feat(key): add generic key CPL

kad/key/test/util_test.go

@@ -0,0 +1,22 @@
+package test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/probe-lab/go-libdht/kad/key"
+	"github.com/probe-lab/go-libdht/kad/key/bit256"
+	"github.com/probe-lab/go-libdht/kad/key/bitstr"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGenericCommonPrefixLength(t *testing.T) {
+	k0 := bit256.NewKey(make([]byte, 32))             // 0000...0000
+	k1 := bitstr.Key(strings.Repeat("0", 256))        // 0000...0000
+	k2 := bitstr.Key("01" + strings.Repeat("0", 254)) // 0100...0000
+	k3 := bitstr.Key("000")                           // 0001
+
+	require.Equal(t, 256, key.CommonPrefixLength(k0, k1)) // keys are identical
+	require.Equal(t, 1, key.CommonPrefixLength(k0, k2))   // only first bit is common
+	require.Equal(t, 3, key.CommonPrefixLength(k0, k3))   // no common bits
+}

kad/key/util.go

@@ -18,6 +18,22 @@ func Equal[K kad.Key[K]](a, b K) bool {
 	return a.Compare(b) == 0
 }
 
+// CommonPrefixLength returns the length of the common prefix of two keys.
+// Note that keys can be of different types, and different lengths.
+func CommonPrefixLength[K0 kad.Key[K0], K1 kad.Key[K1]](a K0, b K1) int {
+	minLen := a.BitLen()
+	if b.BitLen() < minLen {
+		minLen = b.BitLen()
+	}
+
+	for i := 0; i < minLen; i++ {
+		if a.Bit(i) != b.Bit(i) {
+			return i
+		}
+	}
+	return minLen
+}
+
 // BitString returns a string containing the binary representation of a key.
 func BitString[K kad.Key[K]](k K) string {
 	if bs, ok := any(k).(interface{ BitString() string }); ok {