// Dependency-free line diff (LCS) for the CMS version comparison view.
// Input lines are plain text; output entries are typed add/del/ctx rows.

function diffLines(oldLines, newLines) {
  const n = oldLines.length;
  const m = newLines.length;
  // LCS lengths DP (files are small, a few hundred lines — O(n*m) is fine)
  const dp = Array.from({ length: n + 1 }, () => new Array(m + 1).fill(0));
  for (let i = n - 1; i >= 0; i--) {
    for (let j = m - 1; j >= 0; j--) {
      dp[i][j] = oldLines[i] === newLines[j] ? dp[i + 1][j + 1] + 1 : Math.max(dp[i + 1][j], dp[i][j + 1]);
    }
  }
  const out = [];
  let i = 0;
  let j = 0;
  while (i < n && j < m) {
    if (oldLines[i] === newLines[j]) {
      out.push({ type: 'ctx', text: oldLines[i] });
      i++;
      j++;
    } else if (dp[i + 1][j] >= dp[i][j + 1]) {
      out.push({ type: 'del', text: oldLines[i] });
      i++;
    } else {
      out.push({ type: 'add', text: newLines[j] });
      j++;
    }
  }
  while (i < n) { out.push({ type: 'del', text: oldLines[i] }); i++; }
  while (j < m) { out.push({ type: 'add', text: newLines[j] }); j++; }
  return out;
}

function escapeHtml(value) {
  return String(value)
    .replace(/&/g, '&amp;')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;');
}

// Keep only ±contextAround context lines around changes to keep pages small.
function trimContext(entries, contextAround = 3) {
  const keep = new Array(entries.length).fill(false);
  entries.forEach((e, idx) => {
    if (e.type !== 'ctx') {
      for (let k = Math.max(0, idx - contextAround); k <= Math.min(entries.length - 1, idx + contextAround); k++) keep[k] = true;
    }
  });
  const out = [];
  let skipping = false;
  entries.forEach((e, idx) => {
    if (keep[idx]) { out.push(e); skipping = false; }
    else if (!skipping) { out.push({ type: 'skip', text: '…' }); skipping = true; }
  });
  return out;
}

function renderDiffHtml(oldText, newText) {
  const entries = trimContext(diffLines(oldText.split('\n'), newText.split('\n')));
  return entries.map(e => `<div class="diff-${e.type}">${escapeHtml(e.text) || '&nbsp;'}</div>`).join('\n');
}

module.exports = { diffLines, trimContext, renderDiffHtml, escapeHtml };