quicktex/quicktex/s3tc/bc4/BC4Block.h

/*  Python-rgbcx Texture Compression Library
    Copyright (C) 2021 Andrew Cassidy <drewcassidy@me.com>
    Partially derived from rgbcx.h written by Richard Geldreich <richgel99@gmail.com>
    and licenced under the public domain

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#pragma once

#include <array>
#include <cassert>
#include <cstdint>
#include <cstdlib>

#include "../../util.h"

namespace quicktex::s3tc {

class alignas(8) BC4Block {
   public:
    static constexpr int Width = 4;
    static constexpr int Height = 4;

    using SelectorArray = std::array<std::array<uint8_t, Width>, Height>;
    using AlphaPair = std::pair<uint8_t, uint8_t>;

    constexpr BC4Block() {
        static_assert(sizeof(BC4Block) == 8);
        static_assert(sizeof(std::array<BC4Block, 10>) == 8 * 10);
        static_assert(alignof(BC4Block) >= 8);
        alpha0 = alpha1 = 0;
        _selectors = {0, 0, 0, 0, 0, 0};
    }

    BC4Block(uint8_t valpha0, uint8_t valpha1, const SelectorArray& selectors) {
        alpha0 = valpha0;
        alpha1 = valpha1;
        SetSelectors(selectors);
    }

    constexpr AlphaPair GetAlphas() const { return AlphaPair(alpha0, alpha1); }

    void SetAlphas(AlphaPair as) {
        alpha0 = as.first;
        alpha1 = as.second;
    }

    constexpr uint64_t GetSelectorBits() const {
        auto packed = Pack<uint8_t, uint64_t, 8, SelectorSize>(_selectors);
        assert(packed <= SelectorsPackedMax);
        return packed;
    }

    void SetSelectorBits(uint64_t packed) {
        assert(packed <= SelectorsPackedMax);
        _selectors = Unpack<uint64_t, uint8_t, 8, SelectorSize>(packed);
    }

    constexpr SelectorArray GetSelectors() const {
        auto rows = Unpack<uint64_t, uint16_t, SelectorBits * Width, Height>(GetSelectorBits());
        auto unpacked = MapArray(rows, Unpack<uint16_t, uint8_t, SelectorBits, Width>);
        return unpacked;
    }

    void SetSelectors(const SelectorArray& unpacked) {
        auto rows = MapArray(unpacked, Pack<uint8_t, uint16_t, SelectorBits, Width>);
        auto packed = Pack<uint16_t, uint64_t, SelectorBits * Width, Height>(rows);
        SetSelectorBits(packed);
    }

    constexpr bool Is6Value() const { return alpha0 <= alpha1; }

    static constexpr std::array<uint8_t, 8> GetValues6(unsigned l, unsigned h) {
        return {static_cast<uint8_t>(l),
                static_cast<uint8_t>(h),
                static_cast<uint8_t>((l * 4 + h) / 5),
                static_cast<uint8_t>((l * 3 + h * 2) / 5),
                static_cast<uint8_t>((l * 2 + h * 3) / 5),
                static_cast<uint8_t>((l + h * 4) / 5),
                0,
                255};
    }

    static constexpr std::array<uint8_t, 8> GetValues8(unsigned l, unsigned h) {
        return {static_cast<uint8_t>(l),
                static_cast<uint8_t>(h),
                static_cast<uint8_t>((l * 6 + h) / 7),
                static_cast<uint8_t>((l * 5 + h * 2) / 7),
                static_cast<uint8_t>((l * 4 + h * 3) / 7),
                static_cast<uint8_t>((l * 3 + h * 4) / 7),
                static_cast<uint8_t>((l * 2 + h * 5) / 7),
                static_cast<uint8_t>((l + h * 6) / 7)};
    }

    static constexpr std::array<uint8_t, 8> GetValues(unsigned l, unsigned h) { return l > h ? GetValues8(l, h) : GetValues6(l, h); }

    static constexpr size_t SelectorSize = 6;
    static constexpr uint8_t SelectorBits = 3;
    static constexpr uint64_t SelectorsPackedMax = (1ULL << (8U * SelectorSize)) - 1U;

    uint8_t alpha0;
    uint8_t alpha1;

   private:
    std::array<uint8_t, SelectorSize> _selectors;
};
}  // namespace quicktex::s3tc