bits.hpp

// Copyright 2020 Pokitec
// All rights reserved.
 
#pragma once
 
#include "TTauri/Foundation/exceptions.hpp"
#include <nonstd/span>
#include <cstddef>
 
namespace tt {
 
[[nodiscard]] inline bool get_bit(nonstd::span<std::byte const> buffer, ssize_t &index) noexcept
{
    auto byte_index = index >> 3;
    auto bit_index = static_cast<uint8_t>(index & 7);
    ++index;
 
    tt_assume(byte_index < ssize(buffer));
    return static_cast<bool>(
        static_cast<int>(buffer[byte_index] >> bit_index) & 1
    );
} 
 
[[nodiscard]] inline int get_bits(nonstd::span<std::byte const> buffer, ssize_t &index, int length) noexcept
{
    auto value = 0;
 
    auto todo = length;
    auto done = 0;
    while (todo) {
        auto byte_index = index >> 3;
        auto bit_index = static_cast<int>(index & 7);
        tt_assume(byte_index < ssize(buffer));
 
        auto available_bits = 8 - bit_index;
        auto nr_bits = std::min(available_bits, todo);
 
        auto mask = (1 << nr_bits) - 1;
 
        auto tmp = static_cast<int>(buffer[byte_index] >> bit_index) & mask;
        value |= tmp << done;
 
        todo -= nr_bits;
        done += nr_bits;
        index += nr_bits;
    }
 
    return value;
} 
 
}