windows/serialInterface: Further refinements to simplify and solve fo…

…r a severe branching misprediction

Co-Authored-By: L. E. Segovia <[email protected]>
Co-Authored-By: dragonmux <[email protected]>

src/windows/serialInterface.cxx

@@ -8,6 +8,7 @@
 #include <string_view>
 #include <fmt/format.h>
 #include <substrate/console>
+#include <substrate/index_sequence>
 #include "windows/serialInterface.hxx"
 #include "usbDevice.hxx"
 #include "bmp.hxx"
@@ -287,7 +288,7 @@ void serialInterface_t::refillBuffer() const
 // NOLINTNEXTLINE(readability-convert-member-functions-to-static)
 std::string serialInterface_t::readPacket() const
 {
-	std::array<char, bmp_t::maxPacketSize + 1U> packet{};
+	std::array<char, bmp_t::maxPacketSize> packet{};
 	size_t length{0};
 	// Try gathering a '#' terminated response
 	while (length < packet.size())
@@ -297,29 +298,33 @@ std::string serialInterface_t::readPacket() const
 			refillBuffer();
 
 		// Look for an end of message marker
-		size_t responseLength{0U};
-		while (readBufferOffset + responseLength < readBufferFullness && length + responseLength < readBuffer.size())
+		const auto responseLength
 		{
-			// If we've found one then stop scanning
-			if (readBuffer[readBufferOffset + responseLength++] == '#')
-				break;
-		}
+			[&]()
+			{
+				for (const auto offset : substrate::indexSequence_t{readBufferOffset, readBufferFullness})
+				{
+					if (readBuffer[offset] == '#')
+						return offset;
+				}
+				return readBufferFullness;
+			}()
+		};
 		// We now either have a remote end of message marker, or need all the data from the buffer
-		memcpy(packet.data() + length, readBuffer.data() + readBufferOffset, responseLength);
+		std::memcpy(packet.data() + length, readBuffer.data() + readBufferOffset, responseLength);
 		readBufferOffset += responseLength;
-		length += responseLength - 1U;
+		length += responseLength;
 		// If it's a remote end of message marker, break out the loop
-		if (packet[length] == '#')
+		if (responseLength != readBufferFullness)
+		{
+			++readBufferOffset;
 			break;
-		++length;
+		}
 	}
 
-	// Adjust the length to remove the beginning '&' (the ending '#' is already taken care of in the read loop)
-	--length;
-	// Make a new std::string of an appropriate length
-	std::string result(length + 1U, '\0');
-	// And copy the result string in, returning it
-	std::memcpy(result.data(), packet.data() + 1U, length);
+	// Make a new std::string of an appropriate length, copying the data in to return it
+	// Skip the first byte to remove the beginning '&' (the ending '#' is already taken care of in the read loop)
+	std::string result{packet.data() + 1U, length};
 	console.debug("Remote read: "sv, result);
 	return result;
 }