otype_hmtx.hpp

// Copyright Take Vos 2023.
// 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 "otype_utilities.hpp"
#include "../utility/module.hpp"
#include <span>
#include <cstddef>
 
namespace hi { inline namespace v1 {
 
[[nodiscard]] inline auto
otype_hmtx_get(std::span<std::byte const> bytes, hi::glyph_id glyph_id, uint16_t num_horizontal_metrics, float em_scale)
{
    struct entry_type {
        otype_fuword_buf_t advance_width;
        otype_fword_buf_t left_side_bearing;
    };
 
    struct return_type {
        float advance_width;
        float left_side_bearing;
    };
 
    hi_axiom(num_horizontal_metrics >= 1);
 
    auto offset = 0_uz;
    hilet horizontal_metrics = implicit_cast<entry_type>(offset, bytes, num_horizontal_metrics);
 
    if (*glyph_id < num_horizontal_metrics) {
        hilet& entry = horizontal_metrics[*glyph_id];
        return return_type{entry.advance_width * em_scale, entry.left_side_bearing * em_scale};
    }
 
    // In mono-type fonts the advance_width is repeating from the last entry and only the left_side_bearing is needed.
    hilet advance_width = horizontal_metrics.back().advance_width * em_scale;
 
    // The rest of the bytes in this table form the left_side_bearings.
    hilet num_left_side_bearing = (bytes.size() - offset) / sizeof(otype_fword_buf_t);
    hilet left_side_bearings = implicit_cast<otype_fword_buf_t>(offset, bytes, num_left_side_bearing);
 
    hilet left_side_bearing_index = *glyph_id - num_horizontal_metrics;
    hilet left_side_bearing = hi_check_at(left_side_bearings, left_side_bearing_index) * em_scale;
    return return_type{advance_width, left_side_bearing};
}
 
}} // namespace hi::v1