239 lines
9.9 KiB
Rust
239 lines
9.9 KiB
Rust
//! Fonts carried by exported documents (SVG `@font-face`): what a font's licence allows (its
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//! OS/2 `fsType` flags) and font files holding only the characters a document uses.
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use std::borrow::Cow;
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use skrifa::MetadataProvider;
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use skrifa::raw::TableProvider;
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use skrifa::raw::types::Tag;
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use crate::FontFace;
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/// What a font's licence lets a document do with it (OS/2 `fsType`).
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#[derive(Clone, Copy, Debug, PartialEq, Eq)]
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pub enum Embedding {
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/// The font may be embedded, as a subset.
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Subset,
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/// The font may be embedded whole only (no subsetting).
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Whole,
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/// Restricted licence, or bitmap embedding only: the outlines may not be embedded.
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Forbidden,
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}
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impl Embedding {
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/// The permission an OS/2 `fsType` value grants. Bits 0–3 are the usage permissions (none:
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/// installable; 2: restricted; 4: preview and print; 8: editable), the most permissive one
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/// winning when several are set; bit 8 forbids subsetting and bit 9 allows bitmaps only.
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pub fn from_fs_type(fs: u16) -> Self {
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let restricted = fs & 0x0002 != 0 && fs & 0x000c == 0;
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if restricted || fs & 0x0200 != 0 {
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Self::Forbidden
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} else if fs & 0x0100 != 0 {
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Self::Whole
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} else {
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Self::Subset
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}
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}
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}
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/// A font file a document carries.
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#[derive(Clone, Debug)]
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pub struct EmbeddedFont {
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/// An OpenType file of the one face (`cmap`, `OS/2` and the other tables a viewer needs).
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pub data: Vec<u8>,
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/// CFF outlines (`font/otf`) rather than TrueType ones (`font/ttf`).
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pub cff: bool,
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/// Only the glyphs of the characters asked for; false: the whole face, as its licence asks
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/// (or when it can't be subset).
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pub subset: bool,
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}
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const CMAP: Tag = Tag::new(b"cmap");
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const OS2: Tag = Tag::new(b"OS/2");
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const HEAD: Tag = Tag::new(b"head");
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impl FontFace {
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/// What the face's licence allows (a face without an `OS/2` table states no restriction).
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pub fn embedding(&self) -> Embedding {
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self.skrifa().and_then(|f| f.os2().ok()).map_or(Embedding::Subset, |os2| Embedding::from_fs_type(os2.fs_type()))
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}
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/// The face as a font file to embed: the glyphs of `chars` alone when its licence allows
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/// subsetting, else the whole face. Ligatures, kerning and other layout features are left out
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/// of subsets (viewers draw each character's own glyph). `None` when the licence forbids
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/// embedding or the font can't be read.
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pub fn embed(&self, chars: &[char]) -> Option<EmbeddedFont> {
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let font = self.skrifa()?;
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let cff = [b"CFF ", b"CFF2"].iter().any(|t| font.table_data(Tag::new(t)).is_some());
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let whole = || Some(EmbeddedFont { data: sfnt(font.table_directory.sfnt_version(), tables(&font))?, cff, subset: false });
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match self.embedding() {
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Embedding::Forbidden => None,
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Embedding::Whole => whole(),
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Embedding::Subset => match subset(self, &font, chars) {
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Some(data) => Some(EmbeddedFont { data, cff, subset: true }),
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// Outlines the subsetter can't read (CFF2): what the licence allows anyway.
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None => whole(),
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},
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}
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}
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}
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/// Every table of `font`, by tag.
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fn tables<'a>(font: &skrifa::FontRef<'a>) -> Vec<(Tag, Cow<'a, [u8]>)> {
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font.table_directory.table_records().iter().filter_map(|r| font.table_data(r.tag()).map(|d| (r.tag(), Cow::Borrowed(d.as_bytes())))).collect()
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}
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/// `face` reduced to the glyphs of `chars`, with a `cmap` mapping them and the face's `OS/2`.
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fn subset(face: &FontFace, font: &skrifa::FontRef<'_>, chars: &[char]) -> Option<Vec<u8>> {
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let charmap = font.charmap();
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let mut remapper = subsetter::GlyphRemapper::new();
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let mut map: Vec<(u32, u16)> = chars
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.iter()
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.filter_map(|c| {
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let gid = u16::try_from(charmap.map(*c)?.to_u32()).ok().filter(|g| *g != 0)?;
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Some((*c as u32, remapper.remap(gid)))
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})
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.collect();
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map.sort_unstable();
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map.dedup_by_key(|m| m.0);
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let data = subsetter::subset(face.data(), face.index(), &remapper).ok()?;
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let sub = skrifa::FontRef::new(&data).ok()?;
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let mut t: Vec<(Tag, Cow<'_, [u8]>)> = tables(&sub).into_iter().filter(|(tag, _)| *tag != CMAP && *tag != OS2).collect();
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t.push((CMAP, Cow::Owned(cmap(&map)?)));
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if let Some(os2) = font.table_data(OS2) {
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t.push((OS2, Cow::Owned(os2_for(os2.as_bytes(), &map))));
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}
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sfnt(sub.table_directory.sfnt_version(), t)
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}
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/// The face's `OS/2` table with its first and last character fitted to the subset's.
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fn os2_for(os2: &[u8], map: &[(u32, u16)]) -> Vec<u8> {
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let mut out = os2.to_vec();
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let bmp = |c: u32| u16::try_from(c).unwrap_or(u16::MAX).to_be_bytes();
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if let (Some(first), Some(last), Some(slot)) = (map.first(), map.last(), out.get_mut(64..68)) {
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slot[..2].copy_from_slice(&bmp(first.0));
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slot[2..].copy_from_slice(&bmp(last.0));
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}
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out
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}
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/// Runs of consecutive characters mapped to consecutive glyphs: (first char, last char, first
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/// glyph).
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fn runs(map: &[(u32, u16)]) -> Vec<(u32, u32, u16)> {
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let mut out: Vec<(u32, u32, u16)> = Vec::new();
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for &(c, g) in map {
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match out.last_mut() {
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Some((start, end, g0)) if c == *end + 1 && u32::from(g) == u32::from(*g0) + (c - *start) => *end = c,
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_ => out.push((c, c, g)),
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}
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}
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out
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}
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/// A `cmap` table mapping `map`'s characters (sorted, unique) to their glyphs: a format 4
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/// subtable for the Basic Multilingual Plane and, when characters lie beyond it (or it would be
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/// too long), a format 12 one for all of them. `None` when there is nothing to map.
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pub(crate) fn cmap(map: &[(u32, u16)]) -> Option<Vec<u8>> {
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let all = runs(map);
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let last = all.last()?.1;
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let mut bmp: Vec<(u32, u32, u16)> = runs(&map.iter().copied().filter(|(c, _)| *c < 0xffff).collect::<Vec<_>>());
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// The closing segment every format 4 subtable ends with.
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bmp.push((0xffff, 0xffff, 0));
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let f4_len = 16 + 8 * bmp.len();
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let f4 = f4_len <= usize::from(u16::MAX);
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let f12 = !f4 || last >= 0xffff;
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let mut subtables: Vec<(u16, Vec<u8>)> = Vec::new();
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if f4 {
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let seg_x2 = u16::try_from(bmp.len() * 2).ok()?;
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let sel = bmp.len().ilog2() as u16;
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let range = 2u16.checked_pow(u32::from(sel))?.checked_mul(2)?;
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let mut s = Vec::with_capacity(f4_len);
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for v in [4, u16::try_from(f4_len).ok()?, 0, seg_x2, range, sel, seg_x2 - range] {
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s.extend(v.to_be_bytes());
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}
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let u = |c: u32| u16::try_from(c).unwrap_or(u16::MAX);
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s.extend(bmp.iter().flat_map(|(_, end, _)| u(*end).to_be_bytes()));
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s.extend(0u16.to_be_bytes());
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s.extend(bmp.iter().flat_map(|(start, _, _)| u(*start).to_be_bytes()));
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// The closing segment maps to glyph 0: 0xffff + 1.
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s.extend(bmp.iter().flat_map(|(start, end, g)| if *end == 0xffff { 1u16 } else { g.wrapping_sub(u(*start)) }.to_be_bytes()));
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s.extend(bmp.iter().flat_map(|_| 0u16.to_be_bytes()));
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subtables.push((1, s));
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}
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if f12 {
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let n = u32::try_from(all.len()).ok()?;
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let mut s = Vec::with_capacity(16 + 12 * all.len());
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s.extend(12u16.to_be_bytes());
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s.extend(0u16.to_be_bytes());
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for v in [16 + 12 * n, 0, n] {
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s.extend(v.to_be_bytes());
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}
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for (start, end, g) in &all {
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for v in [*start, *end, u32::from(*g)] {
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s.extend(v.to_be_bytes());
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}
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}
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subtables.push((10, s));
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}
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// Header and encoding records (Windows platform: 1 = BMP, 10 = full repertoire).
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let mut out = Vec::new();
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out.extend(0u16.to_be_bytes());
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out.extend(u16::try_from(subtables.len()).ok()?.to_be_bytes());
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let mut offset = 4 + 8 * subtables.len();
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for (encoding, s) in &subtables {
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out.extend(3u16.to_be_bytes());
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out.extend(encoding.to_be_bytes());
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out.extend(u32::try_from(offset).ok()?.to_be_bytes());
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offset += s.len();
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}
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for (_, s) in subtables {
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out.extend(s);
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}
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Some(out)
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}
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/// The sum of `data` as big-endian 32-bit words (zero-padded), as table directories record it.
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fn checksum(data: &[u8]) -> u32 {
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let (words, rest) = data.as_chunks::<4>();
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let mut last = [0u8; 4];
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last[..rest.len()].copy_from_slice(rest);
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words.iter().chain([&last]).fold(0u32, |s, w| s.wrapping_add(u32::from_be_bytes(*w)))
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}
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/// An OpenType file of `tables` (unique tags) with checksums and the `head` checksum adjustment.
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fn sfnt(version: u32, mut tables: Vec<(Tag, Cow<'_, [u8]>)>) -> Option<Vec<u8>> {
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tables.sort_by_key(|t| t.0);
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tables.dedup_by_key(|t| t.0);
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let n = u16::try_from(tables.len()).ok().filter(|n| *n > 0)?;
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let sel = n.ilog2() as u16;
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let range = 2u16.checked_pow(u32::from(sel))?.checked_mul(16)?;
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let mut out = Vec::with_capacity(12 + 16 * tables.len() + tables.iter().map(|t| t.1.len() + 3).sum::<usize>());
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out.extend(version.to_be_bytes());
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for v in [n, range, sel, n.checked_mul(16)? - range] {
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out.extend(v.to_be_bytes());
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}
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let mut offset = 12 + 16 * tables.len();
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let mut head_at = None;
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for (tag, data) in &mut tables {
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if *tag == HEAD {
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// The adjustment is summed as zero; it is set once the whole file is written.
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data.to_mut().get_mut(8..12)?.fill(0);
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head_at = Some(offset + 8);
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}
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out.extend(tag.to_be_bytes());
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out.extend(checksum(data).to_be_bytes());
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out.extend(u32::try_from(offset).ok()?.to_be_bytes());
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out.extend(u32::try_from(data.len()).ok()?.to_be_bytes());
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offset += data.len().next_multiple_of(4);
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}
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for (_, data) in &tables {
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out.extend_from_slice(data);
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out.resize(out.len().next_multiple_of(4), 0);
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}
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if let Some(at) = head_at {
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let adjust = 0xb1b0_afba_u32.wrapping_sub(checksum(&out));
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out.get_mut(at..at + 4)?.copy_from_slice(&adjust.to_be_bytes());
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}
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Some(out)
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}
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