PAIR: Add Shuffle (#75)

pkg/pair/pair.go

@@ -5,6 +5,7 @@ import (
  "crypto/sha512"
  "errors"
  "hash"
+ mrandv2 "math/rand/v2"
 
  "github.com/gtank/ristretto255"
 )
@@ -119,3 +120,13 @@ func (pk *PrivateKey) Decrypt(ciphertext []byte) ([]byte, error) {
 
  return cipher.MarshalText()
 }
+
+// Shuffle shuffles the data in place by using the Fisher-Yates algorithm.
+// Note that ideally, it should be called with less than 2^32-1 (4 billion) elements.
+func Shuffle(data [][]byte) {
+ // NOTE: since go 1.20, math.Rand seeds the global random number generator.
+ // V2 uses ChaCha8 generator as the global one.
+ mrandv2.Shuffle(len(data), func(i, j int) {
+ data[i], data[j] = data[j], data[i]
+ })
+}

pkg/pair/pair_test.go

@@ -1,8 +1,10 @@
 package pair
 
 import (
+ "bytes"
  "crypto/rand"
  "crypto/sha512"
+ "slices"
  "strings"
  "testing"
 
@@ -59,3 +61,45 @@ func TestPAIR(t *testing.T) {
  t.Fatalf("want: %s, got: %s", string(ciphertext), string(decrypted))
  }
 }
+
+func genData(n int) [][]byte {
+ data := make([][]byte, n)
+ for i := 0; i < n; i++ {
+ // marshaled ristretto255.Scalar is 44 bytes
+ data[i] = make([]byte, 44)
+ rand.Read(data[i])
+ }
+ return data
+}
+
+func TestShuffle(t *testing.T) {
+ data := genData(1 << 10) // 1k
+ orig := make([][]byte, len(data))
+ copy(orig, data)
+
+ // shuffle the data in place
+ Shuffle(data)
+
+ once := make([][]byte, len(data))
+ copy(once, data)
+
+ if slices.EqualFunc(data, orig, bytes.Equal) {
+ t.Fatalf("data not shuffled")
+ }
+
+ // shuffle again
+ Shuffle(data)
+
+ if slices.EqualFunc(data, once, bytes.Equal) {
+ t.Fatalf("data not shuffled")
+ }
+}
+
+func BenchmarkShuffleOneMillionIDs(b *testing.B) {
+ data := genData(1 << 20) // 1m
+ b.ResetTimer()
+
+ for i := 0; i < b.N; i++ {
+ Shuffle(data)
+ }
+}