serenity/AK/IntegralMath.h

/*
 * Copyright (c) 2022, Leon Albrecht <[email protected]>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#pragma once

#include <AK/BuiltinWrappers.h>
#include <AK/Concepts.h>
#include <AK/Types.h>

namespace AK {

template<Integral T>
constexpr T exp2(T exponent)
{
    return static_cast<T>(1) << exponent;
}

template<Integral T>
constexpr T log2(T x)
{
    return x ? (8 * sizeof(T) - 1) - count_leading_zeroes(static_cast<MakeUnsigned<T>>(x)) : 0;
}

template<Integral T>
constexpr T ceil_log2(T x)
{
    if (x <= 1)
        return 0;

    return AK::log2(x - 1) + 1;
}

template<Integral I>
constexpr I pow(I base, I exponent)
{
    // https://en.wikipedia.org/wiki/Exponentiation_by_squaring
    if (exponent < 0)
        return 0;
    if (exponent == 0)
        return 1;

    I res = 1;
    while (exponent > 0) {
        if (exponent & 1)
            res *= base;
        base *= base;
        exponent /= 2u;
    }
    return res;
}

template<auto base, UnsignedIntegral U = decltype(base)>
constexpr bool is_power_of(U x)
{
    if constexpr (base == 1)
        return x == 1;
    else if constexpr (base == 2)
        return is_power_of_two(x);

    if (base == 0 && x == 0)
        return true;
    if (base == 0 || x == 0)
        return false;

    while (x != 1) {
        if (x % base != 0)
            return false;
        x /= base;
    }
    return true;
}

template<UnsignedIntegral T>
constexpr T reinterpret_as_octal(T decimal)
{
    T result = 0;
    T n = 0;
    while (decimal > 0) {
        result += pow<T>(8, n++) * (decimal % 10);
        decimal /= 10;
    }
    return result;
}

template<UnsignedIntegral T>
constexpr MakeSigned<T> sign_extend(T value, u8 bits)
{
    // C++ considers the shift by sizeof(T) * 8 UB, and it doesn’t make logical sense to sign-extend 0 bits anyways.
    VERIFY(bits > 0);
    auto shift = sizeof(T) * 8 - bits;
    return static_cast<MakeSigned<T>>(value << shift) >> shift;
}

}