Input: UDP Client to provide motion and touch controls

An implementation of the cemuhook motion/touch protocol, this adds the
ability for users to connect several different devices to citra to send
direct motion and touch data to citra.

src/input_common/udp/protocol.cpp

@@ -0,0 +1,79 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <cstddef>
+#include <cstring>
+#include "common/logging/log.h"
+#include "input_common/udp/protocol.h"
+
+namespace InputCommon::CemuhookUDP {
+
+static const size_t GetSizeOfResponseType(Type t) {
+    switch (t) {
+    case Type::Version:
+        return sizeof(Response::Version);
+    case Type::PortInfo:
+        return sizeof(Response::PortInfo);
+    case Type::PadData:
+        return sizeof(Response::PadData);
+    }
+    return 0;
+}
+
+namespace Response {
+
+/**
+ * Returns Type if the packet is valid, else none
+ *
+ * Note: Modifies the buffer to zero out the crc (since thats the easiest way to check without
+ * copying the buffer)
+ */
+boost::optional<Type> Validate(u8* data, size_t size) {
+    if (size < sizeof(Header)) {
+        LOG_ERROR(Input, "Invalid UDP packet received");
+        return boost::none;
+    }
+    Header header;
+    std::memcpy(&header, data, sizeof(Header));
+    if (header.magic != SERVER_MAGIC) {
+        LOG_ERROR(Input, "UDP Packet has an unexpected magic value");
+        return boost::none;
+    }
+    if (header.protocol_version != PROTOCOL_VERSION) {
+        LOG_ERROR(Input, "UDP Packet protocol mismatch");
+        return boost::none;
+    }
+    if (header.type < Type::Version || header.type > Type::PadData) {
+        LOG_ERROR(Input, "UDP Packet is an unknown type");
+        return boost::none;
+    }
+
+    // Packet size must equal sizeof(Header) + sizeof(Data)
+    // and also verify that the packet info mentions the correct size. Since the spec includes the
+    // type of the packet as part of the data, we need to include it in size calculations here
+    // ie: payload_length == sizeof(T) + sizeof(Type)
+    const size_t data_len = GetSizeOfResponseType(header.type);
+    if (header.payload_length != data_len + sizeof(Type) || size < data_len + sizeof(Header)) {
+        LOG_ERROR(
+            Input,
+            "UDP Packet payload length doesn't match. Received: {} PayloadLength: {} Expected: {}",
+            size, header.payload_length, data_len + sizeof(Type));
+        return boost::none;
+    }
+
+    const u32 crc32 = header.crc;
+    boost::crc_32_type result;
+    // zero out the crc in the buffer and then run the crc against it
+    std::memset(&data[offsetof(Header, crc)], 0, sizeof(u32_le));
+
+    result.process_bytes(data, data_len + sizeof(Header));
+    if (crc32 != result.checksum()) {
+        LOG_ERROR(Input, "UDP Packet CRC check failed. Offset: {}", offsetof(Header, crc));
+        return boost::none;
+    }
+    return header.type;
+}
+} // namespace Response
+
+} // namespace InputCommon::CemuhookUDP