Add initial fixed point number implementation

CMakeLists.txt

@@ -3,7 +3,7 @@ cmake_minimum_required(VERSION 3.5)
 project(mtl LANGUAGES CXX C ASM)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
-set(TARGET_SRC_FILES src/memcpy.s src/string.cpp)
+set(TARGET_SRC_FILES src/memcpy.s src/string.cpp src/fixed.s)
 set(TARGET_PUB_INCLUDE_FILES include/mtl/utility.hpp include/mtl/string.hpp include/mtl/fsm.hpp)
 
 add_library(${PROJECT_NAME} STATIC ${TARGET_SRC_FILES} ${TARGET_PUB_INCLUDE_FILES})

include/mtl/fixed.hpp

@@ -0,0 +1,83 @@
+#pragma once
+
+#include <cstdint>
+
+/**
+ * \brief Fixed point multiply assembly implementation
+ *
+ * DO NOT USE DIRECTLY! Use fixed::operator* instead
+ */
+extern "C" int32_t fixed_mul(int32_t x, int32_t y);
+
+namespace mtl {
+/**
+ * \brief 32-bit Fixed point number
+ *
+ * Uses a base of 64. ie. the lower 6 bits are after the decimal place,
+ * the other 26 bits are before the decimal place.
+ *
+ * Valid values are in the range ~[-33'554'431.01, 33'554'432.98]
+ *
+ * Has a maximum error of +/- 1/128 (~0.0078), integers are always
+ * exactly.
+ */
+class fixed {
+	int32_t x;
+
+	/**
+	 * \brief Raw constructor
+	 *
+	 * Creates a new fixed point number with the raw data of x.
+	 * DO NOT use to set the fixed number to an integer value, use
+	 * the public constructor instead.
+	 */
+	constexpr fixed(int32_t _x, bool) : x(_x) {}
+
+public:
+	constexpr fixed() : x(0) {}
+	/**
+	 * \brief 32-bit integer constructor
+	 *
+	 * Creates a new fixed point number with the value of the integer.
+	 * Must be within the range represented by fixed point numbers, see
+	 * the class description for more detail.
+	 */
+	constexpr fixed(int32_t _i) : x(_i * 64) {}
+	/**
+	 * \brief Floating point constructor
+	 *
+	 * Creates a new fixed point number with the closest number to
+	 * the floating point number. Must be within the range represented by
+	 * fixed point numbers, see the class description for more detail.
+	 */
+	constexpr fixed(float _f)
+		// 0.5 offset accounts for truncating to integer, round instead
+		: x((_f * 64) + 0.5f) {}
+	
+	/**
+	 * \brief Fixed point addition
+	 *
+	 * Addition with fixed point numbers is the same as with a 32-bit
+	 * integer, so should be extremely quick.
+	 */
+	fixed operator+(fixed rhs) const {
+		return fixed(x + rhs.x, true);
+	}
+
+	/**
+	 * \brief Fixed point multiplication
+	 *
+	 * Uses an assembly implementation to multiply the two numbers.
+	 * Not as quick as an integer multiplication. Use sparringly.
+	 *
+	 * Tested on the MGBA Gameboy Advance emulator, takes around 90
+	 * cycles when the assembly routine is placed in IWRAM.
+	 * The Gameboy Advance uses an armv7tdmi, and IWRAM is the fastest
+	 * available RAM.
+	 */
+	fixed operator*(fixed rhs) const {
+		return fixed(fixed_mul(x, rhs.x), true);
+	}
+};
+
+} // namespace mtl

src/fixed.s

@@ -0,0 +1,11 @@
+.section .iwram, "ax", %progbits
+.arm
+.align 2
+
+.global mtl_fixed_mul
+.type mtl_fixed_mul STT_FUNC
+mtl_fixed_mul:
+smull	r2, r3, r0, r1
+lsr	r0, r2, #6
+orr	r0, r3, lsl #26
+bx	lr