Let's use rapidjson instead of roll-your-own json. Added helpers to keep allocations minimized.

src/CMakeLists.txt

@@ -1,6 +1,6 @@
 include_directories(${PROJECT_SOURCE_DIR}/include)
 
-set(BASE_RPC_SRC ${PROJECT_SOURCE_DIR}/include/discord-rpc.h discord-rpc.cpp rpc_connection.h rpc_connection.cpp yolojson.h connection.h backoff.h)
+set(BASE_RPC_SRC ${PROJECT_SOURCE_DIR}/include/discord-rpc.h discord-rpc.cpp rpc_connection.h rpc_connection.cpp serialization.h serialization.cpp connection.h backoff.h)
 
 if(WIN32)
     add_library(discord-rpc STATIC ${BASE_RPC_SRC} connection_win.cpp)

src/discord-rpc.cpp

@@ -1,10 +1,8 @@
 #include "discord-rpc.h"
 
-#include "rpc_connection.h"
-#include "yolojson.h"
 #include "backoff.h"
-
+#include "rpc_connection.h"
-#include "rapidjson/document.h"
+#include "serialization.h"
 
 #include <atomic>
 #include <chrono>
@@ -126,7 +124,7 @@ extern "C" void Discord_Initialize(const char* applicationId, DiscordEventHandle
     };
     Connection->onDisconnect = [](int err, const char* message) {
         LastErrorCode = err;
-        StringCopy(LastErrorMessage, message, sizeof(LastErrorMessage));
+        StringCopy(LastErrorMessage, message);
         WasJustDisconnected.exchange(true);
         UpdateReconnectTime();
     };
@@ -155,9 +153,7 @@ extern "C" void Discord_UpdatePresence(const DiscordRichPresence* presence)
 {
     auto qmessage = SendQueueGetNextAddMessage();
     if (qmessage) {
-        char* jsonWrite = qmessage->buffer;
+        qmessage->length = JsonWriteRichPresenceObj(qmessage->buffer, sizeof(qmessage->buffer), presence);
-        JsonWriteRichPresenceObj(jsonWrite, presence);
-        qmessage->length = jsonWrite - qmessage->buffer;
         SendQueueCommitMessage();
         SignalIOActivity();
     }

src/rpc_connection.cpp

@@ -1,5 +1,5 @@
 #include "rpc_connection.h"
-#include "yolojson.h"
+#include "serialization.h"
 
 #include <atomic>
 
@@ -9,7 +9,7 @@ static RpcConnection Instance;
 /*static*/ RpcConnection* RpcConnection::Create(const char* applicationId)
 {
     Instance.connection = BaseConnection::Create();
-    StringCopy(Instance.appId, applicationId, sizeof(Instance.appId));
+    StringCopy(Instance.appId, applicationId);
     return &Instance;
 }
 
@@ -35,9 +35,7 @@ void RpcConnection::Open()
     }
 
     sendFrame.opcode = Opcode::Handshake;
-    char* json = sendFrame.message;
+    sendFrame.length = JsonWriteHandshakeObj(sendFrame.message, sizeof(sendFrame.message), RpcVersion, appId);
-    JsonWriteHandshakeObj(json, RpcVersion, appId);
-    sendFrame.length = json - sendFrame.message;
         
     if (connection->Write(&sendFrame, sizeof(MessageFrameHeader) + sendFrame.length)) {
         state = State::Connected;
@@ -97,7 +95,7 @@ bool RpcConnection::Read(rapidjson::Document& message)
             message.ParseInsitu(readFrame.message);
             lastErrorCode = message["code"].GetInt();
             const auto& m = message["message"];
-            StringCopy(lastErrorMessage, m.GetString(), sizeof(lastErrorMessage));
+            StringCopy(lastErrorMessage, m.GetString());
             Close();
             return false;
         }
@@ -105,13 +103,11 @@ bool RpcConnection::Read(rapidjson::Document& message)
             message.ParseInsitu(readFrame.message);
             return true;
         case Opcode::Ping:
-        {
             readFrame.opcode = Opcode::Pong;
             if (!connection->Write(&readFrame, sizeof(MessageFrameHeader) + readFrame.length)) {
                 Close();
             }
             break;
-        }
         case Opcode::Pong:
             break;
         default:

src/serialization.cpp

@@ -0,0 +1,202 @@
+#include "connection.h"
+#include "discord-rpc.h"
+
+#include "rapidjson/writer.h"
+#include "rapidjson/stringbuffer.h"
+
+// I want to use as few allocations as I can get away with, and to do that with RapidJson, you need to supply some of
+// your own allocators for stuff rather than use the defaults
+
+class LinearAllocator {
+public:
+    char* buffer_;
+    char* end_;
+    LinearAllocator() {
+        assert(0); // needed for some default case in rapidjson, should not use
+    }
+    LinearAllocator(char* buffer, size_t size) : buffer_(buffer), end_(buffer + size) {}
+    static const bool kNeedFree = false;
+    void* Malloc(size_t size)
+    {
+        char* res = buffer_;
+        buffer_ += size;
+        if (buffer_ > end_) {
+            buffer_ = res;
+            return nullptr;
+        }
+        return res;
+    }
+    void* Realloc(void* originalPtr, size_t originalSize, size_t newSize)
+    {
+        if (newSize == 0) {
+            return nullptr;
+        }
+        // allocate how much you need in the first place
+        assert(!originalPtr && !originalSize);
+        return Malloc(newSize);
+    }
+    static void Free(void* ptr) { /* shrug */ }
+};
+
+template<int Size>
+class FixedLinearAllocator : public LinearAllocator {
+public:
+    char fixedBuffer_[Size];
+    FixedLinearAllocator() : LinearAllocator(fixedBuffer_, Size) {}
+    static const bool kNeedFree = false;
+};
+
+// wonder why this isn't a thing already, maybe I missed it
+class DirectStringBuffer {
+public:
+    typedef typename char Ch;
+    char* buffer_;
+    char* end_;
+    char* current_;
+
+    DirectStringBuffer(char* buffer, size_t maxLen)
+        : buffer_(buffer)
+        , end_(buffer + maxLen)
+        , current_(buffer)
+    {}
+
+    void Put(char c)
+    {
+        if (current_ < end_) {
+            *current_++ = c;
+        }
+    }
+    void Flush() {}
+    size_t GetSize() const
+    {
+        return current_ - buffer_;
+    }
+};
+
+using Encoding = rapidjson::UTF8<char>;
+// Writer appears to need about 16 bytes per nested object level (with 64bit size_t)
+using WriterAllocator = FixedLinearAllocator<2048>;
+constexpr size_t WriterNestingLevels = 2048 / 16;
+using JsonWriter = rapidjson::Writer<DirectStringBuffer, Encoding, Encoding, WriterAllocator, rapidjson::kWriteNoFlags>;
+
+// it's ever so slightly faster to not have to strlen the key
+template<typename T>
+void WriteKey(JsonWriter& w, T& k) {
+    w.Key(k, sizeof(T) - 1);
+}
+
+template<typename T>
+void WriteOptionalString(JsonWriter& w, T& k, const char* value) {
+    if (value) {
+        w.Key(k, sizeof(T) - 1);
+        w.String(value);
+    }
+}
+
+size_t JsonWriteRichPresenceObj(char* dest, size_t maxLen, const DiscordRichPresence* presence)
+{
+    DirectStringBuffer sb(dest, maxLen);
+    WriterAllocator wa;
+    JsonWriter writer(sb, &wa, WriterNestingLevels);
+
+    // const args = {pid, activity};
+    // this.socket.write(encode(OPCODES.FRAME, { nonce: uuid(), cmd : 'SET_ACTIVITY', args })
+
+    writer.StartObject();
+
+    WriteKey(writer, "args");
+    writer.StartObject();
+
+    WriteKey(writer, "activity");
+    writer.StartObject();
+
+    WriteOptionalString(writer, "state", presence->state);
+    WriteOptionalString(writer, "details", presence->details);
+
+    if (presence->startTimestamp || presence->endTimestamp) {
+        WriteKey(writer, "timestamps");
+        writer.StartObject();
+
+        if (presence->startTimestamp) {
+            WriteKey(writer, "start");
+            writer.Int64(presence->startTimestamp);
+        }
+
+        if (presence->endTimestamp) {
+            WriteKey(writer, "end");
+            writer.Int64(presence->endTimestamp);
+        }
+
+        writer.EndObject();
+    }
+
+    if (presence->largeImageKey || presence->largeImageText || presence->smallImageKey || presence->smallImageText) {
+        WriteKey(writer, "assets");
+        writer.StartObject();
+
+        WriteOptionalString(writer, "large_image", presence->largeImageKey);
+        WriteOptionalString(writer, "large_text", presence->largeImageText);
+        WriteOptionalString(writer, "small_image", presence->smallImageKey);
+        WriteOptionalString(writer, "small_text", presence->smallImageText);
+
+        writer.EndObject();
+    }
+
+    if (presence->partyId || presence->partySize || presence->partyMax) {
+        WriteKey(writer, "party");
+        writer.StartObject();
+
+        WriteOptionalString(writer, "id", presence->partyId);
+        if (presence->partySize) {
+            writer.StartArray();
+
+            writer.Int(presence->partySize);
+            if (0 < presence->partyMax) {
+                writer.Int(presence->partyMax);
+            }
+
+            writer.EndArray();
+        }
+
+        writer.EndObject();
+    }
+
+    if (presence->matchSecret || presence->joinSecret || presence->spectateSecret) {
+        WriteKey(writer, "secrets");
+        writer.StartObject();
+
+        WriteOptionalString(writer, "match", presence->matchSecret);
+        WriteOptionalString(writer, "join", presence->joinSecret);
+        WriteOptionalString(writer, "spectate", presence->spectateSecret);
+
+        writer.EndObject();
+    }
+
+    writer.Key("instance");
+    writer.Bool(presence->instance != 0);
+
+    writer.EndObject(); // activity
+
+    writer.EndObject(); // args
+
+    writer.EndObject(); // top level
+
+    return sb.GetSize();
+}
+
+size_t JsonWriteHandshakeObj(char* dest, size_t maxLen, int version, const char* applicationId)
+{
+    DirectStringBuffer sb(dest, maxLen);
+    WriterAllocator wa;
+    JsonWriter writer(sb, &wa, WriterNestingLevels);
+
+    writer.StartObject();
+    WriteKey(writer, "v");
+    writer.Int(version);
+    WriteKey(writer, "client_id");
+    writer.String(applicationId);
+    writer.EndObject();
+
+    return sb.GetSize();
+}
+

src/serialization.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <stdint.h>
+
+// if only there was a standard library function for this
+template<size_t Len>
+inline size_t StringCopy(char (&dest)[Len], const char* src) {
+    if (!dest || !src || !Len) {
+        return 0;
+    }
+    size_t copied;
+    char* out = dest;
+    for (copied = 1; *src && copied < Len; ++copied) {
+        *out++ = *src++;
+    }
+    *out = 0;
+    return copied - 1;
+}
+
+struct DiscordRichPresence;
+
+size_t JsonWriteRichPresenceObj(char* dest, size_t maxLen, const DiscordRichPresence* presence);
+size_t JsonWriteHandshakeObj(char* dest, size_t maxLen, int version, const char* applicationId);

src/yolojson.h

@@ -1,207 +0,0 @@
-#pragma once
-
-#include "connection.h"
-#include "discord-rpc.h"
-
-/*
-    This is as simple of a json writing thing as possible; does not try to keep you
-    from overflowing buffer, so make sure you have room.
-*/
-
-// if only there was a standard library function for this
-inline size_t StringCopy(char* dest, const char* src, size_t maxBytes = UINT32_MAX) {
-    if (!dest || !src || !maxBytes) {
-        return 0;
-    }
-    size_t copied;
-    for (copied = 1; *src && copied < maxBytes; ++copied) {
-        *dest++ = *src++;
-    }
-    *dest = 0;
-    return copied - 1;
-}
-
-inline void JsonWriteEscapedString(char*& dest, const char* src)
-{
-    for (char c = *src++; c; c = *src++) {
-        switch (c) {
-        case '\"':
-        case '\\':
-            *dest++ = '\\';
-            *dest++ = c;
-            break;
-        case '\b':
-            *dest++ = '\\';
-            *dest++ = 'b';
-            break;
-        case '\f':
-            *dest++ = '\\';
-            *dest++ = 'f';
-            break;
-        case '\n':
-            *dest++ = '\\';
-            *dest++ = 'n';
-            break;
-        case '\r':
-            *dest++ = '\\';
-            *dest++ = 'r';
-            break;
-        case '\t':
-            *dest++ = '\\';
-            *dest++ = 't';
-            break;
-        default:
-            *dest++ = c;
-            break;
-        }
-    }
-}
-
-template<typename T> void JsonWriteNumber(char*& dest, T number)
-{
-    if (!number) {
-        *dest++ = '0';
-        return;
-    }
-    if (number < 0) {
-        *dest++ = '-';
-        number = -number;
-    }
-    char temp[32];
-    int place = 0;
-    while (number) {
-        auto digit = number % 10;
-        number = number / 10;
-        temp[place++] = '0' + (char)digit;
-    }
-    for (--place; place >= 0; --place) {
-        *dest++ = temp[place];
-    }
-    *dest = 0;
-}
-
-inline void JsonWritePropName(char*& dest, const char* name)
-{
-    *dest++ = '"';
-    dest += StringCopy(dest, name);
-    *dest++ = '"';
-    *dest++ = ':';
-    *dest++ = ' ';
-}
-
-inline void JsonWritePropSep(char*& dest)
-{
-    *dest++ = ',';
-    *dest++ = ' ';
-}
-
-inline void JsonWriteStringProp(char*& dest, const char* name, const char* value)
-{
-    JsonWritePropName(dest, name);
-    *dest++ = '"';
-    JsonWriteEscapedString(dest, value);
-    *dest++ = '"';
-    JsonWritePropSep(dest);
-}
-
-template<typename T>
-void JsonWriteNumberAsStringProp(char*& dest, const char* name, T value)
-{
-    JsonWritePropName(dest, name);
-    *dest++ = '"';
-    JsonWriteNumber(dest, value);
-    *dest++ = '"';
-    JsonWritePropSep(dest);
-}
-
-template<typename T>
-void JsonWriteNumberProp(char*& dest, const char* name, T value)
-{
-    JsonWritePropName(dest, name);
-    JsonWriteNumber(dest, value);
-    JsonWritePropSep(dest);
-}
-
-inline void JsonWriteBoolProp(char*& dest, const char* name, bool value)
-{
-    JsonWritePropName(dest, name);
-    dest += StringCopy(dest, value ? "true" : "false");
-    JsonWritePropSep(dest);
-}
-
-inline void JsonWriteRichPresenceObj(char*& dest, const DiscordRichPresence* presence)
-{
-    *dest++ = '{';
-
-    if (presence->state) {
-        JsonWriteStringProp(dest, "state", presence->state);
-    }
-
-    if (presence->details) {
-        JsonWriteStringProp(dest, "details", presence->details);
-    }
-
-    if (presence->startTimestamp) {
-        JsonWriteNumberAsStringProp(dest, "start_timestamp", presence->startTimestamp);
-    }
-
-    if (presence->endTimestamp) {
-        JsonWriteNumberAsStringProp(dest, "end_timestamp", presence->endTimestamp);
-    }
-
-    if (presence->largeImageKey) {
-        JsonWriteStringProp(dest, "large_image_key", presence->largeImageKey);
-    }
-
-    if (presence->largeImageText) {
-        JsonWriteStringProp(dest, "large_image_text", presence->largeImageText);
-    }
-
-    if (presence->smallImageKey) {
-        JsonWriteStringProp(dest, "small_image_key", presence->smallImageKey);
-    }
-
-    if (presence->smallImageText) {
-        JsonWriteStringProp(dest, "small_image_text", presence->smallImageText);
-    }
-
-    if (presence->partyId) {
-        JsonWriteStringProp(dest, "party_id", presence->partyId);
-    }
-
-    if (presence->partyMax) {
-        JsonWriteNumberProp(dest, "party_size", presence->partySize);
-        JsonWriteNumberProp(dest, "party_max", presence->partyMax);
-    }
-
-    if (presence->matchSecret) {
-        JsonWriteStringProp(dest, "match_secret", presence->matchSecret);
-    }
-
-    if (presence->joinSecret) {
-        JsonWriteStringProp(dest, "join_secret", presence->joinSecret);
-    }
-
-    if (presence->spectateSecret) {
-        JsonWriteStringProp(dest, "spectate_secret", presence->spectateSecret);
-    }
-
-    JsonWriteBoolProp(dest, "instance", presence->instance != 0);
-
-    dest -= 1;
-    *(dest - 1) = '}';
-    *dest = 0;
-}
-
-inline void JsonWriteHandshakeObj(char*& dest, int version, const char* applicationId)
-{
-    *dest++ = '{';
-
-    JsonWriteNumberProp(dest, "v", version);
-    JsonWriteStringProp(dest, "client_id", applicationId);
-
-    dest -= 1;
-    *(dest - 1) = '}';
-    *dest = 0;
-}
-