[andiwand]

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Refs #764.

Summary

This adds decision 7 to docs/design/lazy-loading.md, a proposal for level 3 of a pdf.

Measured on Core_v5.1.pdf (2,985 pages):

step	time	memory
core: parse	0.5 s
core: whole-document view	4.9 s	761 MB peak
core: one page view	5–37 ms
browser: first frame at level 1	1.8 s, nearly all html parsing	2.0 GB
document.html	127 MB: 5.4 MB head (2.3 MB fonts), 122 MB pages

For a server, the time is in core. For an offline file, core works once and the reader waits for the browser.

• Option A, the page side: as the sheets do in feat(html): load the later chunks of a sheet on demand #1255. Keep the full pass, so the head has every class and font, and make the later pages scripts that re-render with the kept registries.
  • The page is about 5.5 MB.
  • Core still takes 4.9 s before the page goes out.
  • It is a small change.
• Option B, a page on its own: render the first page only, and each later page alone. Its cost is in proportion to what the reader sees: about 40 ms for the page.
  • Class names come from a hash of the declaration, so they are the same in every render.
  • Fonts become shared files. Without that, each page carries its fonts, about 0.5 MB per page here. In dual_layer, the default, a font is already the same for every page. Only single_layer must re-encode a font from its whole ToUnicode map.
  • It is a large change to the pdf writer.

The doc recommends B, because the time is in core. A is the smaller step. The browser cost is the same for both.

Review fixes

• Rebased onto main after the squash of feat(html): load the later chunks of a sheet on demand #1255.
• The proposal said that the writer re-encodes each font with the code points of the runs it meets. Only single_layer does that. In dual_layer, the default, reencode_to_pua maps every glyph to a private-use code point, so a font file is already the same for every page. Option B is a little smaller for the default mode.
• The class examples used the prefix x, which the writer does not have. They now use c.