import fs from 'fs'
import crypto from 'crypto'

const CHUNK_SIZE = 64 * 1024 // 64 KB

/**
 * Computes a stable partial-content fingerprint for a file.
 * Uses SHA-256 of the file size + first 64 KB of content.
 * Fast enough for large video files (~instant) and collision-resistant
 * for real-world media libraries.
 *
 * Returns null if the file cannot be read (missing, permission error, etc.).
 */
export function computeFingerprint(absolutePath: string): string | null {
  try {
    const stat = fs.statSync(absolutePath)
    const size = stat.size
    const chunkLen = Math.min(CHUNK_SIZE, size)
    const buf = Buffer.alloc(chunkLen)
    if (chunkLen > 0) {
      const fd = fs.openSync(absolutePath, 'r')
      try {
        fs.readSync(fd, buf, 0, chunkLen, 0)
      } finally {
        fs.closeSync(fd)
      }
    }
    return crypto
      .createHash('sha256')
      .update(`${size}:`)
      .update(buf)
      .digest('hex')
  } catch {
    return null
  }
}