// A line diff, for showing what a proposed policy actually changes before anyone saves it.
//
// Hand-rolled rather than a dependency: this is one screen showing one document, the inputs are a few
// hundred lines at most, and the alternative is adding a package to the frozen lockfile for forty lines of
// well-understood algorithm. If a second surface ever needs a diff, that trade flips.

export type DiffLine = { kind: 'same' | 'add' | 'remove'; text: string };

/** Longest common subsequence table over the two line arrays. O(n·m) — fine at document scale. */
function lcsLengths(a: string[], b: string[]): number[][] {
  const table: number[][] = Array.from({ length: a.length + 1 }, () => new Array<number>(b.length + 1).fill(0));
  for (let i = a.length - 1; i >= 0; i--) {
    for (let j = b.length - 1; j >= 0; j--) {
      table[i]![j] = a[i] === b[j] ? table[i + 1]![j + 1]! + 1 : Math.max(table[i + 1]![j]!, table[i]![j + 1]!);
    }
  }
  return table;
}

/** Every line of both documents, in order, tagged with what happened to it. */
export function diffLines(before: string, after: string): DiffLine[] {
  const a = before.split('\n');
  const b = after.split('\n');
  const table = lcsLengths(a, b);

  const out: DiffLine[] = [];
  let i = 0;
  let j = 0;
  while (i < a.length && j < b.length) {
    if (a[i] === b[j]) {
      out.push({ kind: 'same', text: a[i]! });
      i++;
      j++;
    } else if (table[i + 1]![j]! >= table[i]![j + 1]!) {
      out.push({ kind: 'remove', text: a[i]! });
      i++;
    } else {
      out.push({ kind: 'add', text: b[j]! });
      j++;
    }
  }
  while (i < a.length) out.push({ kind: 'remove', text: a[i++]! });
  while (j < b.length) out.push({ kind: 'add', text: b[j++]! });
  return out;
}

/**
 * Drop long runs of unchanged lines, keeping `context` either side of every change.
 *
 * A policy is mostly unchanged by any one edit, and an unabridged diff buries the three lines that matter.
 * `null` marks each elision so the view can draw a gap rather than pretend the lines are adjacent.
 */
export function collapseUnchanged(lines: DiffLine[], context = 3): (DiffLine | null)[] {
  const keep = new Array<boolean>(lines.length).fill(false);
  lines.forEach((line, index) => {
    if (line.kind === 'same') return;
    for (let k = Math.max(0, index - context); k <= Math.min(lines.length - 1, index + context); k++) keep[k] = true;
  });

  const out: (DiffLine | null)[] = [];
  let gap = false;
  lines.forEach((line, index) => {
    if (keep[index]) {
      out.push(line);
      gap = false;
    } else if (!gap) {
      out.push(null);
      gap = true;
    }
  });
  return out;
}

export const diffCounts = (lines: DiffLine[]) => ({
  added: lines.filter((l) => l.kind === 'add').length,
  removed: lines.filter((l) => l.kind === 'remove').length,
});
