exstrings: add fast string utilities

Author: tulir

exstrings/stringutil.go

@@ -0,0 +1,32 @@
+// Copyright (c) 2025 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package exstrings
+
+import (
+	"crypto/sha256"
+	"crypto/subtle"
+	"unsafe"
+)
+
+// UnsafeBytes returns a byte slice that points to the same memory as the input string.
+//
+// The returned byte slice must not be modified.
+func UnsafeBytes(str string) []byte {
+	return unsafe.Slice(unsafe.StringData(str), len(str))
+}
+
+// SHA256 returns the SHA-256 hash of the input string without copying the string.
+func SHA256(str string) [32]byte {
+	return sha256.Sum256(UnsafeBytes(str))
+}
+
+// ConstantTimeEqual compares two strings using [subtle.ConstantTimeCompare] without copying the strings.
+//
+// Note that ConstantTimeCompare is not constant time if the strings are of different length.
+func ConstantTimeEqual(a, b string) bool {
+	return subtle.ConstantTimeCompare(UnsafeBytes(a), UnsafeBytes(b)) == 1
+}