split duplicate grouping and hash helpers

internal/service/duplicate_hash.go

@@ -0,0 +1,62 @@
+// Package service — duplicate sparse file hashing.
+package service
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+)
+
+const sampleSize = 1 << 20 // 1 MiB per sample window
+
+// SparseFileHash computes the head+mid+tail SHA-256 of a file, suffixed with
+// the file size so two files that happen to collide on the sample window
+// but differ in length are still distinguishable.
+func SparseFileHash(path string) (string, error) {
+	if path == "" {
+		return "", errors.New("empty path")
+	}
+	f, err := os.Open(path) // #nosec G304 -- path is selected from configured media library files for duplicate detection.
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+	st, err := f.Stat()
+	if err != nil {
+		return "", err
+	}
+	size := st.Size()
+	h := sha256.New()
+	if size <= int64(sampleSize)*3 {
+		if _, err := io.Copy(h, f); err != nil {
+			return "", err
+		}
+		return fmt.Sprintf("%s-%d", hex.EncodeToString(h.Sum(nil)), size), nil
+	}
+	buf := make([]byte, sampleSize)
+	// head
+	if _, err := io.ReadFull(f, buf); err != nil {
+		return "", err
+	}
+	h.Write(buf)
+	// middle
+	if _, err := f.Seek(size/2-int64(sampleSize)/2, io.SeekStart); err != nil {
+		return "", err
+	}
+	if _, err := io.ReadFull(f, buf); err != nil {
+		return "", err
+	}
+	h.Write(buf)
+	// tail
+	if _, err := f.Seek(size-int64(sampleSize), io.SeekStart); err != nil {
+		return "", err
+	}
+	if _, err := io.ReadFull(f, buf); err != nil {
+		return "", err
+	}
+	h.Write(buf)
+	return fmt.Sprintf("%s-%d", hex.EncodeToString(h.Sum(nil)), size), nil
+}