/**
 * trackChangesModel — F7 pure, vscode-free render engine (spec §6.2, INV-22/23).
 *
 * `renderTrackChanges(baselineText, currentText)` returns the annotated HTML body
 * for the preview webview: a block-level diff (LCS over normalized blocks) with
 * word-level `<ins>`/`<del>` refinement inside changed PROSE blocks, and CODE /
 * MERMAID fences kept ATOMIC (INV-23 — never word-refined, never partially
 * rendered). Deterministic: same inputs → identical HTML (INV-22). No vscode, no
 * DOM — `markdown-it` and `diff` are libraries; mermaid runs later in the webview.
 */
import MarkdownIt from "markdown-it";
import { diffArrays, diffWords } from "diff";

export type BlockType = "prose" | "code" | "mermaid";

export interface Block {
  /** Verbatim source of the block (no surrounding blank lines). */
  raw: string;
  /** Normalized match key: lowercased, whitespace-collapsed. */
  key: string;
  type: BlockType;
}

function normalize(raw: string): string {
  return raw.trim().replace(/\s+/g, " ").toLowerCase();
}

function makeBlock(raw: string, type: BlockType): Block {
  return { raw, key: normalize(raw), type };
}

/** Split markdown into top-level blocks; fenced code/mermaid stay whole. */
export function splitBlocks(text: string): Block[] {
  const lines = text.split(/\r?\n/);
  const blocks: Block[] = [];
  let buf: string[] = [];
  const flushProse = () => {
    const raw = buf.join("\n");
    if (raw.trim()) blocks.push(makeBlock(raw, "prose"));
    buf = [];
  };
  let i = 0;
  while (i < lines.length) {
    const line = lines[i];
    const fence = line.match(/^(\s*)(`{3,}|~{3,})(.*)$/);
    if (fence) {
      flushProse();
      const marker = fence[2][0]; // ` or ~
      const info = fence[3].trim().split(/\s+/)[0].toLowerCase();
      const fenceLines = [line];
      i++;
      while (i < lines.length) {
        fenceLines.push(lines[i]);
        const closed = lines[i].trim().startsWith(marker.repeat(3));
        i++;
        if (closed) break;
      }
      blocks.push(makeBlock(fenceLines.join("\n"), info === "mermaid" ? "mermaid" : "code"));
      continue;
    }
    if (line.trim() === "") {
      flushProse();
      i++;
      continue;
    }
    buf.push(line);
    i++;
  }
  flushProse();
  return blocks;
}

export type BlockOp =
  | { kind: "unchanged"; block: Block }
  | { kind: "added"; block: Block }
  | { kind: "removed"; block: Block }
  | { kind: "changed"; block: Block; before: Block; atomic: boolean };

/**
 * Diff two block sequences. Matching is by normalized key (jsdiff `diffArrays`);
 * adjacent removed-then-added runs are paired element-wise into `changed` ops, the
 * surplus staying `removed` / `added`. A `changed` op is ATOMIC (INV-23) when
 * either side is a code/mermaid fence — rendered whole, never word-refined.
 */
export function diffBlocks(baselineText: string, currentText: string): BlockOp[] {
  const before = splitBlocks(baselineText);
  const after = splitBlocks(currentText);
  const changes = diffArrays(
    before.map((b) => b.key),
    after.map((b) => b.key),
  );

  const ops: BlockOp[] = [];
  let bi = 0; // index into `before`
  let ci = 0; // index into `after`
  for (let n = 0; n < changes.length; n++) {
    const ch = changes[n];
    const count = ch.count ?? ch.value.length;
    if (!ch.added && !ch.removed) {
      for (let k = 0; k < count; k++) ops.push({ kind: "unchanged", block: after[ci++] });
      bi += count;
      continue;
    }
    if (ch.removed) {
      const next = changes[n + 1];
      const addCount = next?.added ? (next.count ?? next.value.length) : 0;
      const paired = Math.min(count, addCount);
      for (let k = 0; k < paired; k++) {
        const beforeBlk = before[bi++];
        const afterBlk = after[ci++];
        const atomic = beforeBlk.type !== "prose" || afterBlk.type !== "prose";
        ops.push({ kind: "changed", block: afterBlk, before: beforeBlk, atomic });
      }
      for (let k = paired; k < count; k++) ops.push({ kind: "removed", block: before[bi++] });
      for (let k = paired; k < addCount; k++) ops.push({ kind: "added", block: after[ci++] });
      if (next?.added) n++; // consumed the paired added run
      continue;
    }
    // a lone added run (no preceding removed)
    for (let k = 0; k < count; k++) ops.push({ kind: "added", block: after[ci++] });
  }
  return ops;
}