//! `OS/2` — OS/2 and Windows Metrics table.
//!
//! Contains font-wide metrics used for line spacing, weight classification,
//! and typographic alignment.
//! Reference: <https://learn.microsoft.com/en-us/typography/opentype/spec/os2>

use crate::font::parse::Reader;
use crate::font::FontError;

/// Parsed `OS/2` table.
#[derive(Debug)]
pub struct Os2Table {
    /// Table version (0–5).
    pub version: u16,
    /// Average weighted advance width of lowercase letters and space.
    pub x_avg_char_width: i16,
    /// Visual weight class (100–900).
    pub us_weight_class: u16,
    /// Visual width class (1–9).
    pub us_width_class: u16,
    /// Font embedding licensing rights.
    pub fs_type: u16,
    /// Typographic ascender.
    pub s_typo_ascender: i16,
    /// Typographic descender (typically negative).
    pub s_typo_descender: i16,
    /// Typographic line gap.
    pub s_typo_line_gap: i16,
    /// Strikeout stroke size in font design units.
    pub y_strikeout_size: i16,
    /// Strikeout stroke position above baseline.
    pub y_strikeout_position: i16,
    /// Subscript vertical offset.
    pub y_subscript_y_offset: i16,
    /// Superscript vertical offset.
    pub y_superscript_y_offset: i16,
    /// Height of lowercase 'x' (version >= 2, else 0).
    pub sx_height: i16,
    /// Height of uppercase letters (version >= 2, else 0).
    pub s_cap_height: i16,
}

impl Os2Table {
    /// Parse the `OS/2` table from raw bytes.
    pub fn parse(data: &[u8]) -> Result<Os2Table, FontError> {
        let r = Reader::new(data);

        // Minimum OS/2 table is 78 bytes (version 0).
        if r.len() < 78 {
            return Err(FontError::MalformedTable("OS/2: too short"));
        }

        let version = r.u16(0)?;
        let x_avg_char_width = r.i16(2)?;
        let us_weight_class = r.u16(4)?;
        let us_width_class = r.u16(6)?;
        let fs_type = r.u16(8)?;

        // Subscript/superscript offsets: offsets 14 and 18 respectively
        // ySubscriptXSize(10), ySubscriptYSize(12), ySubscriptXOffset(14)
        let y_subscript_y_offset = r.i16(16)?;
        // ySuperscriptXSize(18), ySuperscriptYSize(20), ySuperscriptXOffset(22)
        let y_superscript_y_offset = r.i16(24)?;

        let y_strikeout_size = r.i16(26)?;
        let y_strikeout_position = r.i16(28)?;

        // sTypoAscender is at offset 68, sTypoDescender at 70, sTypoLineGap at 72
        let s_typo_ascender = r.i16(68)?;
        let s_typo_descender = r.i16(70)?;
        let s_typo_line_gap = r.i16(72)?;

        // sxHeight and sCapHeight are available in version >= 2 at offsets 86 and 88.
        let (sx_height, s_cap_height) = if version >= 2 && r.len() >= 96 {
            (r.i16(86)?, r.i16(88)?)
        } else {
            (0, 0)
        };

        Ok(Os2Table {
            version,
            x_avg_char_width,
            us_weight_class,
            us_width_class,
            fs_type,
            s_typo_ascender,
            s_typo_descender,
            s_typo_line_gap,
            y_strikeout_size,
            y_strikeout_position,
            y_subscript_y_offset,
            y_superscript_y_offset,
            sx_height,
            s_cap_height,
        })
    }
}