/**
 * Lightweight markdown tokenizer for the editor highlight mirror.
 *
 * Pure module — safe to import client-side. Splits the source into lines and
 * returns, per line, segments tagged with a Tailwind class for the mirror
 * layer behind the editor textarea. Not a full parser: fence state is tracked
 * across lines, everything else is line-local.
 */

export interface HighlightSegment {
  readonly text: string;
  readonly className?: string;
}

const HL = {
  heading: "font-semibold text-sky-200",
  bold: "font-semibold text-foreground",
  italic: "italic text-foreground",
  strike: "line-through text-muted-foreground/70",
  code: "rounded bg-muted px-0.5 text-emerald-300",
  wikilink: "text-sky-300 underline underline-offset-2",
  link: "text-sky-300 underline underline-offset-2",
  quote: "italic text-muted-foreground/80",
  directive: "text-violet-300",
  fence: "text-muted-foreground/60",
  codeBlock: "text-emerald-300/50",
  listMarker: "text-amber-300",
} as const;

const INLINE_RE =
  /(`[^`\n]+`)|(\*\*[^*\n]+\*\*)|(__[^_\n]+__)|(~~[^~\n]+~~)|(\*[^*\n]+\*)|((?<![A-Za-z0-9])_[^_\n]+_(?![A-Za-z0-9]))|(\[\[[^\]\n]+\]\])|(\[[^\]\n]*\]\([^)\n]+\))/g;

const INLINE_CLASSES = [
  HL.code,
  HL.bold,
  HL.bold,
  HL.strike,
  HL.italic,
  HL.italic,
  HL.wikilink,
  HL.link,
] as const;

function tokenizeInline(text: string): HighlightSegment[] {
  const segments: HighlightSegment[] = [];
  let cursor = 0;
  for (const match of text.matchAll(INLINE_RE)) {
    const start = match.index ?? 0;
    if (start > cursor) segments.push({ text: text.slice(cursor, start) });
    const groupIndex =
      match.findIndex((capture, index) => index > 0 && capture !== undefined) - 1;
    segments.push({ text: match[0], className: INLINE_CLASSES[groupIndex] });
    cursor = start + match[0].length;
  }
  if (cursor < text.length) {
    segments.push({ text: text.slice(cursor) });
  }
  return segments;
}

const FENCE_RE = /^\s{0,3}(```|~~~)/;
const HEADING_RE = /^\s{0,3}#{1,6}\s/;
const QUOTE_RE = /^\s{0,3}>/;
const DIRECTIVE_RE = /^\s{0,3}:::/;
const HRULE_RE = /^\s{0,3}(?:-{3,}|\*{3,}|_{3,})\s*$/;
const LIST_MARKER_RE = /^(\s*)([-*+]|\d+[.)])(\s+)/;

export function highlightMarkdownLines(source: string): HighlightSegment[][] {
  const lines: HighlightSegment[][] = [];
  let inFence = false;
  for (const line of source.split("\n")) {
    if (FENCE_RE.test(line)) {
      lines.push([{ text: line, className: HL.fence }]);
      inFence = !inFence;
      continue;
    }
    if (inFence) {
      lines.push([{ text: line, className: HL.codeBlock }]);
      continue;
    }
    if (HEADING_RE.test(line)) {
      lines.push([{ text: line, className: HL.heading }]);
      continue;
    }
    if (QUOTE_RE.test(line)) {
      lines.push([{ text: line, className: HL.quote }]);
      continue;
    }
    if (DIRECTIVE_RE.test(line)) {
      lines.push([{ text: line, className: HL.directive }]);
      continue;
    }
    if (HRULE_RE.test(line)) {
      lines.push([{ text: line, className: HL.fence }]);
      continue;
    }
    const listMatch = line.match(LIST_MARKER_RE);
    if (listMatch) {
      const markerEnd = listMatch[0].length;
      lines.push([
        { text: listMatch[0], className: HL.listMarker },
        ...tokenizeInline(line.slice(markerEnd)),
      ]);
      continue;
    }
    lines.push(tokenizeInline(line));
  }
  return lines;
}