontend', 'textbooks');
const files = ['algebra_11_ch1.html','algebra_11_ch3.html','geometry_11_ch3.html',
  'geometry_11_ch2.html','geometry_11_ch1.html','algebra_11_ch2.html','algebra_8.html',
  'algebra_7_ch4.html','geometry_11_ch4.html'];

const report = [];
report.push('MODE: ' + (APPLY ? 'APPLY' : 'DRY-RUN'));
let grandFixes = 0;

for (const f of files) {
  const p = path.join(dir, f);
  const t = fs.readFileSync(p, 'utf8');
  const before = katexErrors(t);

  const perCmd = {};
  let fixes = 0;
  const next = t.replace(/(\\{4,})([A-Za-z]+)/g, (whole, bs, word) => {
    if (bs.length % 4 !== 0) return whole;        // 6,10,... rowbreak+command -> keep
    if (!CMDSET.has(word)) return whole;          // x / begin / unknown -> keep
    fixes++;
    perCmd[word] = (perCmd[word] || 0) + 1;
    return '\\\\' + word;                         // collapse to two backslashes
  });

  // validate by emulating browser render of the FIXED text
  const after = katexErrors(next);
  grandFixes += fixes;
  report.push('');
  report.push(f + ': fixes=' + fixes + '  katexErrors before=' + before + ' after=' + after +
    (fixes ? '  cmds=' + JSON.stringify(perCmd) : ''));
  if (after !== null && before !== null && after > before)
    report.push('  !! WARNING: katex errors INCREASED — not writing this file');

  if (APPLY && fixes > 0 && !(after > before)) fs.writeFileSync(p, next, 'utf8');
}

report.push('');
report.push('TOTAL fixes: ' + grandFixes);
fs.writeFileSync(path.join(__dirname, 'fix_overescaped_latex.report.txt'), report.join('\n'), 'utf8');
console.log(report.join('\n'));