Kern

A Japanese-capable typesetting engine in Common Lisp — box/glue/penalty at the core, JLReq-grade Japanese on top, and a small S-expression document layer you can actually write in.

Kern focuses on the parts most engines skimp on: line-length adjustment (追い込み・追い出し) to JIS X 4051 §4.19, ruby (振り仮名) of every kind, and vertical writing (縦書き) — each checked against LuaLaTeX + jlreq, which reads the very same jfm-jlreq.lua data Kern does, so a disagreement points at the engine, not the data.

(:document (:size 14 :line-width 294)
  (:h1 (:group "組版" "くみはん") "エンジン" (:group "試験" "しけん"))
  (:p "吾輩は" (:ruby "猫" "ねこ") "である。"
      (:ruby "名" "な") (:ruby "前" "まえ") "はまだ" (:ruby "無" "な") "い。")
  (:h2 "ルビの" (:ruby "種" "しゅ") "類")
  (:p "これは" (:group "組版" "くみはん") "エンジンである。"
      (:jukugo "二十" ("に" "じゅう")) "も" (:em "強調") "もできる。"))

read is the parser — a document is just Lisp data. The same document set :direction :vertical typesets top-to-bottom in right-to-left columns, with ruby and emphasis dots moving to the right of each character on their own (the layout is written in terms of "advance along the writing direction", so vertical is a coordinate change, not a rewrite).

horizontal	vertical (:direction :vertical)

What works

• Line breaking: Knuth-Plass with fitness classes and pruning.
• §4.19 line adjustment: the spec's priority-ordered compression and expansion (欧文間 → 中点 → 括弧 → 和欧間, then character gaps), verified glue-by-glue against luatexja. Kern follows the spec's priority ordering where luatexja's default uses plain proportional distribution.
• Kinsoku (禁則) as a UAX #14 + JLReq class-pair table; punctuation spacing (約物・和欧間アキ) from the JFM.
• Ruby: mono / group (even-distributed) / jukugo (two modes) / overhang into adjacent kana with except-kanji and line-boundary suppression — all matched to luatexja numerically and visually.
• Emphasis (圏点) via the ruby mechanism.
• Vertical writing: upright tategaki, right-to-left columns; ruby and emphasis land correctly on the right.
• PDF output through cl-pdf, with font subsetting and /ToUnicode (searchable, copy-pasteable).
• Document layer: :document / :p / :h1 / :h2 / :ruby / :group / :jukugo / :em, first-line indent, and :font — font mixing down to a single character ((:font :gothic "この語だけ")), composing with ruby.

Deferred, and marked as such in DESIGN.md: vertical punctuation corner placement and bracket rotation (needs the vertical JFM), 縦中横, breaking inside a jukugo, and Latin micro-typography (protrusion / hz).

Design

The core (items / linebreak / setglue / layout) knows no language and no writing direction — language rules arrive compiled into penalties and glue, and "advance" means "along the writing direction". Only the JFM data, punctuation offsets, and the backend's coordinate map are direction- or language-specific. See DESIGN.md for the full record, including the ground-truth comparisons under compare/.

Running

Needs SBCL (via Roswell). Two things live outside the repo; setup.sh fetches both (needs git and curl):

sh setup.sh

• vendor/cl-pdf — cl-pdf with two fixes the PDF backend needs, from the snmsts/cl-pdf fork (local-fixes). Upstream PRs mbattyani/cl-pdf#47 and #48; once merged the Quicklisp cl-pdf works and this is unnecessary. See DESIGN.md.
• vendor/jlreq/jfm-jlreq.lua — the character-class and spacing data Kern reads at run time, from abenori/jlreq (BSD-2).

The core and its tests need only vendor/jlreq/jfm-jlreq.lua (read at run time) — no cl-pdf, no font. (asdf:test-system "kern") passes with just that. cl-pdf and a single-face .ttf Japanese font (游明朝 by default; switchable, see below) are only for the PDF demos.

;; fetch vendor deps once: sh setup.sh
;; then load everything and run a demo
ros run -- --load load.lisp
(kern::run-document-pdf)           ; the horizontal document above
(kern::run-vertical-document-pdf)  ; the same, vertical
(kern::run-ja-pdf)                 ; a longer justified sample

;; fonts are pluggable — the engine only ever asks a font for metrics
;; through generic functions, so any single-face .ttf works:
(kern::run-document-pdf :ttf #P"/path/to/another.ttf")

;; and a document can mix fonts mid-text via :fonts + (:font key …):
;;   (:document (:fonts (:db "…/yumindb.ttf"))
;;     (:p "本文は明朝、" (:font :db "この語だけ太字") "、また明朝。"))

Tests need only the core — no cl-pdf, no fonts:

(asdf:test-system "kern")          ; 113 checks (§4.19, ruby, document)

License

MIT (LICENSE). Bundled third-party components keep their own licenses — see THIRD-PARTY.md. The character-class and spacing data comes from abenori/jlreq's jfm-jlreq.lua (BSD-2).

The name: Kern — kerning, the adjustment of space between type. The engine's core is exactly that box/glue space computation.