rgbds/src/opmath.cpp

/* SPDX-License-Identifier: MIT */

// Mathematical operators that don't reuse C++'s behavior

#include <stdint.h>

#include "opmath.hpp"

int32_t op_divide(int32_t dividend, int32_t divisor)
{
	// Adjust division to floor toward negative infinity,
	// not truncate toward zero
	return dividend / divisor - ((dividend % divisor < 0) != (divisor < 0));
}

int32_t op_modulo(int32_t dividend, int32_t divisor)
{
	int32_t remainder = dividend % divisor;

	// Adjust modulo to have the sign of the divisor,
	// not the sign of the dividend
	return remainder + divisor * ((remainder < 0) != (divisor < 0));
}

int32_t op_exponent(int32_t base, uint32_t power)
{
	int32_t result = 1;

	for (;;) {
		if (power % 2)
			result *= base;
		power /= 2;
		if (!power)
			break;
		base *= base;
	}

	return result;
}

int32_t op_shift_left(int32_t value, int32_t amount)
{
	// Get the easy cases out of the way
	if (amount == 0)
		return value;
	if (value == 0 || amount >= 32)
		return 0;
	if (amount < -31)
		return (value < 0) ? -1 : 0;
	if (amount < 0)
		return op_shift_right(value, -amount);

	// Use unsigned to force a bitwise shift
	// Casting back is OK because the types implement two's complement behavior
	return (uint32_t)value << amount;
}

int32_t op_shift_right(int32_t value, int32_t amount)
{
	// Repeat the easy cases here to avoid INT_MIN funny business
	if (amount == 0)
		return value;
	if (value == 0 || amount < -31)
		return 0;
	if (amount > 31)
		return (value < 0) ? -1 : 0;
	if (amount < 0)
		return op_shift_left(value, -amount);

	if (value > 0)
		return (uint32_t)value >> amount;

	// Calculate an OR mask for sign extension
	// 1->0x80000000, 2->0xC0000000, ..., 31->0xFFFFFFFE
	uint32_t amount_high_bits = -(UINT32_C(1) << (32 - amount));

	// The C++ standard leaves shifting right negative values
	// undefined, so use a left shift manually sign-extended
	return ((uint32_t)value >> amount) | amount_high_bits;
}

int32_t op_shift_right_unsigned(int32_t value, int32_t amount)
{
	// Repeat the easy cases here to avoid INT_MIN funny business
	if (amount == 0)
		return value;
	if (value == 0 || amount < -31 || amount > 31)
		return 0;
	if (amount < 0)
		return op_shift_left(value, -amount);

	return (uint32_t)value >> amount;
}