export class DiffEngine {
  build(oldText, newText) {
    const a = (oldText ?? "").replace(/\r\n/g, "\n").split("\n");
    const b = (newText ?? "").replace(/\r\n/g, "\n").split("\n");
    const lcs = this.#lcsTable(a, b);
    const ops = [];

    let i = 0;
    let j = 0;
    while (i < a.length && j < b.length) {
      if (a[i] === b[j]) {
        ops.push({ kind: "equal", oldLine: a[i], newLine: b[j], oldIndex: i, newIndex: j });
        i += 1;
        j += 1;
      } else if (lcs[i + 1][j] >= lcs[i][j + 1]) {
        ops.push({ kind: "remove", oldLine: a[i], oldIndex: i, newIndex: j });
        i += 1;
      } else {
        ops.push({ kind: "add", newLine: b[j], oldIndex: i, newIndex: j });
        j += 1;
      }
    }

    while (i < a.length) {
      ops.push({ kind: "remove", oldLine: a[i], oldIndex: i, newIndex: j });
      i += 1;
    }
    while (j < b.length) {
      ops.push({ kind: "add", newLine: b[j], oldIndex: i, newIndex: j });
      j += 1;
    }

    return ops.map((op, index) => ({ ...op, id: index }));
  }

  #lcsTable(a, b) {
    const rows = a.length + 1;
    const cols = b.length + 1;
    const table = Array.from({ length: rows }, () => Array(cols).fill(0));
    for (let i = rows - 2; i >= 0; i -= 1) {
      for (let j = cols - 2; j >= 0; j -= 1) {
        if (a[i] === b[j]) table[i][j] = table[i + 1][j + 1] + 1;
        else table[i][j] = Math.max(table[i + 1][j], table[i][j + 1]);
      }
    }
    return table;
  }
}