export interface Mention {
  name: string;
  start: number;
  end: number;
}

// Scan prose only: escaped characters, inline code and fenced code are literal.
// The same scanner powers submission and completion so their boundaries agree.
export function findMentions(text: string, partial = false): Mention[] {
  const mentions: Mention[] = [];
  let fence: { char: string; length: number } | undefined;
  let inlineTicks = 0;
  for (let i = 0; i < text.length;) {
    if (i === 0 || text[i - 1] === "\n") {
      const lineEnd = text.indexOf("\n", i);
      const end = lineEnd < 0 ? text.length : lineEnd;
      const line = text.slice(i, end);
      const marker = /^ {0,3}(`{3,}|~{3,})(.*)$/.exec(line);
      if (fence) {
        if (marker && marker[1][0] === fence.char && marker[1].length >= fence.length && !marker[2].trim()) {
          fence = undefined;
        }
        i = end < text.length ? end + 1 : end;
        continue;
      }
      if (marker && inlineTicks === 0) {
        fence = { char: marker[1][0], length: marker[1].length };
        i = end < text.length ? end + 1 : end;
        continue;
      }
    }
    if (text[i] === "`") {
      let end = i + 1;
      while (text[end] === "`") end++;
      const length = end - i;
      if (inlineTicks === 0) inlineTicks = length;
      else if (inlineTicks === length) inlineTicks = 0;
      i = end;
      continue;
    }
    if (inlineTicks) { i++; continue; }
    if (text[i] === "\\") { i += 2; continue; }
    if (text[i] === "$" && (i === 0 || !/[a-zA-Z0-9_$/\\.\-]/.test(text[i - 1]))) {
      const name = /^[a-zA-Z0-9_-]*/.exec(text.slice(i + 1))![0];
      const end = i + 1 + name.length;
      const next = text[end];
      if ((name || partial) && (next === undefined || !/[$/\\]/.test(next))) {
        mentions.push({ name, start: i, end });
      }
      i = end;
      continue;
    }
    i++;
  }
  return mentions;
}

export function completionPrefix(lines: string[], line: number, col: number): string | undefined {
  const text = [...lines.slice(0, line), (lines[line] ?? "").slice(0, col)].join("\n");
  const last = findMentions(text, true).at(-1);
  return last?.end === text.length ? `$${last.name}` : undefined;
}