Reader: show how much another translation differs from the one being read

- Word-level comparison (ignores case and punctuation): differing words are
  marked in both columns of the side-by-side view
- Chapter summary ("NET is 18% different from BSB · 37 of 39 verses differ")
  with links to the most different verses, and a percentage on each verse
- Compare tab on a verse shows each translation's difference from the one
  being read, with the differing words marked
- "Highlight differences" setting; your own highlights and search matches
  always win where they overlap a difference mark

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/utils/textDiff.js

@@ -0,0 +1,90 @@
+// How two wordings of the same verse differ: which words are not shared, and how alike they are overall.
+// Comparison ignores case and punctuation ("light,”" matches "Light"), and works on whole words.
+
+// Words with their position in the original text: [{ word: 'light', start, end }]
+export function tokenize(text) {
+  const out = [];
+  const re = /[\p{L}\p{N}][\p{L}\p{N}’'-]*/gu;
+  let m;
+  while ((m = re.exec(text ?? '')) !== null) {
+    out.push({ word: m[0].toLowerCase().replace(/[’']s?$/u, '').replace(/[’'-]/gu, ''), start: m.index, end: m.index + m[0].length });
+  }
+  return out;
+}
+
+// Longest common subsequence of two word lists -> the indexes kept in each (classic DP; verses are short).
+function commonWords(a, b) {
+  const n = a.length;
+  const m = b.length;
+  const dp = Array.from({ length: n + 1 }, () => new Uint16Array(m + 1));
+  for (let i = n - 1; i >= 0; i -= 1) {
+    for (let j = m - 1; j >= 0; j -= 1) {
+      dp[i][j] = a[i].word === b[j].word ? dp[i + 1][j + 1] + 1 : Math.max(dp[i + 1][j], dp[i][j + 1]);
+    }
+  }
+  const inA = new Set();
+  const inB = new Set();
+  let i = 0;
+  let j = 0;
+  while (i < n && j < m) {
+    if (a[i].word === b[j].word) { inA.add(i); inB.add(j); i += 1; j += 1; }
+    else if (dp[i + 1][j] >= dp[i][j + 1]) i += 1;
+    else j += 1;
+  }
+  return { inA, inB };
+}
+
+// Joins neighbouring changed words into one range, so a changed phrase is one highlight, not many.
+function ranges(tokens, kept, text) {
+  const out = [];
+  tokens.forEach((t, i) => {
+    if (kept.has(i)) return;
+    const last = out[out.length - 1];
+    if (last && /^\s*$/.test(text.slice(last.end, t.start))) last.end = t.end;
+    else out.push({ start: t.start, end: t.end });
+  });
+  return out;
+}
+
+// -> { a: [{ start, end }], b: [{ start, end }], similarity: 0..1, same: bool }
+// `a` and `b` are the character ranges (in each text) of words the other does not have.
+export function diffVerses(textA, textB) {
+  const a = tokenize(textA);
+  const b = tokenize(textB);
+  if (a.length === 0 && b.length === 0) return { a: [], b: [], similarity: 1, same: true };
+  const { inA, inB } = commonWords(a, b);
+  const similarity = (2 * inA.size) / (a.length + b.length);
+  return { a: ranges(a, inA, textA), b: ranges(b, inB, textB), similarity, same: inA.size === a.length && inB.size === b.length };
+}
+
+// A whole chapter: pairs of [mainVerse, otherVerse] by verse number ->
+// { similarity, differing, total, verses: [{ number, similarity, ...ranges }] sorted most different first }
+export function diffChapter(mainVerses, otherVerses) {
+  const other = new Map(otherVerses.map((v) => [v.number, v]));
+  let shared = 0;
+  let words = 0;
+  const verses = [];
+  for (const v of mainVerses) {
+    const o = other.get(v.number);
+    if (!o) continue;
+    const d = diffVerses(v.text, o.text);
+    const na = tokenize(v.text).length;
+    const nb = tokenize(o.text).length;
+    shared += (d.similarity * (na + nb)) / 2;
+    words += (na + nb) / 2;
+    verses.push({ number: v.number, ...d });
+  }
+  return {
+    similarity: words === 0 ? 1 : shared / words,
+    total: verses.length,
+    differing: verses.filter((v) => !v.same).length,
+    verses,
+    mostDifferent: [...verses].filter((v) => !v.same).sort((x, y) => x.similarity - y.similarity).slice(0, 3),
+  };
+}
+
+// "12% different" for a similarity of 0.88; "identical words" at 1.
+export const differenceLabel = (similarity) => {
+  const pct = Math.round((1 - similarity) * 100);
+  return pct === 0 && similarity < 1 ? '<1% different' : pct === 0 ? 'Same words' : `${pct}% different`;
+};