const ERROR_RE = /\b(?:error|failed|failure|missing|undefined|exception)\b/i;
const LOCATION_RE = /\b(?:at\s+.+:\d+(?::\d+)?)\b|(?:[A-Za-z0-9_./-]+\.(?:ts|tsx|js|mjs|json|md):\d+(?::\d+)?)/i;
const MAX_SUMMARY_LENGTH = 320;
const MAX_LINES = 2;

function unique(lines: string[]): string[] {
  return lines.filter((line, index) => lines.indexOf(line) === index);
}

function pickSalientLines(content: string): string[] {
  const lines = content
    .split(/\n+/)
    .map((line) => line.trim())
    .filter(Boolean);

  if (lines.length === 0) {
    return [];
  }

  const important = unique(lines.filter((line) => ERROR_RE.test(line)));
  const location = unique(lines.filter((line) => LOCATION_RE.test(line)));
  const fallback = unique(lines);

  const selected: string[] = [];
  for (const line of [...important, ...location, ...fallback]) {
    if (selected.includes(line)) {
      continue;
    }

    selected.push(line);
    if (selected.length >= MAX_LINES) {
      break;
    }
  }

  return selected;
}

export function distillToolResult(input: { toolName?: string; content: string }): string | undefined {
  const picked = pickSalientLines(input.content);
  if (picked.length === 0) {
    return undefined;
  }

  const prefix = `[distilled ${input.toolName ?? "tool"} output]`;
  return `${prefix} ${picked.join("; ")}`.slice(0, MAX_SUMMARY_LENGTH);
}