math/big: Add fuzzer

src/math/big/fuzz.go

@@ -0,0 +1,74 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gofuzz
+// +build gofuzz
+
+package big
+
+import (
+	"strings"
+)
+
+func FuzzCmp(data []byte) int {
+	if !IsDivisibleBy(len(data), 2) {
+		return -1
+	}
+	half := len(data) / 2
+
+	halfOne := data[:half]
+	x, err := natFromString(string(halfOne))
+	if err != nil {
+		return 0
+	}
+
+	halfTwo := data[half:]
+	y, err := natFromString(string(halfTwo))
+	if err != nil {
+		return 0
+	}
+
+	x.cmp(y)
+	return 1
+}
+
+func FuzzExpNN(data []byte) int {
+	if !IsDivisibleBy(len(data), 3) {
+		return -1
+	}
+	firstThird := len(data) / 3
+	x, err := natFromString(string(data[:firstThird]))
+	if err != nil {
+		return 0
+	}
+
+	secondThird := firstThird * 2
+	y, err := natFromString(string(data[firstThird:secondThird]))
+	if err != nil {
+		return 0
+	}
+
+	z, err := natFromString(string(data[secondThird:]))
+	if err != nil {
+		return 0
+	}
+
+	p := nat(nil).expNN(x, y, z)
+
+	const n = 165
+	p = p.shl(p, n)
+	return 1
+}
+
+func IsDivisibleBy(n int, divisibleby int) bool {
+	return (n % divisibleby) == 0
+}
+
+func natFromString(s string) (nat, error) {
+	x, _, _, err := nat(nil).scan(strings.NewReader(s), 0, false)
+	if err != nil {
+		return nil, err
+	}
+	return x, nil
+}