First commit. Version 0.1.0

src/system_fonts.rs

@@ -0,0 +1,133 @@
+// SPDX-License-Identifier: LGPL-2.1-only
+// Copyright (C) 2026 Liberux Labs, S. L. <info@liberux.net>
+
+//! Lazy per-glyph fallback font resolution.
+//!
+//! The default font ([`crate::theme::fallback::FALLBACK_FONT`], Sora)
+//! covers Latin and a portion of extended Latin; everything outside
+//! that band — Cyrillic, Devanagari, Arabic, Hebrew, Thai, the CJK
+//! ideographic block — relies on a chain of system Noto fonts loaded
+//! on demand: a fallback file is read off disk the first time some
+//! glyph asks for it and the resulting `Arc<Font>` is cached
+//! process-wide.
+//!
+//! Each `(path, face)` entry in [`FALLBACK_FONT_CANDIDATES`] gets its
+//! own `OnceLock<Option<Arc<Font>>>` slot. `Some(font)` means the
+//! file was read and parsed successfully; `None` means it's missing
+//! or unparseable, locked in for the rest of the process so we don't
+//! re-`stat` the same path on every glyph miss. The `OnceLock`
+//! handles concurrent first-loads correctly: at most one thread runs
+//! the closure for a given slot, the rest wait for the result.
+//!
+//! Renderers (`SoftwareCanvas` and `GlesCanvas`) consult this module
+//! through [`lookup`] in their `font_for_char` paths.
+
+use std::sync::{ Arc, OnceLock };
+
+use fontdue::{ Font, FontSettings };
+
+/// Per-script fallback font path with the TrueType-collection face
+/// index fontdue should load (most files are single-face → 0; CJK
+/// `.ttc` archives carry many faces, see the SC face on the canonical
+/// Adobe-built `NotoSansCJK-Regular.ttc`).
+struct FallbackFontSpec
+{
+	path: &'static str,
+	face: u32,
+}
+
+/// Ordered fallback chain consulted on a glyph miss. Order matters:
+/// the first slot whose font owns a non-zero glyph index for the
+/// codepoint wins, so the broadest families come first (Noto Sans
+/// covers Cyrillic, Greek, extended Latin) and the script-specific
+/// + CJK packs trail. DejaVu is the last resort — most distros carry
+/// it under one path or another.
+const FALLBACK_FONT_CANDIDATES: &[ FallbackFontSpec ] =
+&[
+	// Noto Sans — Cyrillic, Greek, extended Latin.
+	FallbackFontSpec { path: "/usr/share/fonts/truetype/noto/NotoSans-Regular.ttf",       face: 0 },
+	FallbackFontSpec { path: "/usr/share/fonts/noto/NotoSans-Regular.ttf",                face: 0 },
+
+	// Devanagari (Hindi).
+	FallbackFontSpec { path: "/usr/share/fonts/truetype/noto/NotoSansDevanagari-Regular.ttf", face: 0 },
+	FallbackFontSpec { path: "/usr/share/fonts/noto/NotoSansDevanagari-Regular.ttf",          face: 0 },
+
+	// Arabic, Hebrew, Thai — common scripts cheap to keep.
+	FallbackFontSpec { path: "/usr/share/fonts/truetype/noto/NotoSansArabic-Regular.ttf",  face: 0 },
+	FallbackFontSpec { path: "/usr/share/fonts/truetype/noto/NotoSansHebrew-Regular.ttf",  face: 0 },
+	FallbackFontSpec { path: "/usr/share/fonts/truetype/noto/NotoSansThai-Regular.ttf",    face: 0 },
+
+	// CJK packs ship as `.ttc`. The collection layout on the canonical
+	// Adobe-built `NotoSansCJK-Regular.ttc` is 0=JP, 1=KR, 2=SC, 3=TC,
+	// 4=HK; we want CJK shared ideographs available in any locale, so
+	// any of the faces is fine — pick SC (face 2) as the broadest
+	// baseline. ~30 MB on disk; under the lazy loader this only fires
+	// when a user-visible string actually contains a CJK codepoint.
+	FallbackFontSpec { path: "/usr/share/fonts/opentype/noto/NotoSansCJK-Regular.ttc",     face: 2 },
+	FallbackFontSpec { path: "/usr/share/fonts/opentype/noto-cjk/NotoSansCJK-Regular.ttc", face: 2 },
+	FallbackFontSpec { path: "/usr/share/fonts/noto-cjk/NotoSansCJK-Regular.ttc",          face: 2 },
+
+	// Last resort — DejaVu has broad-but-shallow coverage of most
+	// scripts and ships almost everywhere.
+	FallbackFontSpec { path: "/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf",            face: 0 },
+];
+
+/// Per-slot lazy state. `Vec` length matches
+/// [`FALLBACK_FONT_CANDIDATES`]; each `OnceLock` resolves
+/// independently so the act of looking up a Devanagari codepoint
+/// doesn't drag the CJK pack into memory. `None` inside a resolved
+/// slot means the file was missing or fontdue rejected it — a sticky
+/// negative result so subsequent misses skip the slot in O(1).
+fn slots() -> &'static [ OnceLock<Option<Arc<Font>>> ]
+{
+	static SLOTS: OnceLock<Vec<OnceLock<Option<Arc<Font>>>>> = OnceLock::new();
+	SLOTS.get_or_init( ||
+	{
+		( 0..FALLBACK_FONT_CANDIDATES.len() )
+			.map( |_| OnceLock::new() )
+			.collect()
+	} )
+}
+
+/// Try to load and parse the fallback font at slot `idx`. Each
+/// `OnceLock` wraps `Option<Arc<Font>>` so a missing or malformed
+/// file is recorded as `None` and never re-attempted.
+fn slot_font( idx: usize ) -> Option<Arc<Font>>
+{
+	let slot = &slots()[ idx ];
+	slot.get_or_init( ||
+	{
+		let spec  = &FALLBACK_FONT_CANDIDATES[ idx ];
+		let bytes = std::fs::read( spec.path ).ok()?;
+		let opts  = FontSettings { collection_index: spec.face, ..FontSettings::default() };
+		Font::from_bytes( bytes.as_slice(), opts ).ok().map( Arc::new )
+	} )
+		.as_ref()
+		.map( Arc::clone )
+}
+
+/// Find the first fallback font that has a non-zero glyph index for
+/// `ch`, loading it from disk on the first hit and caching the
+/// `Arc<Font>` for the rest of the process. Returns `None` if no
+/// installed fallback covers the codepoint — the caller then paints
+/// the primary font's `.notdef` rather than dropping the glyph.
+///
+/// Side effect: walking the chain may load and cache a slot even if
+/// it doesn't end up covering `ch` (`lookup_glyph_index` reads the
+/// `cmap` table, which requires the font to be parsed). That's
+/// acceptable — the slot is cached on the first encounter regardless,
+/// and most coverage gaps in early slots are the small Noto Sans
+/// scripts (Devanagari, Arabic, …) whose total weight is a fraction
+/// of the CJK pack everyone was paying for unconditionally.
+pub fn lookup( ch: char ) -> Option<Arc<Font>>
+{
+	for idx in 0..FALLBACK_FONT_CANDIDATES.len()
+	{
+		let Some( font ) = slot_font( idx ) else { continue };
+		if font.lookup_glyph_index( ch ) != 0
+		{
+			return Some( font );
+		}
+	}
+	None
+}