nigig-org/crates/apps/pdf/pdf-graphics/tests/subset_real_font.rs

//! Subsetting a real TrueType font.
//!
//! The unit tests in `subset.rs` cover refusals and edge cases with
//! hand-built table stubs. They cannot catch the failure that actually
//! happened: a subset that is *structurally valid* — correct table
//! directory, checksums, every glyph decoding — and whose glyphs are in the
//! wrong place. Writing a zero left side bearing shifted every outline
//! horizontally, and `hmtx` was indexed by new glyph id against the
//! original font, so glyphs took other glyphs' metrics.
//!
//! So these tests assert **geometry against the source font**, not that the
//! subset parses.
//!
//! Skipped when the system font is absent rather than failing, because the
//! font is a property of the machine and not of this crate. CI installs
//! `fonts-dejavu-core`.
//!
//! See `REVIEWS/adr/0019-pdf-document-creation.md`.

use std::collections::BTreeSet;

use nigig_pdf_graphics::sfnt::SfntFont;
use nigig_pdf_graphics::subset::{glyph_index, subset_truetype, SubsetError};

const FONT: &str = "/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf";

fn font_bytes() -> Option<Vec<u8>> {
    std::fs::read(FONT).ok()
}

/// Read a glyph's advance and left side bearing straight from a font's
/// `hmtx`, independently of the subsetter.
fn metrics_of(data: &[u8], gid: u16) -> (u16, i16) {
    let font = SfntFont::parse(data).expect("parses");
    let metrics = font.metrics().expect("metrics");
    (
        font.advance(gid, &metrics).expect("advance"),
        font.left_side_bearing(gid, &metrics).expect("lsb"),
    )
}

#[test]
fn a_subset_keeps_only_the_glyphs_asked_for() {
    let Some(data) = font_bytes() else { return };
    let chars: BTreeSet<char> = "Hello".chars().collect();
    let subset = subset_truetype(&data, &chars).expect("subsets");

    // H, e, l, o plus .notdef. No composites are involved here, so the
    // count is exact.
    assert_eq!(
        subset.glyph_map.len(),
        5,
        "expected .notdef plus four distinct letters, got {:?}",
        subset.to_unicode
    );
    assert!(
        subset.data.len() < data.len() / 50,
        "a five-glyph subset of a 750 KB font should be tiny, got {} bytes",
        subset.data.len()
    );
    // .notdef must survive: a TrueType font without glyph 0 is invalid.
    assert_eq!(subset.glyph_map.get(&0), Some(&0));
}

#[test]
fn every_subset_glyph_keeps_its_advance_and_bearing() {
    // This is the regression. The outlines were intact and every letter
    // was drawn shifted left by its own left side bearing, because the
    // rebuilt hmtx wrote a zero bearing and read advances by the new glyph
    // id from the *original* font.
    let Some(data) = font_bytes() else { return };
    let chars: BTreeSet<char> = "HWdelor,!".chars().collect();
    let subset = subset_truetype(&data, &chars).expect("subsets");

    for (&old_gid, &new_gid) in &subset.glyph_map {
        let expected = metrics_of(&data, old_gid);
        let actual = metrics_of(&subset.data, new_gid);
        assert_eq!(
            expected, actual,
            "glyph {old_gid} -> {new_gid}: expected (advance, lsb) {expected:?}, got {actual:?}"
        );
    }
}

#[test]
fn a_composite_glyph_brings_its_components_with_it() {
    // "é" is a composite of "e" and an accent. Subsetting without
    // following the reference leaves a glyph that renders as nothing.
    let Some(data) = font_bytes() else { return };
    let mut chars = BTreeSet::new();
    chars.insert('é');
    let subset = subset_truetype(&data, &chars).expect("subsets");

    assert!(
        subset.glyph_map.len() > 2,
        "é is composite, so its components must be kept too; got {:?}",
        subset.glyph_map
    );

    // Each component named inside the subset must exist in the subset.
    let font = SfntFont::parse(&subset.data).expect("subset parses");
    let maxp = font.table(b"maxp").expect("maxp");
    let num_glyphs = u16::from_be_bytes([maxp[4], maxp[5]]);
    assert_eq!(num_glyphs as usize, subset.glyph_map.len());
}

#[test]
fn widths_are_in_pdf_thousandths_and_match_the_source() {
    let Some(data) = font_bytes() else { return };
    let chars: BTreeSet<char> = "Hi".chars().collect();
    let subset = subset_truetype(&data, &chars).expect("subsets");

    let source = SfntFont::parse(&data).expect("parses");
    let metrics = source.metrics().expect("metrics");
    let scale = 1000.0 / metrics.units_per_em as f64;

    for (&old_gid, &new_gid) in &subset.glyph_map {
        let expected = source.advance(old_gid, &metrics).expect("advance") as f64 * scale;
        let actual = subset.widths[new_gid as usize];
        assert!(
            (expected - actual).abs() < 0.05,
            "glyph {new_gid}: expected width {expected:.2}, got {actual:.2}"
        );
    }
    // A width of zero for a visible letter is the silent-empty failure.
    let h = subset.glyph_for('H').expect("H is in the subset");
    assert!(
        subset.widths[h as usize] > 100.0,
        "a capital H cannot be {} units wide",
        subset.widths[h as usize]
    );
}

#[test]
fn identity_h_encoding_is_two_bytes_per_glyph() {
    let Some(data) = font_bytes() else { return };
    let chars: BTreeSet<char> = "AB".chars().collect();
    let subset = subset_truetype(&data, &chars).expect("subsets");

    let encoded = subset.encode("AB");
    assert_eq!(encoded.len(), 4, "two glyphs, two bytes each");

    let a = subset.glyph_for('A').expect("A present");
    let b = subset.glyph_for('B').expect("B present");
    assert_eq!(
        encoded,
        vec![(a >> 8) as u8, a as u8, (b >> 8) as u8, b as u8]
    );
}

#[test]
fn a_character_outside_the_subset_is_dropped_not_rendered_as_notdef() {
    let Some(data) = font_bytes() else { return };
    let chars: BTreeSet<char> = "A".chars().collect();
    let subset = subset_truetype(&data, &chars).expect("subsets");

    assert!(subset.glyph_for('Z').is_none());
    // "AZA" keeps only the two A's rather than emitting .notdef boxes.
    assert_eq!(subset.encode("AZA").len(), 4);
}

#[test]
fn text_width_sums_the_glyph_advances() {
    let Some(data) = font_bytes() else { return };
    let chars: BTreeSet<char> = "il".chars().collect();
    let subset = subset_truetype(&data, &chars).expect("subsets");

    let one = subset.text_width("i", 12.0);
    let two = subset.text_width("ii", 12.0);
    assert!(one > 0.0, "a glyph must have a width");
    assert!(
        (two - one * 2.0).abs() < 1e-9,
        "two of the same glyph must be twice as wide"
    );
    // Scaling with the point size.
    assert!((subset.text_width("i", 24.0) - one * 2.0).abs() < 1e-9);
}

#[test]
fn the_subset_is_byte_identical_between_runs() {
    // A generated PDF that differs run to run cannot be diffed in review
    // or cached. The subset tag is derived from the glyph set for exactly
    // this reason.
    let Some(data) = font_bytes() else { return };
    let chars: BTreeSet<char> = "Reproducible".chars().collect();
    let first = subset_truetype(&data, &chars).expect("subsets");
    let second = subset_truetype(&data, &chars).expect("subsets");
    assert_eq!(first.data, second.data);
}

#[test]
fn glyph_lookup_finds_a_known_character() {
    let Some(data) = font_bytes() else { return };
    let font = SfntFont::parse(&data).expect("parses");
    // Assert against a value read from the font, not a hardcoded id.
    let a = glyph_index(&font, 'A').expect("A must be in DejaVu Sans");
    let b = glyph_index(&font, 'B').expect("B must be in DejaVu Sans");
    assert_ne!(a, 0, "A must not map to .notdef");
    assert_ne!(a, b, "different characters need different glyphs");
    // A character DejaVu Sans does not contain.
    assert!(glyph_index(&font, '\u{10FFFF}').is_none());
}

#[test]
fn subsetting_a_cff_font_is_refused_rather_than_silently_wrong() {
    // Nothing on this machine is guaranteed to be CFF, so this drives the
    // refusal through the public entry point with a synthetic OTTO header.
    let mut data = Vec::new();
    data.extend_from_slice(b"OTTO");
    data.extend_from_slice(&1u16.to_be_bytes());
    data.extend_from_slice(&[0; 6]);
    data.extend_from_slice(b"CFF ");
    data.extend_from_slice(&0u32.to_be_bytes());
    data.extend_from_slice(&28u32.to_be_bytes());
    data.extend_from_slice(&4u32.to_be_bytes());
    data.extend_from_slice(&[0; 4]);
    assert_eq!(
        subset_truetype(&data, &BTreeSet::new()).unwrap_err(),
        SubsetError::CffNotSupported
    );
}