Expose the murmur3 partitioning function directly

I need to compute it in external code.

session.go

@@ -19,6 +19,7 @@ import (
 	"unicode"
 
 	"github.com/gocql/gocql/internal/lru"
+	"github.com/gocql/gocql/internal/murmur"
 )
 
 // Session is the interface used by users to interact with the database.
@@ -2122,6 +2123,42 @@ func createRoutingKey(routingKeyInfo *routingKeyInfo, values []interface{}) ([]b
 	return routingKey, nil
 }
 
+var identityIndexes = []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+// Murmur3Token computes a murmur3 token for the given values.
+func Murmur3Token(types []TypeInfo, values []interface{}) (int64, error) {
+	if len(values) == 0 {
+		return 0, fmt.Errorf("gocql: no values provided")
+	}
+
+	if len(types) != len(values) {
+		return 0, fmt.Errorf("gocql: types and values length mismatch")
+	}
+
+	var indexes []int
+	if n := len(types); n <= len(identityIndexes) {
+		indexes = identityIndexes[:n]
+	} else {
+		indexes = make([]int, n)
+		for i := range indexes {
+			indexes[i] = i
+		}
+	}
+
+	rki := &routingKeyInfo{
+		types:   types,
+		indexes: indexes,
+		names:   make([]string, len(types)), // used in error messages
+	}
+
+	routingKey, err := createRoutingKey(rki, values)
+	if err != nil {
+		return 0, err
+	}
+
+	return murmur.Murmur3H1(routingKey), nil
+}
+
 func (b *Batch) borrowForExecution() {
 	// empty, because Batch has no equivalent of Query.Release()
 	// that would race with speculative executions.

session_test.go

@@ -333,3 +333,108 @@ func TestIsUseStatement(t *testing.T) {
 		}
 	}
 }
+
+func TestMurmur3Token(t *testing.T) {
+	tests := map[string]struct {
+		types    []TypeInfo
+		values   []interface{}
+		expected int64
+		err      string
+	}{
+		"8351-2882-581-20036": {
+			types: []TypeInfo{
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+			},
+			values: []interface{}{
+				int16(8351),
+				int16(2882),
+				int16(581),
+				int16(20036),
+			},
+			expected: -9223371846324820981,
+		},
+		"3852-744-522-20116": {
+			types: []TypeInfo{
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+			},
+			values: []interface{}{
+				int16(3852),
+				int16(744),
+				int16(522),
+				int16(20116),
+			},
+			expected: -9223370757649630452,
+		},
+		"5212-813-19933-0": {
+			types: []TypeInfo{
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+			},
+			values: []interface{}{
+				int16(5212),
+				int16(813),
+				int16(19933),
+				int16(0),
+			},
+			expected: 5363655924167674765,
+		},
+		"empty": {
+			types: []TypeInfo{
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+			},
+			values: nil,
+			err:    "gocql: no values provided",
+		},
+		"mismatched length": {
+			types: []TypeInfo{
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+			},
+			values: []interface{}{
+				int16(5212),
+				int16(813),
+				int16(19933),
+				int16(0),
+			},
+			err: "gocql: types and values length mismatch",
+		},
+		"invalid value": {
+			types: []TypeInfo{
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+				NewNativeType(protoVersion4, TypeSmallInt, ""),
+			},
+			values: []interface{}{
+				int16(5212),
+				"hello there",
+			},
+			err: "can not marshal string into smallint: strconv.ParseInt: parsing \"hello there\": invalid syntax",
+		},
+	}
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			got, err := Murmur3Token(test.types, test.values)
+			if test.err != "" {
+				if err == nil {
+					t.Fatalf("expected error %q, got nil", test.err)
+				}
+				if gotErr := err.Error(); gotErr != test.err {
+					t.Fatalf("expected error %q, got %q", test.err, gotErr)
+				}
+			} else {
+				if err != nil {
+					t.Fatalf("unexpected error %q", err)
+				}
+				if got != test.expected {
+					t.Fatalf("expected %v, got %v", test.expected, got)
+				}
+			}
+		})
+	}
+}