export type FileStamp = { mtimeMs: number; size: number }

/**
 * Small bounded cache of values derived from file contents, keyed by path and
 * invalidated when the file's mtime or size changes. Oldest entries are evicted first.
 */
export function createStatCache<T>(maxEntries = 500) {
  const entries = new Map<string, FileStamp & { value: T }>()

  return {
    /** Returns `{ value }` on a hit (the value itself may be null), `undefined` on a miss or stale entry. */
    get(filePath: string, stamp: FileStamp): { value: T } | undefined {
      const entry = entries.get(filePath)
      if (!entry) return undefined
      if (entry.mtimeMs !== stamp.mtimeMs || entry.size !== stamp.size) {
        entries.delete(filePath)
        return undefined
      }
      return { value: entry.value }
    },
    set(filePath: string, stamp: FileStamp, value: T): void {
      entries.delete(filePath)
      entries.set(filePath, { mtimeMs: stamp.mtimeMs, size: stamp.size, value })
      while (entries.size > maxEntries) {
        const oldest = entries.keys().next().value
        if (oldest === undefined) break
        entries.delete(oldest)
      }
    },
    get size(): number {
      return entries.size
    },
  }
}