labels: add stringlabels version of StableHash

Inlined the `Range()` implementation to avoid adding overhead.

model/labels/sharding.go

@@ -1,3 +1,5 @@
+//go:build !stringlabels
+
 package labels
 
 import (

model/labels/sharding_string.go

@@ -0,0 +1,41 @@
+//go:build stringlabels
+
+package labels
+
+import (
+	"github.com/cespare/xxhash/v2"
+)
+
+// StableHash is a labels hashing implementation which is guaranteed to not change over time.
+// This function should be used whenever labels hashing backward compatibility must be guaranteed.
+func StableHash(ls Labels) uint64 {
+	// Use xxhash.Sum64(b) for fast path as it's faster.
+	b := make([]byte, 0, 1024)
+	var h *xxhash.Digest
+	for i := 0; i < len(ls.data); {
+		var v Label
+		v.Name, i = decodeString(ls.data, i)
+		v.Value, i = decodeString(ls.data, i)
+		if h == nil && len(b)+len(v.Name)+len(v.Value)+2 >= cap(b) {
+			// If labels entry is 1KB+, switch to Write API. Copy in the values up to this point.
+			h = xxhash.New()
+			_, _ = h.Write(b)
+		}
+		if h != nil {
+			_, _ = h.WriteString(v.Name)
+			_, _ = h.Write(seps)
+			_, _ = h.WriteString(v.Value)
+			_, _ = h.Write(seps)
+			continue
+		}
+
+		b = append(b, v.Name...)
+		b = append(b, seps[0])
+		b = append(b, v.Value...)
+		b = append(b, seps[0])
+	}
+	if h != nil {
+		return h.Sum64()
+	}
+	return xxhash.Sum64(b)
+}