export type MarkdownImageSegment =
  | { type: "text"; value: string }
  | { type: "image"; alt?: string; src: string };

// Markdown destinations may include balanced parentheses and backslash escapes.
function readDestination(content: string, open: number) {
  if (content[open] !== "(") return null;
  let depth = 1;
  for (let index = open + 1; index < content.length; index += 1) {
    const character = content[index];
    if (character === "\\") {
      index += 1;
      continue;
    }
    if (character === "(") depth += 1;
    if (character === ")" && --depth === 0) {
      const raw = content.slice(open + 1, index);
      if (!/^https?:\/\//i.test(raw) || /\s/.test(raw)) return null;
      return { url: raw.replace(/\\([()\\])/g, "$1"), end: index + 1 };
    }
  }
  return null;
}

function readImage(content: string, start: number) {
  if (!content.startsWith("![", start)) return null;
  let depth = 1;
  for (let index = start + 2; index < content.length; index += 1) {
    const character = content[index];
    if (character === "\\") {
      index += 1;
      continue;
    }
    if (character === "[") depth += 1;
    if (character !== "]" || --depth !== 0) continue;
    const destination = readDestination(content, index + 1);
    if (!destination) return null;
    return {
      alt: content.slice(start + 2, index).replace(/\\([\[\]\\])/g, "$1"),
      src: destination.url,
      end: destination.end,
    };
  }
  return null;
}

export function splitMarkdownImages(content: string): MarkdownImageSegment[] {
  const segments: MarkdownImageSegment[] = [];
  let textStart = 0;
  let cursor = 0;
  while (cursor < content.length) {
    // Leave Markdown examples in code spans and fences untouched.
    if (content[cursor] === "`") {
      let endOfRun = cursor + 1;
      while (content[endOfRun] === "`") endOfRun += 1;
      const marker = content.slice(cursor, endOfRun);
      const close = content.indexOf(marker, endOfRun);
      cursor = close < 0 ? content.length : close + marker.length;
      continue;
    }
    const lineStart = cursor === 0 || content[cursor - 1] === "\n";
    if (lineStart) {
      const fence = content.slice(cursor).match(/^ {0,3}(~{3,})[^\n]*(?:\n|$)/);
      if (fence) {
        const marker = fence[1];
        const closing = new RegExp(`^ {0,3}~{${marker.length},}\\s*$`);
        let line = cursor + fence[0].length;
        while (line < content.length) {
          const next = content.indexOf("\n", line);
          const end = next < 0 ? content.length : next;
          if (closing.test(content.slice(line, end))) {
            line = next < 0 ? end : end + 1;
            break;
          }
          line = next < 0 ? end : end + 1;
        }
        cursor = line;
        continue;
      }
    }
    if (content[cursor] === "\\") {
      cursor += 2;
      continue;
    }
    const wrapped = content.startsWith("[![", cursor);
    const image = readImage(content, cursor + (wrapped ? 1 : 0));
    if (!image) {
      cursor += 1;
      continue;
    }
    let end = image.end;
    if (wrapped) {
      const outer = content[end] === "]" ? readDestination(content, end + 1) : null;
      // Preserve incomplete, unsafe and different-target links as Markdown so
      // Streamdown applies its normal link sanitization and click behavior.
      if (!outer || outer.url !== image.src) {
        cursor = outer ? outer.end : end;
        continue;
      }
      end = outer.end;
    }
    if (cursor > textStart) {
      segments.push({ type: "text", value: content.slice(textStart, cursor) });
    }
    segments.push({ type: "image", alt: image.alt, src: image.src });
    cursor = end;
    textStart = end;
  }
  if (textStart < content.length) {
    segments.push({ type: "text", value: content.slice(textStart) });
  }
  return segments;
}