/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

//! Access to font metrics from the style system.

#![deny(missing_docs)]

use crate::values::computed::Length;

/// Represents the font metrics that style needs from a font to compute the
/// value of certain CSS units like `ex`.
#[derive(Clone, Debug, PartialEq)]
pub struct FontMetrics {
    /// The x-height of the font.
    pub x_height: Option<Length>,
    /// The zero advance. This is usually writing mode dependent
    pub zero_advance_measure: Option<Length>,
    /// The cap-height of the font.
    pub cap_height: Option<Length>,
    /// The ideographic-width of the font.
    pub ic_width: Option<Length>,
    /// The ascent of the font (a value is always available for this).
    pub ascent: Length,
    /// Script scale down factor for math-depth 1.
    /// https://w3c.github.io/mathml-core/#dfn-scriptpercentscaledown
    pub script_percent_scale_down: Option<f32>,
    /// Script scale down factor for math-depth 2.
    /// https://w3c.github.io/mathml-core/#dfn-scriptscriptpercentscaledown
    pub script_script_percent_scale_down: Option<f32>,
}

impl Default for FontMetrics {
    fn default() -> Self {
        FontMetrics {
            x_height: None,
            zero_advance_measure: None,
            cap_height: None,
            ic_width: None,
            ascent: Length::new(0.0),
            script_percent_scale_down: None,
            script_script_percent_scale_down: None,
        }
    }
}

impl FontMetrics {
    /// Returns the x-height, computing a fallback value if not present
    pub fn x_height_or_default(&self, reference_font_size: Length) -> Length {
        // https://drafts.csswg.org/css-values/#ex
        //
        //     In the cases where it is impossible or impractical to
        //     determine the x-height, a value of 0.5em must be
        //     assumed.
        //
        // (But note we use 0.5em of the used, not computed
        // font-size)
        self.x_height.unwrap_or_else(|| reference_font_size * 0.5)
    }

    /// Returns the zero advance measure, computing a fallback value if not present
    pub fn zero_advance_measure_or_default(
        &self,
        reference_font_size: Length,
        upright: bool,
    ) -> Length {
        // https://drafts.csswg.org/css-values/#ch
        //
        //     In the cases where it is impossible or impractical to
        //     determine the measure of the “0” glyph, it must be
        //     assumed to be 0.5em wide by 1em tall. Thus, the ch
        //     unit falls back to 0.5em in the general case, and to
        //     1em when it would be typeset upright (i.e.
        //     writing-mode is vertical-rl or vertical-lr and
        //     text-orientation is upright).
        //
        // Same caveat about computed vs. used font-size applies
        // above.
        self.zero_advance_measure.unwrap_or_else(|| {
            if upright {
                reference_font_size
            } else {
                reference_font_size * 0.5
            }
        })
    }

    /// Returns the cap-height, computing a fallback value if not present
    pub fn cap_height_or_default(&self) -> Length {
        // https://drafts.csswg.org/css-values/#cap
        //
        //     In the cases where it is impossible or impractical to
        //     determine the cap-height, the font’s ascent must be
        //     used.
        //
        self.cap_height.unwrap_or_else(|| self.ascent)
    }

    /// Returns the ideographic advance measure, computing a fallback value if not present
    pub fn ic_width_or_default(&self, reference_font_size: Length) -> Length {
        // https://drafts.csswg.org/css-values/#ic
        //
        //     In the cases where it is impossible or impractical to
        //     determine the ideographic advance measure, it must be
        //     assumed to be 1em.
        //
        // Same caveat about computed vs. used as for other
        // metric-dependent units.
        self.ic_width.unwrap_or_else(|| reference_font_size)
    }
}

/// Type of font metrics to retrieve.
#[derive(Clone, Debug, PartialEq)]
pub enum FontMetricsOrientation {
    /// Get metrics for horizontal or vertical according to the Context's
    /// writing mode, using horizontal metrics for vertical/mixed
    MatchContextPreferHorizontal,
    /// Get metrics for horizontal or vertical according to the Context's
    /// writing mode, using vertical metrics for vertical/mixed
    MatchContextPreferVertical,
    /// Force getting horizontal metrics.
    Horizontal,
}