feat!: replace djb2 with xored version in asm

acheron.go

@@ -22,7 +22,7 @@ type options struct {
 // New returns a new Acheron instance with the given options, or an error if initialization fails.
 func New(opts ...Option) (*Acheron, error) {
 	options := &options{
-		hashFunction: hashing.DJB2, // default
+		hashFunction: hashing.XorDjb2Hash, // default
 	}
 	for _, o := range opts {
 		o(options)

pkg/hashing/hashing.go

@@ -3,11 +3,5 @@ package hashing
 // HashFunction is a type alias for a function that takes a byte slice and returns a uint64.
 type HashFunction func([]byte) uint64
 
-// DJB2 is an implementation of the djb2 hash function. Ref: http://www.cse.yorku.ca/~oz/hash.html
-func DJB2(s []byte) uint64 {
-	var hash uint64 = 5381
-	for _, c := range s {
-		hash = ((hash << 5) + hash) + uint64(c)
-	}
-	return hash
-}
+// XorDjb2Hash XORes the byte slice and calc its djb2 hash.
+func XorDjb2Hash(s []byte) uint64

pkg/hashing/xordjb2.s

@@ -0,0 +1,50 @@
+#include "textflag.h"
+
+// func XorDjb2Hash(data []byte) uint64
+TEXT ·XorDjb2Hash(SB),NOSPLIT,$0-24
+    MOVQ    data+0(FP), DI
+    MOVQ    DI, R9
+    MOVQ    len+8(FP), SI
+    MOVQ    SI, R10
+
+    // djb2 magic number
+    XORQ    AX, AX
+    MOVL    $5381, AX
+
+    // xor byte
+    XORQ    R15, R15
+    MOVB    $0xf1, R15
+
+    // if data slice is empty, return 0
+    CMPQ    SI, $0
+    JEQ     done
+
+loop_xor:
+    // data[i] = data[i] ^ 0xf1
+    XORB    R15, (DI)
+
+    INCQ    DI
+    DECQ    SI
+    CMPQ    SI, $0
+    JNE     loop_xor
+
+    // restore start ptr and length
+    MOVQ    R9, DI
+    MOVQ    R10, SI
+
+loop_hash:
+    // move nth byte to BX
+    MOVB    (DI), BX
+    INCQ    DI
+
+    // hash = hash * 33 + data[i]
+    IMULQ   $33, AX
+    ADDQ    BX, AX
+
+    DECQ    SI
+    CMPQ    SI, $0
+    JNE     loop_hash
+
+done:
+    MOVQ    AX, ret+24(FP)
+    RET