ghostty-rs/crates/ghostty-font/src/metrics.rs

//! Font metrics for the terminal grid. Port of `font/Metrics.zig`.

use std::{collections::HashMap, str::FromStr};

/// Metrics describing the terminal grid cell for a font.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Metrics {
    /// Recommended cell width and height for a monospace grid using this font.
    pub cell_width: u32,
    pub cell_height: u32,

    /// Distance in pixels from the bottom of the cell to the text baseline.
    pub cell_baseline: u32,

    /// Distance in pixels from the top of the cell to the top of the underline.
    pub underline_position: u32,
    /// Thickness in pixels of the underline.
    pub underline_thickness: u32,

    /// Distance in pixels from the top of the cell to the top of the strikethrough.
    pub strikethrough_position: u32,
    /// Thickness in pixels of the strikethrough.
    pub strikethrough_thickness: u32,

    /// Distance in pixels from the top of the cell to the top of the overline.
    /// Can be negative to adjust the position above the top of the cell.
    pub overline_position: i32,
    /// Thickness in pixels of the overline.
    pub overline_thickness: u32,

    /// Thickness in pixels of box drawing characters.
    pub box_thickness: u32,

    /// The thickness in pixels of the cursor sprite. This has a default value
    /// because it is not determined by fonts but rather by user configuration.
    pub cursor_thickness: u32,

    /// The height in pixels of the cursor sprite.
    pub cursor_height: u32,

    /// The constraint height for nerd fonts icons.
    pub icon_height: f64,

    /// The constraint height for nerd fonts icons limited to a single cell width.
    pub icon_height_single: f64,

    /// The unrounded face width, used in scaling calculations.
    pub face_width: f64,

    /// The unrounded face height, used in scaling calculations.
    pub face_height: f64,

    /// The offset from the bottom of the cell to the bottom
    /// of the face's bounding box, based on the rounded and
    /// potentially adjusted cell height.
    pub face_y: f64,
}

/// Minimum acceptable values for some fields to prevent modifiers
/// from being able to, for example, cause 0-thickness underlines.
mod minimums {
    pub const CELL_WIDTH: u32 = 1;
    pub const CELL_HEIGHT: u32 = 1;
    pub const UNDERLINE_THICKNESS: u32 = 1;
    pub const STRIKETHROUGH_THICKNESS: u32 = 1;
    pub const OVERLINE_THICKNESS: u32 = 1;
    pub const BOX_THICKNESS: u32 = 1;
    pub const CURSOR_THICKNESS: u32 = 1;
    pub const CURSOR_HEIGHT: u32 = 1;
    pub const ICON_HEIGHT: f64 = 1.0;
    pub const ICON_HEIGHT_SINGLE: f64 = 1.0;
    pub const FACE_HEIGHT: f64 = 1.0;
    pub const FACE_WIDTH: f64 = 1.0;
}

/// Metrics extracted from a font face, based on
/// the metadata tables and glyph measurements.
#[derive(Debug, Clone, Copy, PartialEq, Default)]
pub struct FaceMetrics {
    /// Pixels per em, dividing the other values in this struct by this should
    /// yield sizes in ems, to allow comparing metrics from faces of different
    /// sizes.
    pub px_per_em: f64,

    /// The minimum cell width that can contain any glyph in the ASCII range.
    ///
    /// Determined by measuring all printable glyphs in the ASCII range.
    pub cell_width: f64,

    /// The typographic ascent metric from the font.
    /// This represents the maximum vertical position of the highest ascender.
    ///
    /// Relative to the baseline, in px, +Y=up
    pub ascent: f64,

    /// The typographic descent metric from the font.
    /// This represents the minimum vertical position of the lowest descender.
    ///
    /// Relative to the baseline, in px, +Y=up
    ///
    /// Note:
    /// As this value is generally below the baseline, it is typically negative.
    pub descent: f64,

    /// The typographic line gap (aka "leading") metric from the font.
    /// This represents the additional space to be added between lines in
    /// addition to the space defined by the ascent and descent metrics.
    ///
    /// Positive value in px
    pub line_gap: f64,

    /// The TOP of the underline stroke.
    ///
    /// Relative to the baseline, in px, +Y=up
    pub underline_position: Option<f64>,

    /// The thickness of the underline stroke in px.
    pub underline_thickness: Option<f64>,

    /// The TOP of the strikethrough stroke.
    ///
    /// Relative to the baseline, in px, +Y=up
    pub strikethrough_position: Option<f64>,

    /// The thickness of the strikethrough stroke in px.
    pub strikethrough_thickness: Option<f64>,

    /// The height of capital letters in the font, either derived from
    /// a provided cap height metric or measured from the height of the
    /// capital H glyph.
    pub cap_height: Option<f64>,

    /// The height of lowercase letters in the font, either derived from
    /// a provided ex height metric or measured from the height of the
    /// lowercase x glyph.
    pub ex_height: Option<f64>,

    /// The measured height of the bounding box containing all printable
    /// ASCII characters. This can be different from ascent - descent for
    /// two reasons: non-letter symbols like @ and $ often exceed the
    /// the ascender and descender lines; and fonts often bake the line
    /// gap into the ascent and descent metrics (as per, e.g., the Google
    /// Fonts guidelines: https://simoncozens.github.io/gf-docs/metrics.html).
    ///
    /// Positive value in px
    pub ascii_height: Option<f64>,

    /// The width of the character "水" (CJK water ideograph, U+6C34),
    /// if present. This is used for font size adjustment, to normalize
    /// the width of CJK fonts mixed with latin fonts.
    ///
    /// NOTE: IC = Ideograph Character
    pub ic_width: Option<f64>,
}

fn positive(v: Option<f64>) -> Option<f64> {
    v.filter(|v| *v > 0.0)
}

impl FaceMetrics {
    /// Convenience function for getting the line height
    /// (ascent - descent + line_gap).
    #[inline]
    pub fn line_height(&self) -> f64 {
        self.ascent - self.descent + self.line_gap
    }

    /// Convenience function for getting the cap height. If this is not
    /// defined in the font, we estimate it as 75% of the ascent.
    #[inline]
    pub fn cap_height(&self) -> f64 {
        positive(self.cap_height).unwrap_or(0.75 * self.ascent)
    }

    /// Convenience function for getting the ex height. If this is not
    /// defined in the font, we estimate it as 75% of the cap height.
    #[inline]
    pub fn ex_height(&self) -> f64 {
        positive(self.ex_height).unwrap_or_else(|| 0.75 * self.cap_height())
    }

    /// Convenience function for getting the ASCII height. If we
    /// couldn't measure this, we use 1.5 * cap_height as our
    /// estimator, based on measurements across programming fonts.
    #[inline]
    pub fn ascii_height(&self) -> f64 {
        positive(self.ascii_height).unwrap_or_else(|| 1.5 * self.cap_height())
    }

    /// Convenience function for getting the ideograph width. If this is
    /// not defined in the font, we estimate it as the minimum of the
    /// ascii height and two cell widths.
    #[inline]
    pub fn ic_width(&self) -> f64 {
        positive(self.ic_width).unwrap_or_else(|| self.ascii_height().min(2.0 * self.cell_width))
    }

    /// Convenience function for getting the underline thickness. If
    /// this is not defined in the font, we estimate it as 15% of the ex
    /// height.
    #[inline]
    pub fn underline_thickness(&self) -> f64 {
        positive(self.underline_thickness).unwrap_or_else(|| 0.15 * self.ex_height())
    }

    /// Convenience function for getting the strikethrough thickness. If
    /// this is not defined in the font, we set it equal to the
    /// underline thickness.
    #[inline]
    pub fn strikethrough_thickness(&self) -> f64 {
        positive(self.strikethrough_thickness).unwrap_or_else(|| self.underline_thickness())
    }

    // NOTE: The getters below return positions, not sizes, so both
    // positive and negative values are valid, hence no sign validation.

    /// Convenience function for getting the underline position. If
    /// this is not defined in the font, we place it one underline
    /// thickness below the baseline.
    #[inline]
    pub fn underline_position(&self) -> f64 {
        self.underline_position
            .unwrap_or_else(|| -self.underline_thickness())
    }

    /// Convenience function for getting the strikethrough position. If
    /// this is not defined in the font, we center it at half the ex
    /// height, so that it's perfectly centered on lower case text.
    #[inline]
    pub fn strikethrough_position(&self) -> f64 {
        self.strikethrough_position
            .unwrap_or_else(|| f64::midpoint(self.ex_height(), self.strikethrough_thickness()))
    }
}

/// Zig's `@round`: round half away from zero (same as `f64::round`).
#[inline]
fn zround(v: f64) -> f64 {
    v.round()
}

impl Metrics {
    /// All-zero metrics, except `cursor_thickness` which defaults to 1.
    /// (Zig's private `init()`; used by tests and the web_canvas face.)
    pub(crate) fn init() -> Metrics {
        Metrics {
            cell_width: 0,
            cell_height: 0,
            cell_baseline: 0,
            underline_position: 0,
            underline_thickness: 0,
            strikethrough_position: 0,
            strikethrough_thickness: 0,
            overline_position: 0,
            overline_thickness: 0,
            box_thickness: 0,
            cursor_thickness: 1,
            cursor_height: 0,
            icon_height: 0.0,
            icon_height_single: 0.0,
            face_width: 0.0,
            face_height: 0.0,
            face_y: 0.0,
        }
    }

    /// Calculate our metrics based on values extracted from a font.
    ///
    /// Try to pass values with as much precision as possible,
    /// do not round them before using them for this function.
    ///
    /// For any nullable options that are not provided, estimates will be used.
    pub fn calc(face: FaceMetrics) -> Metrics {
        // These are the unrounded advance width and line height values,
        // which are retained separately from the rounded cell width and
        // height values (below), for calculations that need to know how
        // much error there is between the design dimensions of the font
        // and the pixel dimensions of our cells.
        let face_width = face.cell_width;
        let face_height = face.line_height();

        // The cell width and height values need to be integers since they
        // represent pixel dimensions of the grid cells in the terminal.
        //
        // We use round for the cell width to limit the difference from
        // the "true" width value to no more than 0.5px. This is a better
        // approximation of the authorial intent of the font than ceiling
        // would be, and makes the apparent spacing match better between
        // low and high DPI displays.
        //
        // This does mean that it's possible for a glyph to overflow the
        // edge of the cell by a pixel if it has no side bearings, but in
        // reality such glyphs are generally meant to connect to adjacent
        // glyphs in some way so it's not really an issue.
        //
        // The same is true for the height. Some fonts are poorly authored
        // and have a descender on a normal glyph that extends right up to
        // the descent value of the face, and this can result in the glyph
        // overflowing the bottom of the cell by a pixel, which isn't good
        // but if we try to prevent it by increasing the cell height then
        // we get line heights that are too large for most users and even
        // more inconsistent across DPIs.
        //
        // Users who experience such cell-height overflows should:
        //
        // 1. Nag the font author to either redesign the glyph to not go
        //    so low, or else adjust the descent value in the metadata.
        //
        // 2. Add an `adjust-cell-height` entry to their config to give
        //    the cell enough room for the glyph.
        let cell_width = zround(face_width);
        let cell_height = zround(face_height);

        // We split our line gap in two parts, and put half of it on the top
        // of the cell and the other half on the bottom, so that our text never
        // bumps up against either edge of the cell vertically.
        let half_line_gap = face.line_gap / 2.0;

        // NOTE: Unlike all our other metrics, `cell_baseline` is
        // relative to the BOTTOM of the cell rather than the top.
        let face_baseline = half_line_gap - face.descent;
        // We calculate the baseline by trying to center the face vertically
        // in the pixel-rounded cell height, so that before rounding it will
        // be an even distance from the top and bottom of the cell, meaning
        // it either sticks out the same amount or is inset the same amount,
        // depending on whether the cell height was rounded up or down from
        // the line height. We do this by adding half the difference between
        // the cell height and the face height.
        let cell_baseline = zround(face_baseline - (cell_height - face_height) / 2.0);

        // We keep track of the offset from the bottom of the cell
        // to the bottom of the face's "true" bounding box, which at
        // this point, since nothing has been scaled yet, is equivalent
        // to the offset between the baseline we draw at (cell_baseline)
        // and the one the font wants (face_baseline).
        let face_y = cell_baseline - face_baseline;

        // We calculate a top_to_baseline to make following calculations simpler.
        let top_to_baseline = cell_height - cell_baseline;

        // Get the other font metrics or their estimates. See doc comments
        // in FaceMetrics for explanations of the estimation heuristics.
        let cap_height = face.cap_height();
        let underline_thickness = 1.0f64.max(face.underline_thickness().ceil());
        let strikethrough_thickness = 1.0f64.max(face.strikethrough_thickness().ceil());
        let underline_position = zround(top_to_baseline - face.underline_position());
        let strikethrough_position = zround(top_to_baseline - face.strikethrough_position());

        // Same heuristic as the font_patcher script. We store icon_height
        // separately from face_height such that modifiers can apply to the former
        // without affecting the latter.
        let icon_height = face_height;
        let icon_height_single = (2.0 * cap_height + face_height) / 3.0;

        let mut result = Metrics {
            cell_width: cell_width as u32,
            cell_height: cell_height as u32,
            cell_baseline: cell_baseline as u32,
            underline_position: underline_position as u32,
            underline_thickness: underline_thickness as u32,
            strikethrough_position: strikethrough_position as u32,
            strikethrough_thickness: strikethrough_thickness as u32,
            overline_position: 0,
            overline_thickness: underline_thickness as u32,
            box_thickness: underline_thickness as u32,
            cursor_thickness: 1,
            cursor_height: cell_height as u32,
            icon_height,
            icon_height_single,
            face_width,
            face_height,
            face_y,
        };

        // Ensure all metrics are within their allowable range.
        result.clamp();

        result
    }

    /// Apply a set of modifiers.
    pub fn apply(&mut self, mods: &ModifierSet) {
        // Zig iterates a hash map (unspecified order); we iterate in Key
        // declaration order for determinism.
        let mut entries: Vec<(Key, Modifier)> = mods.iter().map(|(k, v)| (*k, *v)).collect();
        entries.sort_by_key(|(k, _)| *k);

        for (key, modifier) in entries {
            match key {
                // We clamp these values to a minimum of 1 to prevent divide-by-zero
                // in downstream operations.
                Key::CellWidth | Key::CellHeight => {
                    let original = if key == Key::CellWidth {
                        self.cell_width
                    } else {
                        self.cell_height
                    };
                    // Compute the new value. If it is the same avoid the work.
                    let new = modifier.apply_u32(original).max(1);
                    if new == original {
                        continue;
                    }

                    if key == Key::CellWidth {
                        self.cell_width = new;
                        continue;
                    }
                    self.cell_height = new;

                    // For cell height, we have to also modify some positions
                    // that are absolute from the top of the cell. The main goal
                    // here is to center the baseline so that text is vertically
                    // centered in the cell.
                    let original_f64 = f64::from(original);
                    let new_f64 = f64::from(new);
                    let diff = new_f64 - original_f64;
                    let half_diff = diff / 2.0;

                    // If the diff is even, the number of pixels we add
                    // will be the same for the top and the bottom, but
                    // if the diff is odd then we want to add the extra
                    // pixel to the edge of the cell that needs it most.
                    //
                    // How much the edge "needs it" depends on whether
                    // the face is higher or lower than it should be to
                    // be perfectly centered in the cell.
                    //
                    // If the face were perfectly centered then face_y
                    // would be equal to half of the difference between
                    // the cell height and the face height.
                    let position_with_respect_to_center =
                        self.face_y - (original_f64 - self.face_height) / 2.0;

                    let (diff_top, diff_bottom) = if position_with_respect_to_center > 0.0 {
                        // The baseline is higher than it should be, so we
                        // add the extra to the top, or if it's a negative
                        // diff it gets added to the bottom because of how
                        // floor and ceil work.
                        (half_diff.ceil(), half_diff.floor())
                    } else {
                        // The baseline is lower than it should be, so we
                        // add the extra to the bottom, or vice versa for
                        // negative diffs.
                        (half_diff.floor(), half_diff.ceil())
                    };

                    // The cell baseline and face_y values are relative to the
                    // bottom of the cell so we add the bottom diff to them.
                    add_float_to_int(&mut self.cell_baseline, diff_bottom);
                    self.face_y += diff_bottom;

                    // These are all relative to the top of the cell.
                    add_float_to_int(&mut self.underline_position, diff_top);
                    add_float_to_int(&mut self.strikethrough_position, diff_top);
                    self.overline_position = self.overline_position.saturating_add(diff_top as i32);
                }
                Key::IconHeight => {
                    self.icon_height = modifier.apply_f64(self.icon_height);
                    self.icon_height_single = modifier.apply_f64(self.icon_height_single);
                }
                Key::CellBaseline => self.cell_baseline = modifier.apply_u32(self.cell_baseline),
                Key::UnderlinePosition => {
                    self.underline_position = modifier.apply_u32(self.underline_position);
                }
                Key::UnderlineThickness => {
                    self.underline_thickness = modifier.apply_u32(self.underline_thickness);
                }
                Key::StrikethroughPosition => {
                    self.strikethrough_position = modifier.apply_u32(self.strikethrough_position);
                }
                Key::StrikethroughThickness => {
                    self.strikethrough_thickness = modifier.apply_u32(self.strikethrough_thickness);
                }
                Key::OverlinePosition => {
                    self.overline_position = modifier.apply_i32(self.overline_position);
                }
                Key::OverlineThickness => {
                    self.overline_thickness = modifier.apply_u32(self.overline_thickness);
                }
                Key::BoxThickness => self.box_thickness = modifier.apply_u32(self.box_thickness),
                Key::CursorThickness => {
                    self.cursor_thickness = modifier.apply_u32(self.cursor_thickness);
                }
                Key::CursorHeight => self.cursor_height = modifier.apply_u32(self.cursor_height),
                Key::IconHeightSingle => {
                    self.icon_height_single = modifier.apply_f64(self.icon_height_single);
                }
                Key::FaceWidth => self.face_width = modifier.apply_f64(self.face_width),
                Key::FaceHeight => self.face_height = modifier.apply_f64(self.face_height),
                Key::FaceY => self.face_y = modifier.apply_f64(self.face_y),
            }
        }

        // Prevent modifiers from pushing metrics out of their allowable range.
        self.clamp();
    }

    /// Clamp all metrics to their allowable range.
    fn clamp(&mut self) {
        self.cell_width = self.cell_width.max(minimums::CELL_WIDTH);
        self.cell_height = self.cell_height.max(minimums::CELL_HEIGHT);
        self.underline_thickness = self.underline_thickness.max(minimums::UNDERLINE_THICKNESS);
        self.strikethrough_thickness = self
            .strikethrough_thickness
            .max(minimums::STRIKETHROUGH_THICKNESS);
        self.overline_thickness = self.overline_thickness.max(minimums::OVERLINE_THICKNESS);
        self.box_thickness = self.box_thickness.max(minimums::BOX_THICKNESS);
        self.cursor_thickness = self.cursor_thickness.max(minimums::CURSOR_THICKNESS);
        self.cursor_height = self.cursor_height.max(minimums::CURSOR_HEIGHT);
        self.icon_height = self.icon_height.max(minimums::ICON_HEIGHT);
        self.icon_height_single = self.icon_height_single.max(minimums::ICON_HEIGHT_SINGLE);
        self.face_height = self.face_height.max(minimums::FACE_HEIGHT);
        self.face_width = self.face_width.max(minimums::FACE_WIDTH);
    }
}

/// Helper function for adding an f64 to a u32.
///
/// Performs saturating addition or subtraction
/// depending on the sign of the provided float.
///
/// The f64 is asserted to have an integer value.
#[inline]
fn add_float_to_int(int: &mut u32, float: f64) {
    debug_assert!((float.floor() - float).abs() < f64::EPSILON);
    *int = if float >= 0.0 {
        int.saturating_add(float as u32)
    } else {
        int.saturating_sub((-float) as u32)
    };
}

/// A set of modifiers to apply to metrics. We use a hash map because
/// we expect most metrics to be unmodified and want to take up as
/// little space as possible.
pub type ModifierSet = HashMap<Key, Modifier>;

/// A modifier to apply to a metrics value (canonical definition in
/// `ghostty-config`, which parses and formats the `adjust-*` options).
pub use ghostty_config::metric_modifier::MetricModifier as Modifier;

/// Key is an enum of all the available metrics keys (every u32/i32/f64
/// field of [`Metrics`]).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub enum Key {
    CellWidth,
    CellHeight,
    CellBaseline,
    UnderlinePosition,
    UnderlineThickness,
    StrikethroughPosition,
    StrikethroughThickness,
    OverlinePosition,
    OverlineThickness,
    BoxThickness,
    CursorThickness,
    CursorHeight,
    IconHeight,
    IconHeightSingle,
    FaceWidth,
    FaceHeight,
    FaceY,
}

impl Key {
    /// Every key, in declaration order.
    pub const ALL: [Key; 17] = [
        Key::CellWidth,
        Key::CellHeight,
        Key::CellBaseline,
        Key::UnderlinePosition,
        Key::UnderlineThickness,
        Key::StrikethroughPosition,
        Key::StrikethroughThickness,
        Key::OverlinePosition,
        Key::OverlineThickness,
        Key::BoxThickness,
        Key::CursorThickness,
        Key::CursorHeight,
        Key::IconHeight,
        Key::IconHeightSingle,
        Key::FaceWidth,
        Key::FaceHeight,
        Key::FaceY,
    ];

    /// The snake_case field name, matching the Zig `@tagName`.
    pub fn name(&self) -> &'static str {
        match self {
            Key::CellWidth => "cell_width",
            Key::CellHeight => "cell_height",
            Key::CellBaseline => "cell_baseline",
            Key::UnderlinePosition => "underline_position",
            Key::UnderlineThickness => "underline_thickness",
            Key::StrikethroughPosition => "strikethrough_position",
            Key::StrikethroughThickness => "strikethrough_thickness",
            Key::OverlinePosition => "overline_position",
            Key::OverlineThickness => "overline_thickness",
            Key::BoxThickness => "box_thickness",
            Key::CursorThickness => "cursor_thickness",
            Key::CursorHeight => "cursor_height",
            Key::IconHeight => "icon_height",
            Key::IconHeightSingle => "icon_height_single",
            Key::FaceWidth => "face_width",
            Key::FaceHeight => "face_height",
            Key::FaceY => "face_y",
        }
    }
}

impl FromStr for Key {
    type Err = ();
    fn from_str(s: &str) -> Result<Self, ()> {
        Ok(match s {
            "cell_width" => Key::CellWidth,
            "cell_height" => Key::CellHeight,
            "cell_baseline" => Key::CellBaseline,
            "underline_position" => Key::UnderlinePosition,
            "underline_thickness" => Key::UnderlineThickness,
            "strikethrough_position" => Key::StrikethroughPosition,
            "strikethrough_thickness" => Key::StrikethroughThickness,
            "overline_position" => Key::OverlinePosition,
            "overline_thickness" => Key::OverlineThickness,
            "box_thickness" => Key::BoxThickness,
            "cursor_thickness" => Key::CursorThickness,
            "cursor_height" => Key::CursorHeight,
            "icon_height" => Key::IconHeight,
            "icon_height_single" => Key::IconHeightSingle,
            "face_width" => Key::FaceWidth,
            "face_height" => Key::FaceHeight,
            "face_y" => Key::FaceY,
            _ => return Err(()),
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn metrics_apply_modifiers() {
        let mut set = ModifierSet::new();
        set.insert(Key::CellWidth, Modifier::Percent(1.2));

        let mut m = Metrics::init();
        m.cell_width = 100;
        m.apply(&set);
        assert_eq!(120, m.cell_width);
    }

    #[test]
    fn metrics_adjust_cell_height_smaller() {
        let mut set = ModifierSet::new();
        // We choose numbers such that the subtracted number of pixels is odd,
        // as that's the case that could most easily have off-by-one errors.
        // Here we're removing 25 pixels: 13 on the bottom, 12 on top, split
        // that way because we're simulating a face that's 0.33px higher than
        // it "should" be (due to rounding).
        set.insert(Key::CellHeight, Modifier::Percent(0.75));

        let mut m = Metrics::init();
        m.face_y = 0.33;
        m.cell_baseline = 50;
        m.underline_position = 55;
        m.strikethrough_position = 30;
        m.overline_position = 0;
        m.cell_height = 100;
        m.face_height = 99.67;
        m.cursor_height = 100;
        m.apply(&set);
        assert_eq!(-12.67, m.face_y);
        assert_eq!(75, m.cell_height);
        assert_eq!(37, m.cell_baseline);
        assert_eq!(43, m.underline_position);
        assert_eq!(18, m.strikethrough_position);
        assert_eq!(-12, m.overline_position);
        // Cursor height is separate from cell height and does not follow it.
        assert_eq!(100, m.cursor_height);
    }

    #[test]
    fn metrics_adjust_cell_height_larger() {
        let mut set = ModifierSet::new();
        // We choose numbers such that the added number of pixels is odd,
        // as that's the case that could most easily have off-by-one errors.
        // Here we're adding 75 pixels: 37 on the bottom, 38 on top, split
        // that way because we're simulating a face that's 0.33px higher
        // than it "should" be (due to rounding).
        set.insert(Key::CellHeight, Modifier::Percent(1.75));

        let mut m = Metrics::init();
        m.face_y = 0.33;
        m.cell_baseline = 50;
        m.underline_position = 55;
        m.strikethrough_position = 30;
        m.overline_position = 0;
        m.cell_height = 100;
        m.face_height = 99.67;
        m.cursor_height = 100;
        m.apply(&set);
        assert_eq!(37.33, m.face_y);
        assert_eq!(175, m.cell_height);
        assert_eq!(87, m.cell_baseline);
        assert_eq!(93, m.underline_position);
        assert_eq!(68, m.strikethrough_position);
        assert_eq!(38, m.overline_position);
        // Cursor height is separate from cell height and does not follow it.
        assert_eq!(100, m.cursor_height);
    }

    #[test]
    fn metrics_adjust_icon_height_by_percentage() {
        let mut set = ModifierSet::new();
        set.insert(Key::IconHeight, Modifier::Percent(0.75));

        let mut m = Metrics::init();
        m.icon_height = 100.0;
        m.icon_height_single = 80.0;
        m.face_height = 100.0;
        m.face_y = 1.0;
        m.apply(&set);
        assert_eq!(75.0, m.icon_height);
        assert_eq!(60.0, m.icon_height_single);
        // Face metrics not affected
        assert_eq!(100.0, m.face_height);
        assert_eq!(1.0, m.face_y);
    }

    #[test]
    fn metrics_adjust_icon_height_by_absolute_pixels() {
        let mut set = ModifierSet::new();
        set.insert(Key::IconHeight, Modifier::Absolute(-5));

        let mut m = Metrics::init();
        m.icon_height = 100.0;
        m.icon_height_single = 80.0;
        m.face_height = 100.0;
        m.face_y = 1.0;
        m.apply(&set);
        assert_eq!(95.0, m.icon_height);
        assert_eq!(75.0, m.icon_height_single);
        // Face metrics not affected
        assert_eq!(100.0, m.face_height);
        assert_eq!(1.0, m.face_y);
    }

    #[test]
    fn modifier_parse_absolute() {
        assert_eq!(Modifier::Absolute(100), Modifier::parse("100").unwrap());
        assert_eq!(Modifier::Absolute(-100), Modifier::parse("-100").unwrap());
    }

    #[test]
    fn modifier_parse_percent() {
        assert_eq!(Modifier::Percent(1.2), Modifier::parse("20%").unwrap());
        assert_eq!(Modifier::Percent(0.8), Modifier::parse("-20%").unwrap());
        assert_eq!(Modifier::Percent(1.0), Modifier::parse("0%").unwrap());
    }

    #[test]
    fn modifier_parse_errors() {
        assert!(Modifier::parse("").is_err());
        assert!(Modifier::parse("abc").is_err());
        assert!(Modifier::parse("%").is_err());
        assert_eq!(Ok(Modifier::Percent(0.0)), Modifier::parse("-150%"));
        assert!(Modifier::parse_cli_value(None).is_err());
    }

    #[test]
    fn modifier_percent() {
        assert_eq!(80, Modifier::Percent(0.8).apply_u32(100));
        assert_eq!(180, Modifier::Percent(1.8).apply_u32(100));
    }

    #[test]
    fn modifier_absolute() {
        assert_eq!(0, Modifier::Absolute(-100).apply_u32(100));
        assert_eq!(0, Modifier::Absolute(-120).apply_u32(100));
        assert_eq!(200, Modifier::Absolute(100).apply_u32(100));
    }

    #[test]
    fn key_names_roundtrip() {
        for k in [Key::CellWidth, Key::FaceY, Key::IconHeightSingle] {
            assert_eq!(Ok(k), k.name().parse::<Key>());
        }
    }
}