// A tiny, dependency-free Markdown parser for the READ view (note cards). Capture
// stays plain text; this only formats what's shown. We render the parsed tree as
// Vue vnodes (never v-html), so there is no HTML-injection surface. Deliberately a
// small subset — headings (#..###), unordered/ordered lists, blockquote, fenced
// code, and inline **bold** / *italic* / _italic_ / `code`. Note: headings need a
// space after `#`, so a #tag (no space) is left as plain text and never mistaken for
// a heading.

export interface InlineToken {
  /** `tag` carries the NAME, without the leading `#` — it is both what gets looked up
   *  for a colour and what is rendered, so the renderer puts the `#` back. */
  type: "text" | "bold" | "italic" | "code" | "tag";
  value: string;
}

// Flat (non-discriminated) shape on purpose — keeps template type-checking simple.
export interface Block {
  type: "p" | "h1" | "h2" | "h3" | "quote" | "ul" | "ol" | "pre" | "task";
  inline?: InlineToken[];
  items?: InlineToken[][];
  value?: string;
  /** `task` only: one entry per `items` entry, parallel by position. */
  tasks?: TaskMeta[];
}

export interface TaskMeta {
  /** This item's ordinal among ALL task lines in the body, in document order.
   *  That is the id the server and the native clients address an item by, so a
   *  checkbox can be toggled straight from it. Counted across blocks, not within
   *  one, and unaffected by the card truncating the body — the card only ever
   *  drops lines from the END. */
  index: number;
  checked: boolean;
}

// Order matters: code is matched before emphasis so its contents aren't re-parsed;
// bold (**) before italic (*). Emphasis does not nest (v1).
//
// `[[wiki-links]]` used to lead this alternation. They are gone (note 2897) — this is
// a capture-and-recall surface, and a linking system is organization. `[[text]]` now
// renders as the literal characters someone typed, which is what it always was.
// `#tag` is LAST in the alternation and that is load-bearing twice over. JS tries
// alternatives left to right, so a `#tag` inside backticks is claimed by `code` first
// and stays literal — matching the core, where a fenced block's contents are code.
// And a tag is the one token here that is not delimiter-based, so it must not get a
// chance to start inside `**bold #x**`.
//
// The grammar MIRRORS `line_tags` in core/src/local/derive.rs, which is the definition:
// a `#` at a word boundary (the preceding character is neither a tag character nor
// another `#`, so `a#b` and `##x` are not tags), a letter immediately after it, then
// alphanumerics, `_` and `-`. Rust's `is_alphanumeric` is `Alphabetic | N`, hence the
// property escapes rather than `\w` — and hence the `u` flag.
//
// A heading cannot collide with this: `parseMarkdown` requires a space after the `#`s,
// which `#tag` by definition does not have.
const INLINE_RE =
  /(`[^`]+`)|(\*\*[^*]+\*\*)|(\*[^*]+\*)|(_[^_]+_)|((?<![\p{Alphabetic}\p{N}_#-])#\p{Alphabetic}[\p{Alphabetic}\p{N}_-]*)/gu;

export function parseInline(text: string): InlineToken[] {
  const tokens: InlineToken[] = [];
  let last = 0;
  let m: RegExpExecArray | null;
  INLINE_RE.lastIndex = 0;
  while ((m = INLINE_RE.exec(text)) !== null) {
    if (m.index > last) tokens.push({ type: "text", value: text.slice(last, m.index) });
    const raw = m[0];
    if (m[1]) tokens.push({ type: "code", value: raw.slice(1, -1) });
    else if (m[2]) tokens.push({ type: "bold", value: raw.slice(2, -2) });
    else if (m[5]) tokens.push({ type: "tag", value: raw.slice(1) });
    else tokens.push({ type: "italic", value: raw.slice(1, -1) });
    last = m.index + raw.length;
  }
  if (last < text.length) tokens.push({ type: "text", value: text.slice(last) });
  return tokens;
}

// A checklist item: the third implementation of one grammar, alongside
// core/src/local/derive.rs and src/thoughtsync/notes/checklist.py. A difference
// between any two of them is a checklist that changes shape when it syncs (M304).
//
// `-` and `*` only, deliberately, even though the `ul` matcher below also takes `+`.
// The other two implementations do not take `+`, and one grammar in three places has
// to be one grammar; a `+ [ ] x` line renders as an ordinary bullet everywhere,
// which is at least consistent.
const TASK_RE = /^\s*[-*] +\[([ xX])\](?: +(.*))?$/;

export interface TaskLine {
  checked: boolean;
  text: string;
}

/** One task line's parts, or null when the line is prose.
 *
 *  Exported so the EDITOR's block split (notes/blocks.ts) and this read-view parser
 *  agree by construction rather than by comment. One grammar, one matcher. */
export function parseTaskLine(line: string): TaskLine | null {
  const m = TASK_RE.exec(line);
  return m ? { checked: m[1] !== " ", text: m[2] ?? "" } : null;
}

/** One item as the body line that stores it, in canonical form — `- [x] `, lowercase,
 *  and no trailing space when the item is empty so a round trip does not grow it. */
export function renderTaskLine(text: string, checked: boolean): string {
  const mark = checked ? "x" : " ";
  return text ? `- [${mark}] ${text}` : `- [${mark}]`;
}

export function parseMarkdown(text: string): Block[] {
  const lines = (text ?? "").split("\n");
  const blocks: Block[] = [];
  let paragraph: string[] = [];
  let i = 0;
  // Runs across the whole document, not per block, because that is what the item's
  // id means everywhere else.
  let taskIndex = 0;

  const flushPara = () => {
    if (paragraph.length) {
      blocks.push({ type: "p", inline: parseInline(paragraph.join("\n")) });
      paragraph = [];
    }
  };

  while (i < lines.length) {
    const line = lines[i];

    // Fenced code block: ``` … ``` (kept verbatim, not inline-parsed).
    if (line.trimStart().startsWith("```")) {
      flushPara();
      const buf: string[] = [];
      i++;
      while (i < lines.length && !lines[i].trimStart().startsWith("```")) {
        buf.push(lines[i]);
        i++;
      }
      i++; // consume the closing fence (or fall off the end)
      blocks.push({ type: "pre", value: buf.join("\n") });
      continue;
    }

    // Heading: #, ##, ### followed by a space.
    const h = /^(#{1,3})\s+(.*)$/.exec(line);
    if (h) {
      flushPara();
      const level = h[1].length;
      blocks.push({ type: level === 1 ? "h1" : level === 2 ? "h2" : "h3", inline: parseInline(h[2]) });
      i++;
      continue;
    }

    // Blockquote: consecutive `>` lines merge into one quote.
    if (/^\s*>\s?/.test(line)) {
      flushPara();
      const buf: string[] = [];
      while (i < lines.length && /^\s*>\s?/.test(lines[i])) {
        buf.push(lines[i].replace(/^\s*>\s?/, ""));
        i++;
      }
      blocks.push({ type: "quote", inline: parseInline(buf.join("\n")) });
      continue;
    }

    // Task list, BEFORE the plain bullet below — which would otherwise swallow
    // `- [ ] x` as an ordinary list item and leave the brackets showing. Same
    // ordering reason as `code` being matched before emphasis in INLINE_RE.
    if (parseTaskLine(line)) {
      flushPara();
      const items: InlineToken[][] = [];
      const tasks: TaskMeta[] = [];
      while (i < lines.length) {
        const task = parseTaskLine(lines[i]);
        if (!task) break;
        items.push(parseInline(task.text));
        tasks.push({ index: taskIndex, checked: task.checked });
        taskIndex++;
        i++;
      }
      blocks.push({ type: "task", items, tasks });
      continue;
    }

    // Unordered list: -, *, or + then a space.
    if (/^\s*[-*+]\s+/.test(line)) {
      flushPara();
      const items: InlineToken[][] = [];
      while (i < lines.length && /^\s*[-*+]\s+/.test(lines[i])) {
        items.push(parseInline(lines[i].replace(/^\s*[-*+]\s+/, "")));
        i++;
      }
      blocks.push({ type: "ul", items });
      continue;
    }

    // Ordered list: `1. ` etc.
    if (/^\s*\d+\.\s+/.test(line)) {
      flushPara();
      const items: InlineToken[][] = [];
      while (i < lines.length && /^\s*\d+\.\s+/.test(lines[i])) {
        items.push(parseInline(lines[i].replace(/^\s*\d+\.\s+/, "")));
        i++;
      }
      blocks.push({ type: "ol", items });
      continue;
    }

    // Blank line ends a paragraph; other lines accumulate into one (newlines kept).
    if (line.trim() === "") {
      flushPara();
      i++;
      continue;
    }
    paragraph.push(line);
    i++;
  }

  flushPara();
  return blocks;
}