/**
 * @file Parses git's merge-conflict markers out of a file's lines, and writes
 * the file back with one side (or both) chosen per block — the model behind
 * the Changes panel's conflict view. Handles diff3 style (`|||||||` base
 * section, dropped: it is neither side). Pure, no React.
 * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
 */

export type ConflictChoice = "ours" | "theirs" | "both";

export type ConflictPart =
  | { kind: "text"; lines: string[] }
  | { kind: "conflict"; ours: string[]; theirs: string[]; oursLabel: string; theirsLabel: string };

const OPEN = /^<{7}(?: (.*))?$/;
const BASE = /^\|{7}(?: .*)?$/;
const MID = /^={7}$/;
const CLOSE = /^>{7}(?: (.*))?$/;

/**
 * The file as text runs and conflict blocks, or null when its markers do not
 * balance (a hand edit left half a block) — the caller falls back to the
 * plain editor then rather than guess.
 */
export function parseConflicts(lines: string[]): ConflictPart[] | null {
  const parts: ConflictPart[] = [];
  let text: string[] = [];
  let block: { ours: string[]; theirs: string[]; oursLabel: string } | null = null;
  let section: "ours" | "base" | "theirs" = "ours";

  for (const line of lines) {
    if (!block) {
      const open = line.match(OPEN);
      if (open) {
        if (text.length) parts.push({ kind: "text", lines: text });
        text = [];
        block = { ours: [], theirs: [], oursLabel: open[1] ?? "" };
        section = "ours";
      } else if (CLOSE.test(line) || MID.test(line)) {
        return null;
      } else {
        text.push(line);
      }
      continue;
    }
    if (section === "ours" && BASE.test(line)) section = "base";
    else if (section !== "theirs" && MID.test(line)) section = "theirs";
    else if (section === "theirs" && CLOSE.test(line)) {
      const close = line.match(CLOSE)!;
      parts.push({ kind: "conflict", ...block, theirsLabel: close[1] ?? "" });
      block = null;
    } else if (OPEN.test(line)) return null;
    else if (section === "ours") block.ours.push(line);
    else if (section === "theirs") block.theirs.push(line);
  }
  if (block) return null;
  if (text.length || !parts.length) parts.push({ kind: "text", lines: text });
  return parts;
}

/** The file's lines with block `i` replaced by `choices[i]`; an unchosen block keeps its markers. */
export function applyChoices(
  parts: ConflictPart[],
  choices: (ConflictChoice | undefined)[]
): string[] {
  const out: string[] = [];
  let i = 0;
  for (const p of parts) {
    if (p.kind === "text") {
      out.push(...p.lines);
      continue;
    }
    const c = choices[i++];
    if (c === "ours") out.push(...p.ours);
    else if (c === "theirs") out.push(...p.theirs);
    else if (c === "both") out.push(...p.ours, ...p.theirs);
    else
      out.push(
        `<<<<<<< ${p.oursLabel}`.trimEnd(),
        ...p.ours,
        "=======",
        ...p.theirs,
        `>>>>>>> ${p.theirsLabel}`.trimEnd()
      );
  }
  return out;
}