// Git-format patches, as `git diff` and `jj diff --git` write them, read into files and their hunks.

/** One file's change. */
export type FileDiff = {
  /** Where it is after the change; where it was, for a deletion. */
  path: string;
  /** Where it was before, when it was renamed or copied. */
  from: string | null;
  status: "added" | "deleted" | "modified" | "renamed" | "copied";
  /** Its modes either side, as far as the patch says: such as `100755` once a script is executable. */
  modes: { old: string | null; new: string | null };
  binary: boolean;
  hunks: Hunk[];
};

/**
 * A stretch of the file: the line it starts at on either side, and its lines, each marked `" "`, `"-"` or `"+"`, or
 * `"\\"` when the line before it has no newline.
 */
export type Hunk = { old: number; new: number; lines: string[] };

const HUNK = /^@@ -(\d+)(?:,(\d+))? \+(\d+)(?:,(\d+))? @@/;

/** The files `patch` changes, in its order; whatever surrounds them, such as a commit's header, is skipped. */
export function parsePatch(patch: string): FileDiff[] {
  const lines = patch.split("\n");
  const files: FileDiff[] = [];
  let i = 0;
  while (i < lines.length) {
    const header = lines[i++];
    if (!header.startsWith("diff --git ")) continue;
    const [a, b] = headerPaths(header.slice("diff --git ".length));
    let old: string | null = a;
    let neu: string | null = b;
    let status: FileDiff["status"] = "modified";
    const modes: FileDiff["modes"] = { old: null, new: null };
    let binary = false;
    const hunks: Hunk[] = [];
    for (; i < lines.length && !lines[i].startsWith("diff --git "); i++) {
      const line = lines[i];
      const hunk = line.match(HUNK);
      if (hunk) {
        let oldLeft = hunk[2] === undefined ? 1 : Number(hunk[2]);
        let newLeft = hunk[4] === undefined ? 1 : Number(hunk[4]);
        const body: string[] = [];
        while (i + 1 < lines.length && (oldLeft > 0 || newLeft > 0 || lines[i + 1][0] === "\\")) {
          const next = lines[++i];
          // `diff.suppressBlankEmpty` writes an empty context line without its mark
          const mark = next === "" ? " " : next[0];
          if (mark === " ") oldLeft--, newLeft--;
          else if (mark === "-") oldLeft--;
          else if (mark === "+") newLeft--;
          else if (mark !== "\\") {
            i--;
            break;
          }
          body.push(next === "" ? " " : next);
        }
        hunks.push({ old: Number(hunk[1]), new: Number(hunk[3]), lines: body });
        continue;
      }
      const [, key, value] = line.match(
        /^(new file mode|deleted file mode|old mode|new mode|rename from|rename to|copy from|copy to|index) (.*)$/,
      ) ?? [];
      if (line.startsWith("--- ")) old = sidePath(line.slice(4), "a/");
      else if (line.startsWith("+++ ")) neu = sidePath(line.slice(4), "b/");
      else if (key === "new file mode") (status = "added"), (modes.new = value), (old = null);
      else if (key === "deleted file mode") (status = "deleted"), (modes.old = value), (neu = null);
      else if (key === "old mode") modes.old = value;
      else if (key === "new mode") modes.new = value;
      else if (key === "rename from") (status = "renamed"), (old = unquote(value));
      else if (key === "rename to") neu = unquote(value);
      else if (key === "copy from") (status = "copied"), (old = unquote(value));
      else if (key === "copy to") neu = unquote(value);
      else if (key === "index") {
        const mode = value.split(" ")[1];
        if (mode) modes.old ??= mode, modes.new ??= mode;
      } else if (line.startsWith("Binary files ") || line === "GIT binary patch") binary = true;
    }
    if (status === "modified" && old !== null && neu !== null && old !== neu) status = "renamed";
    files.push({
      path: neu ?? old ?? b,
      from: status === "renamed" || status === "copied" ? old : null,
      status,
      modes,
      binary,
      hunks,
    });
  }
  return files;
}

/** The two paths of `diff --git a/x b/y`, which are ambiguous unquoted; patches say them again where it matters. */
function headerPaths(rest: string): [string, string] {
  if (rest.startsWith('"')) {
    const [a, end] = quoted(rest);
    const after = rest.slice(end + 2);
    return [strip(a, "a/"), strip(after.startsWith('"') ? quoted(after)[0] : after, "b/")];
  }
  if (rest.endsWith('"')) {
    const at = rest.lastIndexOf(' "');
    return [strip(rest.slice(0, at), "a/"), strip(quoted(rest.slice(at + 1))[0], "b/")];
  }
  // unless one was renamed or copied, which says so on its own lines, both sides are the same path
  const half = (rest.length - 1) / 2;
  if (Number.isInteger(half) && rest.slice(2, half) === rest.slice(half + 3)) {
    return [rest.slice(2, half), rest.slice(half + 3)];
  }
  const at = rest.indexOf(" b/");
  return [strip(rest.slice(0, at), "a/"), rest.slice(at + 3)];
}

/** A `---` or `+++` line's path, or `null` for `/dev/null`; git ends those with spaces in them with a tab. */
function sidePath(text: string, prefix: string): string | null {
  const path = unquote(text.replace(/\t$/, ""));
  return path === "/dev/null" ? null : strip(path, prefix);
}

const strip = (path: string, prefix: string) =>
  path.startsWith(prefix) ? path.slice(prefix.length) : path;

const unquote = (text: string) => text.startsWith('"') ? quoted(text)[0] : text;

const ESCAPES: Record<string, number> = {
  a: 7,
  b: 8,
  t: 9,
  n: 10,
  v: 11,
  f: 12,
  r: 13,
  '"': 34,
  "\\": 92,
};

/** A C-quoted string at the start of `text`, as git quotes unusual paths, and the index of its closing quote. */
function quoted(text: string): [string, number] {
  const bytes: number[] = [];
  const encoder = new TextEncoder();
  let i = 1;
  for (; i < text.length && text[i] !== '"'; i++) {
    if (text[i] !== "\\") {
      bytes.push(...encoder.encode(text[i]));
      continue;
    }
    const octal = text.slice(i + 1, i + 4);
    if (/^[0-7]{3}$/.test(octal)) {
      bytes.push(parseInt(octal, 8));
      i += 3;
    } else {
      i++;
      bytes.push(ESCAPES[text[i]] ?? text.charCodeAt(i));
    }
  }
  return [new TextDecoder().decode(new Uint8Array(bytes)), i];
}

/** `file` with only `context` unchanged lines around each change, for files too long to send whole. */
export function narrow(file: FileDiff, context: number): FileDiff {
  const hunks: Hunk[] = [];
  for (const hunk of file.hunks) {
    let oldLine = hunk.old;
    let newLine = hunk.new;
    const changed = hunk.lines.map((line) => line[0] === "-" || line[0] === "+");
    // a missing newline belongs to the line before it
    const near = hunk.lines.map((line, i) =>
      line[0] === "\\" ||
      changed.slice(Math.max(0, i - context), i + context + 1).some(Boolean)
    );
    let current: Hunk | null = null;
    for (const [i, line] of hunk.lines.entries()) {
      if (near[i]) {
        current ??= hunks[hunks.push({ old: oldLine, new: newLine, lines: [] }) - 1];
        current.lines.push(line);
      } else current = null;
      if (line[0] === " " || line[0] === "-") oldLine++;
      if (line[0] === " " || line[0] === "+") newLine++;
    }
  }
  return { ...file, hunks };
}