package sync

import (
	"crypto/md5"
	"encoding/hex"
	"io"
	"os"
	"time"
)

// NeedsSync returns true when src should be copied to dest.
//
// Decision logic:
//   - sizes differ → true (fast path)
//   - src is newer than dest by more than threshold → true
//   - sizes equal and mtime within threshold → delegate to MD5Changed
func NeedsSync(src, dest FileInfo, threshold time.Duration) bool {
	if src.Size != dest.Size {
		return true
	}
	diff := src.ModTime.Sub(dest.ModTime)
	if diff < 0 {
		diff = -diff
	}
	if diff > threshold {
		return src.ModTime.After(dest.ModTime)
	}
	return MD5Changed(src.AbsPath, dest.AbsPath)
}

// MD5File computes the MD5 digest of the file at path using a streaming read.
// Returns the digest as a lowercase hex string.
func MD5File(path string) (string, error) {
	f, err := os.Open(path)
	if err != nil {
		return "", err
	}
	defer f.Close()

	h := md5.New()
	if _, err := io.Copy(h, f); err != nil {
		return "", err
	}
	return hex.EncodeToString(h.Sum(nil)), nil
}

// MD5Changed returns true when the two files have different MD5 hashes.
// Any IO error is treated as "changed" so the file will be re-copied.
func MD5Changed(srcPath, destPath string) bool {
	srcHash, err := MD5File(srcPath)
	if err != nil {
		return true
	}
	destHash, err := MD5File(destPath)
	if err != nil {
		return true
	}
	return srcHash != destHash
}