From ca509a8b97ff46b6cbd3c371784ac5e582a52c99 Mon Sep 17 00:00:00 2001
From: Stenzek <stenzek@gmail.com>
Date: Sat, 25 Jan 2025 15:27:42 +1000
Subject: [PATCH] StringUtil: Add fixed-length DecodeHex()

---
 src/common/string_util.cpp | 29 +++++++++++++++++++++--------
 src/common/string_util.h   |  1 +
 2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/src/common/string_util.cpp b/src/common/string_util.cpp
index 6275d4281..3fd9dea62 100644
--- a/src/common/string_util.cpp
+++ b/src/common/string_util.cpp
@@ -165,21 +165,34 @@ u8 StringUtil::DecodeHexDigit(char ch)
     return 0;
 }
 
-std::optional<std::vector<u8>> StringUtil::DecodeHex(const std::string_view in)
+size_t StringUtil::DecodeHex(std::span<u8> dest, const std::string_view str)
 {
-  std::vector<u8> data;
-  data.reserve(in.size() / 2);
+  if ((str.length() % 2) != 0)
+    return 0;
 
-  for (size_t i = 0; i < in.size() / 2; i++)
+  const size_t bytes = str.length() / 2;
+  if (dest.size() != bytes)
+    return 0;
+
+  for (size_t i = 0; i < bytes; i++)
   {
-    std::optional<u8> byte = StringUtil::FromChars<u8>(in.substr(i * 2, 2), 16);
+    std::optional<u8> byte = StringUtil::FromChars<u8>(str.substr(i * 2, 2), 16);
     if (byte.has_value())
-      data.push_back(*byte);
+      dest[i] = byte.value();
     else
-      return std::nullopt;
+      return i;
   }
 
-  return {data};
+  return bytes;
+}
+
+std::optional<std::vector<u8>> StringUtil::DecodeHex(const std::string_view in)
+{
+  std::optional<std::vector<u8>> ret;
+  ret = std::vector<u8>(in.size() / 2);
+  if (DecodeHex(ret.value(), in) != ret->size())
+    ret.reset();
+  return ret;
 }
 
 std::string StringUtil::EncodeHex(const void* data, size_t length)
diff --git a/src/common/string_util.h b/src/common/string_util.h
index d5803a643..744cba9e2 100644
--- a/src/common/string_util.h
+++ b/src/common/string_util.h
@@ -242,6 +242,7 @@ static inline bool IsWhitespace(char ch)
 
 /// Encode/decode hexadecimal byte buffers
 u8 DecodeHexDigit(char ch);
+size_t DecodeHex(std::span<u8> dest, const std::string_view str);
 std::optional<std::vector<u8>> DecodeHex(const std::string_view str);
 std::string EncodeHex(const void* data, size_t length);
 template<typename T>