polynomial.hpp

// Copyright Take Vos 2022.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#pragma once
 
#include "math.hpp"
#include <numbers>
#include <array>
 
namespace hi::inline v1 {
 
template<typename T, std::size_t N>
class results {
public:
    using value_type = T;
    using array_type = std::array<value_type, N>;
    using const_iterator = typename array_type::const_iterator;
 
    constexpr results() noexcept : _size(0), _v() {}
    constexpr results(results const&) noexcept = default;
    constexpr results(results&&) noexcept = default;
    constexpr results& operator=(results const&) noexcept = default;
    constexpr results& operator=(results&&) noexcept = default;
 
    constexpr results(value_type a) noexcept : _size(1), _v()
    {
        _v[0] = a;
    }
 
    constexpr results(value_type a, value_type b) noexcept : _size(2), _v()
    {
        _v[0] = a;
        _v[1] = b;
    }
 
    constexpr results(value_type a, value_type b, value_type c) noexcept : _size(3), _v()
    {
        _v[0] = a;
        _v[1] = b;
        _v[2] = c;
    }
 
    template<std::size_t O>
    constexpr results(results<T, O> const& other) noexcept requires(O < N) : _size(other._size), _v()
    {
        if constexpr (O > 0) {
            for (auto i = 0_uz; i < other._size; i++) {
                _v[i] = other._v[i];
            }
        }
    }
 
    [[nodiscard]] constexpr std::size_t capacity() const noexcept
    {
        return _capacity;
    }
 
    [[nodiscard]] constexpr std::size_t size() const noexcept
    {
        return _size;
    }
 
    [[nodiscard]] constexpr const_iterator begin() const noexcept
    {
        return _v.begin();
    }
 
    [[nodiscard]] constexpr const_iterator end() const noexcept
    {
        return _v.begin() + size();
    }
 
    [[nodiscard]] constexpr value_type const& operator[](std::size_t index) const noexcept
    {
        hi_axiom(index < _size);
        return _v[index];
    }
 
    [[nodiscard]] constexpr value_type& operator[](std::size_t index) noexcept
    {
        hi_axiom(index < _size);
        return _v[index];
    }
 
    constexpr void add(T a) noexcept
    {
        hi_axiom(_size < _capacity);
        _v[_size++] = a;
    }
 
    [[nodiscard]] constexpr friend results operator-(results lhs, value_type rhs) noexcept
    {
        // For performance reasons work on the whole array. The constructors have
        // initialized the empty elements to 0.0f.
        for (auto i = 0_uz; i < lhs._capacity; i++) {
            lhs._v[i] -= rhs;
        }
        return lhs;
    }
 
private:
    static constexpr std::size_t _capacity = N;
 
    array_type _v;
    std::size_t _size;
 
    template<typename O, std::size_t M>
    friend class results;
};
 
 
template<typename T, std::size_t N>
inline std::ostream& operator<<(std::ostream& os, results<T, N> const& r)
{
    os << "[";
    hi_assert(r.size() <= r.capacity());
    for (auto i = 0_uz; i < r.size(); i++) {
        if (i > 0) {
            os << ", ";
        }
        os << r[i];
    }
    os << "]";
    return os;
}
 
template<typename T>
hi_force_inline constexpr results<T, 1> solvePolynomial(T const& a, T const& b) noexcept
{
    if (a != 0) {
        return {-(b / a)};
    } else if (b == 0) {
        // Any value of x is correct.
        return {0.0f};
    } else {
        // None of the values for x is correct.
        return {};
    }
}
 
template<typename T>
hi_force_inline constexpr results<T, 2> solvePolynomial(T const& a, T const& b, T const& c) noexcept
{
    if (a == 0) {
        return solvePolynomial(b, c);
    } else {
        hilet D = b * b - static_cast<T>(4) * a * c;
        if (D < 0) {
            return {};
        } else if (D == 0) {
            return {-b / (static_cast<T>(2) * a)};
        } else {
            hilet Dsqrt = sqrt(D);
            return {(-b - Dsqrt) / (static_cast<T>(2) * a), (-b + Dsqrt) / (static_cast<T>(2) * a)};
        }
    }
}
 
template<typename T>
hi_force_inline results<T, 3> solveDepressedCubicTrig(T const& p, T const& q) noexcept
{
    constexpr T oneThird = static_cast<T>(1) / static_cast<T>(3);
    constexpr T pi2_3 = (static_cast<T>(2) / static_cast<T>(3)) * std::numbers::pi_v<T>;
    constexpr T pi4_3 = (static_cast<T>(4) / static_cast<T>(3)) * std::numbers::pi_v<T>;
 
    hilet U = oneThird * acos(((static_cast<T>(3) * q) / (static_cast<T>(2) * p)) * sqrt(static_cast<T>(-3) / p));
    hilet V = static_cast<T>(2) * sqrt(-oneThird * p);
 
    hilet t0 = V * cos(U);
    hilet t1 = V * cos(U - pi2_3);
    hilet t2 = V * cos(U - pi4_3);
    return {t0, t1, t2};
}
 
template<typename T>
hi_force_inline results<T, 3> solveDepressedCubicCardano(T const& p, T const& q, T const& D) noexcept
{
    hilet sqrtD = sqrt(D);
    hilet minusHalfQ = static_cast<T>(-0.5) * q;
    hilet v = cbrt(minusHalfQ + sqrtD);
    hilet w = cbrt(minusHalfQ - sqrtD);
    return {v + w};
}
 
template<typename T>
hi_force_inline results<T, 3> solveDepressedCubic(T const& p, T const& q) noexcept
{
    constexpr T oneForth = static_cast<T>(1) / static_cast<T>(4);
    constexpr T oneTwentySeventh = static_cast<T>(1) / static_cast<T>(27);
 
    if (p == 0.0 && q == 0.0) {
        return {static_cast<T>(0)};
 
    } else {
        hilet D = oneForth * q * q + oneTwentySeventh * p * p * p;
 
        if (D < 0 && p != 0.0) {
            // Has three real roots.
            return solveDepressedCubicTrig(p, q);
 
        } else if (D == 0 && p != 0.0) {
            // Has two real roots, or maybe one root
            hilet t0 = (static_cast<T>(3) * q) / p;
            hilet t1 = (static_cast<T>(-3) * q) / (static_cast<T>(2) * p);
            return {t0, t1, t1};
 
        } else {
            // Has one real root.
            return solveDepressedCubicCardano(p, q, D);
        }
    }
}
 
template<typename T>
hi_force_inline constexpr results<T, 3> solvePolynomial(T const& a, T const& b, T const& c, T const& d) noexcept
{
    if (a == 0) {
        return solvePolynomial(b, c, d);
 
    } else {
        hilet p = (static_cast<T>(3) * a * c - b * b) / (static_cast<T>(3) * a * a);
        hilet q = (static_cast<T>(2) * b * b * b - static_cast<T>(9) * a * b * c + static_cast<T>(27) * a * a * d) /
            (static_cast<T>(27) * a * a * a);
 
        hilet r = solveDepressedCubic(p, q);
 
        hilet b_3a = b / (static_cast<T>(3) * a);
 
        return r - b_3a;
    }
}
 
} // namespace hi::inline v1