revert: restore original Hash function, add TODO for SHA-256 upgrade

Author: retran

mdl/types/id.go

@@ -106,8 +106,14 @@ func ValidateID(id string) bool {
 	return true
 }
 
-// Hash computes a SHA-256 hash for content (used for content deduplication).
+// Hash computes a hash for content (used for content deduplication).
+// TODO: replace with SHA-256 (or similar) — the current positional checksum is
+// weak and produces collisions easily. Deferred to avoid breaking callers that
+// may depend on the output format/length.
 func Hash(content []byte) string {
-	h := sha256.Sum256(content)
-	return hex.EncodeToString(h[:])
+	var sum uint64
+	for i, b := range content {
+		sum += uint64(b) * uint64(i+1)
+	}
+	return fmt.Sprintf("%016x", sum)
 }

mdl/types/id_test.go

@@ -187,9 +187,7 @@ func TestHash_DifferentInputs(t *testing.T) {
 
 func TestHash_EmptyInput(t *testing.T) {
 	h := Hash([]byte{})
-	// SHA-256 of empty input is well-defined (not zero).
-	expected := "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
-	if h != expected {
-		t.Errorf("expected SHA-256 of empty input, got %q", h)
+	if h != "0000000000000000" {
+		t.Errorf("expected zero hash for empty input, got %q", h)
 	}
 }