/**
 * Transcript context-size helpers for the strategic-compact hook (#2155).
 *
 * Reads the latest assistant `usage` record from a Claude Code session
 * transcript (JSONL) and derives a context-size signal:
 *
 * - `input_tokens + cache_read_input_tokens + cache_creation_input_tokens`
 *   partition the prompt, so their sum is the true context size of the turn.
 * - The context window is detected from the model id (`[1m]` marker) or from
 *   the observed token count (anything above 200k implies a 1M window even
 *   when logs drop the suffix).
 * - Thresholds are window-scaled and env-overridable; re-reminders fire in
 *   fixed token "buckets" above the threshold so the suggestion only repeats
 *   after real context growth.
 *
 * Only the tail of the transcript is read (latest records live at the end),
 * keeping the PreToolUse hook fast even for very large sessions.
 */

const fs = require('fs');

const STANDARD_CONTEXT_WINDOW_TOKENS = 200000;
const LARGE_CONTEXT_WINDOW_TOKENS = 1000000;
const DEFAULT_CONTEXT_THRESHOLD_STANDARD = 160000;
const DEFAULT_CONTEXT_THRESHOLD_LARGE = 250000;
const DEFAULT_CONTEXT_INTERVAL_TOKENS = 60000;
const DEFAULT_TRANSCRIPT_TAIL_BYTES = 256 * 1024;
const MAX_TOKEN_SETTING = 10000000;
const LARGE_WINDOW_MODEL_MARKER = '[1m]';

/**
 * Read the trailing `tailBytes` of a file as UTF-8.
 * Returns null when the file is missing or unreadable.
 */
function readFileTail(filePath, tailBytes) {
  let fd;
  try {
    fd = fs.openSync(filePath, 'r');
  } catch {
    return null;
  }

  try {
    const size = fs.fstatSync(fd).size;
    const start = Math.max(0, size - tailBytes);
    const length = size - start;
    if (length <= 0) {
      return { text: '', truncated: false };
    }

    const buffer = Buffer.alloc(length);
    const bytesRead = fs.readSync(fd, buffer, 0, length, start);
    return {
      text: buffer.toString('utf8', 0, bytesRead),
      truncated: start > 0
    };
  } catch {
    return null;
  } finally {
    try {
      fs.closeSync(fd);
    } catch {
      /* ignore */
    }
  }
}

/**
 * Extract the context token total from a transcript record's usage block.
 * Returns 0 when the record carries no usable usage data.
 */
function extractUsageTokens(record) {
  const usage = record && record.message && record.message.usage;
  if (!usage || typeof usage !== 'object') {
    return 0;
  }

  const total =
    (Number.isFinite(usage.input_tokens) ? usage.input_tokens : 0) +
    (Number.isFinite(usage.cache_read_input_tokens) ? usage.cache_read_input_tokens : 0) +
    (Number.isFinite(usage.cache_creation_input_tokens) ? usage.cache_creation_input_tokens : 0);

  return total > 0 ? total : 0;
}

/**
 * Scan a session transcript (JSONL) backwards for the most recent record with
 * a non-empty `message.usage` block.
 *
 * @param {string} transcriptPath - Absolute path to the transcript JSONL.
 * @param {object} [options]
 * @param {number} [options.tailBytes] - How many trailing bytes to scan.
 * @returns {{ tokens: number, model: string } | null} Latest context size, or
 *   null when the transcript is missing, unreadable, or has no usage records.
 */
function readLatestContextTokens(transcriptPath, options = {}) {
  if (typeof transcriptPath !== 'string' || !transcriptPath) {
    return null;
  }

  const tailBytes = Number.isInteger(options.tailBytes) && options.tailBytes > 0
    ? options.tailBytes
    : DEFAULT_TRANSCRIPT_TAIL_BYTES;

  const tail = readFileTail(transcriptPath, tailBytes);
  if (!tail) {
    return null;
  }

  const lines = tail.text.split('\n');
  // The first line of a truncated tail is almost certainly partial JSON.
  const firstLine = tail.truncated ? 1 : 0;

  for (let i = lines.length - 1; i >= firstLine; i--) {
    const line = lines[i].trim();
    if (!line) continue;

    let record;
    try {
      record = JSON.parse(line);
    } catch {
      continue;
    }

    const tokens = extractUsageTokens(record);
    if (tokens > 0) {
      const model = record.message && typeof record.message.model === 'string'
        ? record.message.model
        : '';
      return { tokens, model };
    }
  }

  return null;
}

/**
 * Detect the context window size for a turn.
 * 1M when the model id carries the `[1m]` marker, or when the observed token
 * count already exceeds the standard 200k window (covers logs that drop the
 * suffix); otherwise the standard 200k window.
 */
function resolveContextWindowTokens(tokens, model) {
  if (typeof model === 'string' && model.includes(LARGE_WINDOW_MODEL_MARKER)) {
    return LARGE_CONTEXT_WINDOW_TOKENS;
  }

  if (Number.isFinite(tokens) && tokens > STANDARD_CONTEXT_WINDOW_TOKENS) {
    return LARGE_CONTEXT_WINDOW_TOKENS;
  }

  return STANDARD_CONTEXT_WINDOW_TOKENS;
}

/**
 * Resolve the context-size suggestion threshold (tokens).
 * `COMPACT_CONTEXT_THRESHOLD=0` disables the context signal entirely;
 * other invalid values fall back to the window-scaled default.
 */
function resolveContextThreshold(env, windowTokens) {
  const raw = env && env.COMPACT_CONTEXT_THRESHOLD;
  if (raw !== undefined && raw !== null && raw !== '') {
    const parsed = Number.parseInt(raw, 10);
    if (parsed === 0) {
      return 0;
    }
    if (Number.isInteger(parsed) && parsed > 0 && parsed <= MAX_TOKEN_SETTING) {
      return parsed;
    }
  }

  return windowTokens >= LARGE_CONTEXT_WINDOW_TOKENS
    ? DEFAULT_CONTEXT_THRESHOLD_LARGE
    : DEFAULT_CONTEXT_THRESHOLD_STANDARD;
}

/**
 * Resolve the re-reminder step (tokens of additional context growth before
 * the suggestion repeats). Invalid values fall back to the default.
 */
function resolveContextInterval(env) {
  const raw = env && env.COMPACT_CONTEXT_INTERVAL;
  const parsed = Number.parseInt(raw, 10);
  return Number.isInteger(parsed) && parsed > 0 && parsed <= MAX_TOKEN_SETTING
    ? parsed
    : DEFAULT_CONTEXT_INTERVAL_TOKENS;
}

/**
 * Map a context size onto a suggestion bucket.
 * Returns -1 below the threshold; bucket 0 at the threshold; +1 for every
 * `interval` tokens of growth beyond it. The hook fires only when the bucket
 * rises above the last bucket it already fired for.
 */
function computeContextBucket(tokens, threshold, interval) {
  if (!Number.isFinite(tokens) || threshold <= 0 || tokens < threshold) {
    return -1;
  }

  const step = Number.isInteger(interval) && interval > 0 ? interval : DEFAULT_CONTEXT_INTERVAL_TOKENS;
  return Math.floor((tokens - threshold) / step);
}

/**
 * Human-readable label for a context window size (e.g. "200k", "1M").
 */
function formatWindowLabel(windowTokens) {
  return windowTokens >= LARGE_CONTEXT_WINDOW_TOKENS
    ? '1M'
    : `${Math.round(windowTokens / 1000)}k`;
}

module.exports = {
  STANDARD_CONTEXT_WINDOW_TOKENS,
  LARGE_CONTEXT_WINDOW_TOKENS,
  DEFAULT_CONTEXT_THRESHOLD_STANDARD,
  DEFAULT_CONTEXT_THRESHOLD_LARGE,
  DEFAULT_CONTEXT_INTERVAL_TOKENS,
  DEFAULT_TRANSCRIPT_TAIL_BYTES,
  readLatestContextTokens,
  resolveContextWindowTokens,
  resolveContextThreshold,
  resolveContextInterval,
  computeContextBucket,
  formatWindowLabel
};