function splitLines(text: string): string[] {
  return text.match(/[^\n]*\n|[^\n]+$/g) ?? [];
}

/** One parsed piece of a conflicted file: verbatim text, or the index of a conflict block. */
export type Segment = { text: string } | { block: number };
export type ParsedBlock = { ours: string; theirs: string };

const START = /^<{7}(?: |$)/;
const BASE = /^\|{7}(?: |$)/;
const MIDDLE = /^={7}$/;
const END = /^>{7}(?: |$)/;

function stripEol(line: string): string {
  return line.replace(/\r?\n$/, "");
}

/** Splits merged marker text into verbatim text and conflict blocks; null when the markers are malformed. */
export function parseConflictText(text: string): { segments: Segment[]; blocks: ParsedBlock[] } | null {
  const segments: Segment[] = [];
  const blocks: ParsedBlock[] = [];
  let state: "outside" | "ours" | "base" | "theirs" = "outside";
  let outside: string[] = [];
  let ours: string[] = [];
  let theirs: string[] = [];
  for (const line of splitLines(text)) {
    const bare = stripEol(line);
    if (state === "outside") {
      if (START.test(bare)) {
        if (outside.length) segments.push({ text: outside.join("") });
        outside = [];
        ours = [];
        theirs = [];
        state = "ours";
      } else if (BASE.test(bare) || MIDDLE.test(bare) || END.test(bare)) return null;
      else outside.push(line);
    } else if (state === "ours" || state === "base") {
      if (BASE.test(bare) && state === "ours") state = "base";
      else if (MIDDLE.test(bare)) state = "theirs";
      else if (START.test(bare) || END.test(bare) || BASE.test(bare)) return null;
      else if (state === "ours") ours.push(line);
    } else if (END.test(bare)) {
      segments.push({ block: blocks.length });
      blocks.push({ ours: ours.join(""), theirs: theirs.join("") });
      state = "outside";
    } else if (START.test(bare) || MIDDLE.test(bare) || BASE.test(bare)) return null;
    else theirs.push(line);
  }
  if (state !== "outside") return null;
  if (outside.length) segments.push({ text: outside.join("") });
  return { segments, blocks };
}

/** Replaces each conflict block whose choice is non-null, keeping the markers of blocks still undecided. */
export function applyChoices(content: string, choices: Array<string | null>): string | null {
  const parsed = parseConflictText(content);
  if (!parsed || parsed.blocks.length !== choices.length) return null;
  const original = splitBlocks(content);
  return parsed.segments
    .map((segment) => ("text" in segment ? segment.text : (choices[segment.block] ?? original[segment.block])))
    .join("");
}

/** The raw marker text of each block, so undecided blocks round-trip unchanged. */
function splitBlocks(content: string): string[] {
  const blocks: string[] = [];
  let current: string[] | null = null;
  for (const line of splitLines(content)) {
    const bare = stripEol(line);
    if (current === null && START.test(bare)) current = [];
    if (current !== null) current.push(line);
    if (current !== null && END.test(bare)) {
      blocks.push(current.join(""));
      current = null;
    }
  }
  return blocks;
}