packed chunk data

src/td/game/WorldTypes.cpp

@@ -3,7 +3,35 @@
 namespace td {
 namespace game {
 
+const int BITS_IN_BYTE = 8;
+const int BITS_IN_LONG = BITS_IN_BYTE * sizeof(std::uint64_t);
 
+static unsigned int countBits(unsigned int number) {
+	// log function in base 2
+	// take only integer part
+	return static_cast<unsigned int>(std::log2(number) + 1);
+}
+
+TileIndex Chunk::GetTileIndex(std::uint16_t tileNumber) const {
+	const std::uint8_t bitsPerTile = countBits(m_Palette.size());
+
+	const std::size_t startLong = (tileNumber * bitsPerTile) / BITS_IN_LONG;
+	const std::size_t startOffset = (tileNumber * bitsPerTile) % BITS_IN_LONG;
+	const std::size_t endLong = ((tileNumber + 1) * bitsPerTile - 1) / BITS_IN_LONG;
+
+	const Chunk::ChunkData::value_type individualValueMask = ((1 << bitsPerTile) - 1);
+
+	td::game::Chunk::ChunkData::value_type value;
+	if (startLong == endLong) {
+		value = (m_Data[startLong] >> startOffset);
+	} else {
+		int endOffset = BITS_IN_LONG - startOffset;
+		value = (m_Data[startLong] >> startOffset | m_Data[endLong] << endOffset);
+	}
+	value &= individualValueMask;
+
+	return m_Palette.at(value);
+}
 
 }  // namespace game
 }  // namespace td