0.2.64 update

server/internal/credits/fingerprint.go

@@ -0,0 +1,64 @@
+package credits
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// Identifying a media *version*, not an item.
+//
+// An Emby item id survives a file being replaced, which is exactly the case this subsystem
+// must not get wrong: Sonarr swapping a 720p rip for a 1080p one leaves the id alone, and a
+// marker measured against the old file would put Skip Credits somewhere arbitrary in the new
+// one. Keying the marker on item *and* fingerprint means a replaced file simply stops
+// matching — the episode becomes a candidate again, and nothing had to notice the swap or
+// run an invalidation pass to make that happen.
+
+// MediaVersion is what a fingerprint is computed from. Every field is optional except the
+// item id: a source that can only offer runtime still produces a usable fingerprint, it is
+// just a coarser one.
+type MediaVersion struct {
+	ItemID     string
+	RuntimeMs  int64
+	SizeBytes  int64
+	ModifiedAt time.Time
+	// ETag is Emby's own version token where it offers one. It is the strongest field here
+	// because it changes for reasons the other three can miss — a remux to the same length
+	// and near-enough the same size.
+	ETag string
+}
+
+// Fingerprint is a short, stable digest of a media version.
+//
+// Truncated to sixteen bytes because it is an equality key rather than a security claim: it
+// is only ever compared against another fingerprint of the same item, and a full digest
+// would double the width of the busiest index in the schema for no gain.
+func Fingerprint(version MediaVersion) string {
+	parts := []string{
+		strings.TrimSpace(version.ItemID),
+		strconv.FormatInt(version.RuntimeMs, 10),
+		strconv.FormatInt(version.SizeBytes, 10),
+		strings.TrimSpace(version.ETag),
+	}
+	if !version.ModifiedAt.IsZero() {
+		// Second resolution. Filesystems and Emby disagree about sub-second timestamps often
+		// enough that finer granularity would invalidate markers on files nothing had
+		// touched, which is the expensive direction to be wrong in.
+		parts = append(parts, strconv.FormatInt(version.ModifiedAt.UTC().Unix(), 10))
+	}
+	sum := sha256.Sum256([]byte(strings.Join(parts, "|")))
+	return hex.EncodeToString(sum[:16])
+}
+
+// Weak reports a fingerprint built from too little to detect a file swap.
+//
+// Runtime alone does not distinguish a re-encode of the same episode, so a marker stored
+// against a weak fingerprint is one that could survive a replacement it should not have. The
+// service refuses to store those rather than storing a marker it cannot invalidate — the
+// alternative is a wrong Skip Credits position that nothing will ever correct.
+func (v MediaVersion) Weak() bool {
+	return v.SizeBytes <= 0 && strings.TrimSpace(v.ETag) == "" && v.ModifiedAt.IsZero()
+}