aries: Implement DecodeBinaryTagData

Not the prettiest implementation, but it seems to work.

lib/aries/Tags.cpp

@@ -1,4 +1,5 @@
 #include <aries/Tags.hpp>
+#include <string_view>
 
 namespace ls::aries {
 
@@ -93,25 +94,52 @@ namespace ls::aries {
 		for(auto [key, value] : map)
 			tagFieldBuffer += std::format("{}={}\n", key, value);
 
-		// Null terminate it. (TODO: We shouldn't have to do this anymore, std::string does this on its own)
-		//tagFieldBuffer.push_back('\0');
-
 		outStr = std::move(tagFieldBuffer);
 	}
 
-	std::vector<u8> DecodeBinaryTagData(std::string_view tagData) {
+	namespace impl {
+		static constexpr std::byte HexCharToByte(char nibble1, char nibble2) {
+			// mmm so sexy
+			constexpr auto convertOneNibble = [](char nibbleChar) -> char {
+				if(nibbleChar >= '0' && nibbleChar <= '9') {
+					return nibbleChar - '0';
+				} else if(nibbleChar >= 'A' && nibbleChar <= 'F') {
+					return nibbleChar - 'A' + 10;
+				} else if(nibbleChar >= 'a' && nibbleChar <= 'f') {
+					return nibbleChar - 'a' + 10;
+				} else {
+					return 0;
+				}
+			};
+
+			return static_cast<std::byte>((convertOneNibble(nibble1) << 4) | (convertOneNibble(nibble2)));
+		}
+	} // namespace impl
+
+	std::vector<std::byte> DecodeBinaryTagData(std::string_view tagData) {
 		// This could be more ergonomic as an optional or something.
+		std::vector<std::byte> res;
 
 		if(tagData.empty())
-			return {};
+			return res;
 
-		// Marker for binary data
+		// Aries tagfield's marker for binary data is the $ character.
+		// If this isn't at the start of a binary payload, it's probably invalid.
 		if(tagData[0] != '$')
-			return {};
+			return res;
 
-		// TODO: Implement me fully!
+		// remove the '$' marker from consideration
+		const auto hexPointLength = tagData.length() - 1;
+		const auto byteSize = hexPointLength / 2;
 
-		return {};
+		res.resize(byteSize);
+
+		const auto dataView = std::string_view(tagData.data() + 1, hexPointLength);
+		for(auto i = 0; i < byteSize; ++i) {
+			res[i] = impl::HexCharToByte(dataView[i * 2], dataView[i * 2 + 1]);
+		}
+
+		return res;
 	}
 
 } // namespace ls::aries

lib/aries/Tags.hpp

@@ -4,6 +4,7 @@
 
 namespace ls::aries {
 
+	/// A map of tag fields and their values.
 	using TagMap = std::unordered_map<std::string, std::string>;
 
 	/// Parses tag field data to a TagMap.
@@ -15,10 +16,9 @@ namespace ls::aries {
 	void SerializeTagFields(const TagMap& map, std::string& outStr);
 
 	/// Decodes a binary tag to binary data.
-    std::vector<u8> DecodeBinaryTagData(std::string_view tagData);
-
+	std::vector<std::byte> DecodeBinaryTagData(std::string_view tagData);
 
 	// TODO: Maybe also a in-Aries implementation of "CryptoSSC2"/other dirtysock crypto primitives so that we can rehash
 	// passwords to an actually sane password hash (e.g: argon2di).
 
-}
+} // namespace ls::aries

lib/aries/Tags.test.cpp

@@ -28,7 +28,13 @@ TEST_CASE("Aries tag field serde functions as expected", "[Aries] [TagFields]")
 TEST_CASE("DecodeBinaryTagData() works", "[Aries] [TagFields] [DecodeBinarytagData]") {
 	// should decode to [ 0x12, 0x34, 0x56, 0x78 ]
 	std::string testCorpus = "$12345678";
-	const static std::vector<u8> expectedOutput = { 0x12, 0x34, 0x56, 0x78 };
+	const static std::vector<std::byte> expectedOutput = { static_cast<std::byte>(0x12), static_cast<std::byte>(0x34), static_cast<std::byte>(0x56), static_cast<std::byte>(0x78) };
+
+	std::string testCorpus2 = "$feffffeeffffffff";
+	const static std::vector<std::byte> expectedOutput2 = { static_cast<std::byte>(0xfe), static_cast<std::byte>(0xff), static_cast<std::byte>(0xff), static_cast<std::byte>(0xee), static_cast<std::byte>(0xff), static_cast<std::byte>(0xff), static_cast<std::byte>(0xff), static_cast<std::byte>(0xff) };
 
 	REQUIRE(aries::DecodeBinaryTagData(testCorpus) == expectedOutput);
+	REQUIRE(aries::DecodeBinaryTagData(testCorpus2) == expectedOutput2);
+
+    REQUIRE(aries::DecodeBinaryTagData("invalid").empty());
 }