// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: 2026 Martino Ferrari

#pragma once

#include <plot/core/types.hpp>

namespace plot::render::detail {

/// The factor a text run is drawn by: `requested_px / em_px`, a plain ratio.
///
/// It is a ratio rather than a whole multiple because the renderer's built-in
/// face is anti-aliased: a style asking for 12 px from a 13 px bake is drawn at
/// 12/13, where a 1-bit glyph would have had to snap to the nearest multiple of
/// its cell. Everything the renderer stages is scaled by this one factor, so the
/// labels a plot laid out and the glyphs the pipeline draws cannot disagree.
///
/// A backend that cannot state an em -- `em_px()` at or below zero, or NaN --
/// yields a 1:1 scale rather than dividing by it or propagating a NaN into every
/// vertex. The requested size being NaN, zero or negative does the same. The rule
/// lives here, not inline in the renderer, so it is one function and can be
/// asserted without a GL context; it is instrumentation-free and part of no
/// public surface (`*/detail/` headers stay out of the umbrella).
inline f32 text_scale(f32 requested_px, f32 em_px) noexcept {
  const f32 ratio = requested_px / em_px;
  return (em_px > 0.0f && ratio > 0.0f) ? ratio : 1.0f;
}

}  // namespace plot::render::detail