Add a version of absl::HexStringToBytes() that returns a bool

to validate that the input was actually valid hexadecimal data.

Mark the version of absl::HexStringToBytes() that does not validate
the input as deprecated.

Fixes #141

PiperOrigin-RevId: 604495678
Change-Id: Iac3020c33c9dbc6d8e31a43b746783fb345edaa7

absl/strings/escaping.cc

@@ -837,6 +837,24 @@ constexpr char kHexValueLenient[256] = {
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 };
 
+constexpr signed char kHexValueStrict[256] = {
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+     0,  1,  2,  3,  4,  5,  6,  7,  8,  9, -1, -1, -1, -1, -1, -1,  // '0'..'9'
+    -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,  // 'A'..'F'
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,  // 'a'..'f'
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+};
 /* clang-format on */
 
 // This is a templated function so that T can be either a char*
@@ -931,6 +949,30 @@ std::string WebSafeBase64Escape(absl::string_view src) {
   return dest;
 }
 
+bool HexStringToBytes(absl::string_view hex,
+                      absl::Nonnull<std::string*> bytes) {
+  size_t num_bytes = hex.size() / 2;
+  bytes->clear();
+  if (hex.size() != num_bytes * 2) {
+    return false;
+  }
+
+  absl::strings_internal::STLStringResizeUninitialized(bytes, num_bytes);
+  auto hex_p = hex.cbegin();
+  for (std::string::iterator bin_p = bytes->begin();
+       bin_p != bytes->end(); ++bin_p) {
+    int h1 = absl::kHexValueStrict[static_cast<size_t>(*hex_p++)];
+    int h2 = absl::kHexValueStrict[static_cast<size_t>(*hex_p++)];
+    if (h1 == -1 || h2 == -1) {
+      bytes->resize(static_cast<size_t>(bin_p - bytes->begin()));
+      return false;
+    }
+    *bin_p = static_cast<char>((h1 << 4) + h2);
+  }
+
+  return true;
+}
+
 std::string HexStringToBytes(absl::string_view from) {
   std::string result;
   const auto num = from.size() / 2;

absl/strings/escaping.h

@@ -27,6 +27,7 @@
 #include <string>
 #include <vector>
 
+#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
 #include "absl/base/nullability.h"
 #include "absl/strings/ascii.h"
@@ -158,8 +159,19 @@ bool WebSafeBase64Unescape(absl::string_view src,
 
 // HexStringToBytes()
 //
+// Converts the hexadecimal encoded data in `hex` into raw bytes in the `bytes`
+// output string.  If `hex` does not consist of valid hexadecimal data, this
+// function returns false and leaves `bytes` in an unspecified state. Returns
+// true on success.
+ABSL_MUST_USE_RESULT bool HexStringToBytes(absl::string_view hex,
+                                           absl::Nonnull<std::string*> bytes);
+
+// HexStringToBytes()
+//
 // Converts an ASCII hex string into bytes, returning binary data of length
-// `from.size()/2`.
+// `from.size()/2`. The input must be valid hexadecimal data, otherwise the
+// return value is unspecified.
+ABSL_DEPRECATED("Use the HexStringToBytes() that returns a bool")
 std::string HexStringToBytes(absl::string_view from);
 
 // BytesToHexString()

absl/strings/escaping_test.cc

@@ -689,6 +689,35 @@ TEST(Base64, DISABLED_HugeData) {
   EXPECT_EQ(huge, unescaped);
 }
 
+TEST(Escaping, HexStringToBytesBackToHex) {
+  std::string bytes, hex;
+
+  constexpr absl::string_view kTestHexLower =  "1c2f0032f40123456789abcdef";
+  constexpr absl::string_view kTestHexUpper =  "1C2F0032F40123456789ABCDEF";
+  constexpr absl::string_view kTestBytes = absl::string_view(
+      "\x1c\x2f\x00\x32\xf4\x01\x23\x45\x67\x89\xab\xcd\xef", 13);
+
+  EXPECT_TRUE(absl::HexStringToBytes(kTestHexLower, &bytes));
+  EXPECT_EQ(bytes, kTestBytes);
+
+  EXPECT_TRUE(absl::HexStringToBytes(kTestHexUpper, &bytes));
+  EXPECT_EQ(bytes, kTestBytes);
+
+  hex = absl::BytesToHexString(kTestBytes);
+  EXPECT_EQ(hex, kTestHexLower);
+
+  // Length not a multiple of two.
+  EXPECT_FALSE(absl::HexStringToBytes("1c2f003", &bytes));
+
+  // Not hex.
+  EXPECT_FALSE(absl::HexStringToBytes("1c2f00ft", &bytes));
+
+  // Empty input.
+  bytes = "abc";
+  EXPECT_TRUE(absl::HexStringToBytes("", &bytes));
+  EXPECT_EQ("", bytes);  // Results in empty output.
+}
+
 TEST(HexAndBack, HexStringToBytes_and_BytesToHexString) {
   std::string hex_mixed = "0123456789abcdefABCDEF";
   std::string bytes_expected = "\x01\x23\x45\x67\x89\xab\xcd\xef\xAB\xCD\xEF";