improve FNV implementation

crypto/fnv.go

@@ -1,56 +1,80 @@
 package crypto
 
-// Taken and modified from https://golang.org/src/hash/fnv/fnv.go
-// TODO: This implementation uses big ints and is probably horrendously slow.
-
 // Implements FNV-1 and FNV-1a, non-cryptographic hash functions
 // created by Glenn Fowler, Landon Curt Noll, and Phong Vo.
 // See https://en.wikipedia.org/wiki/Fowler-Noll-Vo_hash_function.
 
-import (
-	"hash"
-	"math/big"
-)
+import "hash"
 
 // Hash128 is the common interface implemented by all 128-bit hash functions.
 type Hash128 interface {
 	hash.Hash
-	Sum128() []byte
+	Sum128() (uint64, uint64)
 }
 
 type sum128a struct {
-	*big.Int
+	v0, v1, v2, v3 uint64
 }
 
 var _ Hash128 = &sum128a{}
 
-var offset128 = &big.Int{}
-var prime128 = &big.Int{}
-
-func init() {
-	offset128.SetString("144066263297769815596495629667062367629", 0)
-	prime128.SetString("309485009821345068724781371", 0)
-}
-
 // New128a returns a new 128-bit FNV-1a hash.Hash.
 func New128a() Hash128 {
-	i := &big.Int{}
-	i.Set(offset128)
-	return &sum128a{i}
+	s := &sum128a{}
+	s.Reset()
+	return s
 }
 
-func (s *sum128a) Reset() { s.Set(offset128) }
+func (s *sum128a) Reset() {
+	s.v0 = 0x6295C58D
+	s.v1 = 0x62B82175
+	s.v2 = 0x07BB0142
+	s.v3 = 0x6C62272E
+}
 
-func (s *sum128a) Sum128() []byte { return s.Bytes() }
+func (s *sum128a) Sum128() (uint64, uint64) {
+	return s.v3<<32 | s.v2, s.v1<<32 | s.v0
+}
 
 func (s *sum128a) Write(data []byte) (int, error) {
-	for _, c := range data {
-		s.Xor(s.Int, big.NewInt(int64(c)))
-		s.Mul(s.Int, prime128)
+	var t0, t1, t2, t3 uint64
+	// Taken and slightly modified from github.com/romain-jacotin/quic
+	const fnv128PrimeLow = 0x0000013B
+	const fnv128PrimeShift = 24
 
-		// Truncate the bigint to 128 bits
-		s.SetBytes(s.Bytes()[len(s.Bytes())-16 : len(s.Bytes())])
+	for _, v := range data {
+		// xor the bottom with the current octet
+		s.v0 ^= uint64(v)
+
+		// multiply by the 128 bit FNV magic prime mod 2^128
+		// fnv_prime	= 309485009821345068724781371 (decimal)
+		// 				= 0x0000000001000000000000000000013B (hexadecimal)
+		// 				= 0x00000000 	0x01000000 				0x00000000	0x0000013B (in 4*32 words)
+		//				= 0x0			1<<fnv128PrimeShift	0x0			fnv128PrimeLow
+		//
+		// fnv128PrimeLow = 0x0000013B
+		// fnv128PrimeShift = 24
+
+		// multiply by the lowest order digit base 2^32 and by the other non-zero digit
+		t0 = s.v0 * fnv128PrimeLow
+		t1 = s.v1 * fnv128PrimeLow
+		t2 = s.v2*fnv128PrimeLow + s.v0<<fnv128PrimeShift
+		t3 = s.v3*fnv128PrimeLow + s.v1<<fnv128PrimeShift
+
+		// propagate carries
+		t1 += (t0 >> 32)
+		t2 += (t1 >> 32)
+		t3 += (t2 >> 32)
+
+		s.v0 = t0 & 0xffffffff
+		s.v1 = t1 & 0xffffffff
+		s.v2 = t2 & 0xffffffff
+		s.v3 = t3 // & 0xffffffff
+		// Doing a s.v3 &= 0xffffffff is not really needed since it simply
+		// removes multiples of 2^128.  We can discard these excess bits
+		// outside of the loop when writing the hash in Little Endian.
 	}
+
 	return len(data), nil
 }
 
@@ -59,6 +83,5 @@ func (s *sum128a) Size() int { return 16 }
 func (s *sum128a) BlockSize() int { return 1 }
 
 func (s *sum128a) Sum(in []byte) []byte {
-	b := s.Bytes()
-	return append(in, b[len(b)-s.Size():]...)
+	panic("FNV: not supported")
 }

crypto/fnv_test.go

@@ -9,13 +9,18 @@ import (
 
 var _ = Describe("FNV", func() {
 	It("gives proper null hash", func() {
-		h := crypto.New128a()
-		Expect(h.Sum128()).To(Equal([]byte{0x6c, 0x62, 0x27, 0x2e, 0x07, 0xbb, 0x01, 0x42, 0x62, 0xb8, 0x21, 0x75, 0x62, 0x95, 0xc5, 0x8d}))
+		hash := crypto.New128a()
+		h, l := hash.Sum128()
+		Expect(l).To(Equal(uint64(0x62b821756295c58d)))
+		Expect(h).To(Equal(uint64(0x6c62272e07bb0142)))
 	})
 
-	It("gives hashes", func() {
-		h := crypto.New128a()
-		h.Write([]byte("foobar"))
-		Expect(h.Sum128()).To(Equal([]byte{0x34, 0x3e, 0x16, 0x62, 0x79, 0x3c, 0x64, 0xbf, 0x6f, 0xd, 0x35, 0x97, 0xba, 0x44, 0x6f, 0x18}))
+	It("calculates hash", func() {
+		hash := crypto.New128a()
+		_, err := hash.Write([]byte("foobar"))
+		Expect(err).ToNot(HaveOccurred())
+		h, l := hash.Sum128()
+		Expect(l).To(Equal(uint64(0x6f0d3597ba446f18)))
+		Expect(h).To(Equal(uint64(0x343e1662793c64bf)))
 	})
 })