Add NewFromGoFuzz

Add a helper function that enables using gofuzz (this
project) with go-fuzz (https://github.com/dvyukov/go-fuzz) for continuose
fuzzing. Essentially, it enables translating the fuzzing bytes from
go-fuzz to any Go object using this library.

This change will enable using this project with fuzzing websites such as fuzzit.dev
or fuzzbuzz.io.

I've created an example in: https://github.com/posener/fuzzme

Fixes google#33

fuzz.go

@@ -18,6 +18,8 @@ package fuzz
 
 import (
 	"fmt"
+	"math"
+	"math/big"
 	"math/rand"
 	"reflect"
 	"regexp"
@@ -61,6 +63,33 @@ func NewWithSeed(seed int64) *Fuzzer {
 	return f
 }
 
+// NewFromGoFuzz is a helper function that enables using gofuzz (this
+// project) with go-fuzz (https://github.com/dvyukov/go-fuzz) for continuous
+// fuzzing. Essentially, it enables translating the fuzzing bytes from
+// go-fuzz to any Go object using this library.
+//
+// Example: use go-fuzz to test the function `MyFunc(int)` in the package
+// `mypackage`. Add the file: "mypacakge_fuzz.go" with the content:
+//
+// // +build gofuzz
+// package mypacakge
+// import "github.com/google/go-fuzz"
+// func Fuzz(data []byte) int {
+// 	var i int
+// 	fuzz.NewFromGoFuzz(data).Fuzz(&i)
+// 	MyFunc(i)
+// 	return 0
+// }
+func NewFromGoFuzz(data []byte) *Fuzzer {
+	maxint := big.NewInt(math.MaxInt64)
+	seed := big.NewInt(1)
+	for _, b := range data {
+		seed.Mul(seed, big.NewInt(int64(b)))
+		seed.Mod(seed, maxint)
+	}
+	return NewWithSeed(seed.Int64())
+}
+
 // Funcs adds each entry in fuzzFuncs as a custom fuzzing function.
 //
 // Each entry in fuzzFuncs must be a function taking two parameters.

fuzz_test.go

@@ -504,3 +504,28 @@ func TestFuzz_SkipPattern(t *testing.T) {
 		return 5, true
 	})
 }
+
+func TestNewFromGoFuzz(t *testing.T) {
+	tests := []struct {
+		data []byte
+		want int
+	}{
+		// Tests consistant fuzzing for the same input.
+		{data: []byte("a"), want: 943890139707072619},
+		// Tests that different input corresponds in different output.
+		{data: []byte("b"), want: -7308824908432364150},
+		// Tests long input.
+		{
+			data: []byte("123jhasd0128931`3kljahsdlkasb,mxznbcqlsjerqwporu108378091237ksdjhlakshz"),
+			want: 3588769380299097370,
+		},
+	}
+
+	for _, tt := range tests {
+		var got int
+		NewFromGoFuzz(tt.data).Fuzz(&got)
+		if tt.want != got {
+			t.Errorf("Fuzz(%q) = %d, want: %d", tt.data, got, tt.want)
+		}
+	}
+}