//! `hmtx` — Horizontal Metrics table.
//!
//! Contains per-glyph horizontal metrics (advance width + left side bearing).
//! Reference: <https://learn.microsoft.com/en-us/typography/opentype/spec/hmtx>

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

/// Parsed `hmtx` table.
///
/// Both `advances` and `lsbs` are indexed by glyph ID and have exactly
/// `num_glyphs` entries.
#[derive(Debug)]
pub struct HmtxTable {
    /// Advance widths for each glyph (in font units).
    pub advances: Vec<u16>,
    /// Left side bearings for each glyph (in font units).
    pub lsbs: Vec<i16>,
}

impl HmtxTable {
    /// Parse the `hmtx` table from raw bytes.
    ///
    /// `num_long_hor_metrics` comes from `hhea`, `num_glyphs` from `maxp`.
    pub fn parse(
        data: &[u8],
        num_long_hor_metrics: u16,
        num_glyphs: u16,
    ) -> Result<HmtxTable, FontError> {
        let r = Reader::new(data);
        let n_long = num_long_hor_metrics as usize;
        let n_glyphs = num_glyphs as usize;

        let mut advances = Vec::with_capacity(n_glyphs);
        let mut lsbs = Vec::with_capacity(n_glyphs);

        // First n_long entries are (advance_width: u16, lsb: i16) pairs.
        for i in 0..n_long {
            let offset = i * 4;
            advances.push(r.u16(offset)?);
            lsbs.push(r.i16(offset + 2)?);
        }

        // Remaining glyphs share the last advance width, but have individual lsbs.
        let last_advance = advances.last().copied().unwrap_or(0);
        let remaining = n_glyphs.saturating_sub(n_long);
        let lsb_offset = n_long * 4;
        for i in 0..remaining {
            advances.push(last_advance);
            lsbs.push(r.i16(lsb_offset + i * 2)?);
        }

        Ok(HmtxTable { advances, lsbs })
    }
}