Implement enough functionality to compile & match pokecrystal

include/gfx/rgba.hpp

@@ -0,0 +1,58 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_GFX_RGBA_HPP
+#define RGBDS_GFX_RGBA_HPP
+
+#include <stdint.h>
+
+struct Rgba {
+	uint8_t red;
+	uint8_t green;
+	uint8_t blue;
+	uint8_t alpha;
+
+	Rgba(uint8_t r, uint8_t g, uint8_t b, uint8_t a) : red(r), green(g), blue(b), alpha(a) {}
+	/**
+	 * Constructs the color from a "packed" RGBA representation (0xRRGGBBAA)
+	 */
+	explicit Rgba(uint32_t rgba = 0)
+		: red(rgba >> 24), green(rgba >> 16), blue(rgba >> 8), alpha(rgba) {}
+
+	/**
+	 * Returns this RGBA as a 32-bit number that can be printed in hex (`%08x`) to yield its CSS
+	 * representation
+	 */
+	uint32_t toCSS() const {
+		auto shl = [](uint8_t val, unsigned shift) { return static_cast<uint32_t>(val) << shift; };
+		return shl(red, 24) | shl(green, 16) | shl(blue, 8) | shl(alpha, 0);
+	}
+	friend bool operator!=(Rgba const &lhs, Rgba const &rhs) { return lhs.toCSS() != rhs.toCSS(); }
+
+	/**
+	 * CGB colors are RGB555, so we use bit 15 to signify that the color is transparent instead
+	 * Since the rest of the bits don't matter then, we return 0x8000 exactly.
+	 */
+	static constexpr uint16_t transparent = 0b1'00000'00000'00000;
+
+	/**
+	 * All alpha values strictly below this will be considered transparent
+	 */
+	static constexpr uint8_t opacity_threshold = 0xF0; // TODO: adjust this
+	// TODO: also a transparency threshold, and error out on "middle" values
+	bool isTransparent() const { return alpha < opacity_threshold; }
+	/**
+	 * Computes the equivalent CGB color, respects the color curve depending on options
+	 */
+	uint16_t cgbColor() const;
+
+	bool isGray() const { return red == green && green == blue; }
+	uint8_t grayIndex() const;
+};
+
+#endif /* RGBDS_GFX_RGBA_HPP */