shithub: rgbds

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Implement bit flipping with a lookup table

Should improve performance.
This version is cooler, and also does not suffer from iteration limits

--- a/include/gfx/main.hpp

+++ b/include/gfx/main.hpp

@@ -13,6 +13,7 @@

 #include <limits.h>

 #include <stdint.h>

 #include <string>

+#include <utility>

 #include <vector>

 #include "helpers.h"

@@ -87,12 +88,19 @@

 	uint8_t size() const;

 };

-static constexpr uint8_t flip(uint8_t byte) {

-	// To flip all the bits, we'll flip both nibbles, then each nibble half, etc.

-	byte = (byte & 0b0000'1111) << 4 | (byte & 0b1111'0000) >> 4;

-	byte = (byte & 0b0011'0011) << 2 | (byte & 0b1100'1100) >> 2;

-	byte = (byte & 0b0101'0101) << 1 | (byte & 0b1010'1010) >> 1;

-	return byte;

+namespace detail {

+template<typename T, T... i>

+static constexpr auto flipTable(std::integer_sequence<T, i...>) {

+	return std::array{[](uint8_t byte) {

+		// To flip all the bits, we'll flip both nibbles, then each nibble half, etc.

+		byte = (byte & 0b0000'1111) << 4 | (byte & 0b1111'0000) >> 4;

+		byte = (byte & 0b0011'0011) << 2 | (byte & 0b1100'1100) >> 2;

+		byte = (byte & 0b0101'0101) << 1 | (byte & 0b1010'1010) >> 1;

+		return byte;

+	}(i)...};

 }

+}

+// Flipping tends to happen fairly often, so take a bite out of dcache to speed it up

+static constexpr auto flipTable = detail::flipTable(std::make_integer_sequence<uint16_t, 256>());

 #endif /* RGBDS_GFX_MAIN_HPP */

--- a/src/gfx/process.cpp

+++ b/src/gfx/process.cpp

@@ -652,7 +652,7 @@

 				// Count the line itself as mirrorred; vertical mirroring is

 				// already taken care of because the symmetric line will be XOR'd

 				// the same way. (...which is a problem, but probably benign.)

-				_hash ^= flip(bitplanes >> 8) << 8 | flip(bitplanes & 0xFF);

+				_hash ^= flipTable[bitplanes >> 8] << 8 | flipTable[bitplanes & 0xFF];

 			}

 		}

 	}

@@ -680,7 +680,7 @@

 		// Check if we have horizontal mirroring, which scans the array forward again

 		if (std::equal(_data.begin(), _data.end(), other._data.begin(),

-		               [](uint8_t lhs, uint8_t rhs) { return lhs == flip(rhs); })) {

+		               [](uint8_t lhs, uint8_t rhs) { return lhs == flipTable[rhs]; })) {

 			return MatchType::HFLIP;

 		}

@@ -694,7 +694,7 @@

 			if (lhs != rhs) {

 				hasVFlip = false;

 			}

-			if (lhs != flip(rhs)) {

+			if (lhs != flipTable[rhs]) {

 				hasVHFlip = false;

 			}

 			if (!hasVFlip && !hasVHFlip) {

--- a/src/gfx/reverse.cpp

+++ b/src/gfx/reverse.cpp

@@ -271,8 +271,8 @@

 				uint8_t realY = attribute & 0x40 ? 7 - y : y;

 				uint8_t bitplane0 = tileData[realY * 2], bitplane1 = tileData[realY * 2 + 1];

 				if (attribute & 0x20) { // Handle horizontal flip

-					bitplane0 = flip(bitplane0);

-					bitplane1 = flip(bitplane1);

+					bitplane0 = flipTable[bitplane0];

+					bitplane1 = flipTable[bitplane1];

 				}

 				uint8_t *ptr = &rowPtrs[y][tx * 8 * SIZEOF_PIXEL];

 				for (uint8_t x = 0; x < 8; ++x) {