Emacs-KP: Knuth-Plass Line Breaking for Emacs

中文文档 | Developer Guide

Emacs-kp implements the Knuth-Plass optimal line breaking algorithm with full support for CJK (Chinese, Japanese, Korean) and Latin mixed text typesetting, entirely inside Emacs.

Features

• Optimal line breaking — the Knuth-Plass dynamic program finds the globally optimal set of breaks for a paragraph, not greedy first-fit.
• CJK support — every CJK character is a breakable box; kinsoku rules keep punctuation attached (，。 never start a line, 「《 never end one); dedicated inter-CJK and CJK↔Latin spacing.
• Hyphenation — Frank Liang's algorithm (the TeX algorithm) with 70+ Hunspell pattern dictionaries bundled.
• Pixel-accurate justification — every justified line renders at exactly the requested pixel width, using display (space :width ...) properties; works with variable-width fonts.
• Text properties preserved — faces, colors and other properties survive justification; inserted hyphens inherit the face of the word they break.
• Robust on hard input — unbreakable overlong tokens (URLs, long words at narrow widths) degrade to emergency breaks instead of losing text; every input produces output.
• Optional C module — a dynamic module runs the DP in C with a thread pool that processes paragraphs in parallel (see benchmarks).

Requirements

• Emacs 29.1+ (uses string-pixel-width and object-intervals)
• Optional, for the C module: a C11 compiler and pthreads

Quick Start

(add-to-list 'load-path "/path/to/emacs-kp")
(require 'ekp)

;; Justify a paragraph to 600 pixels
(insert (ekp-pixel-justify "Your paragraph text here..." 600))

;; Find the best width in a range; returns (justified-text . width)
(ekp-pixel-range-justify "Your text" 400 800)

Multiline strings are treated as one paragraph per line; blank lines are preserved.

C module (recommended for long texts)

cd ekp_c && make        # requires C11 compiler, produces ekp.dylib/.so/.dll

(ekp-c-module-load)     ; prints "ekp-c module loaded (version 1.1, N threads)"

Once loaded (and since ekp-use-c-module defaults to t), all justification calls automatically use the C engine. The Elisp and C engines produce identical output; Elisp is the always-available fallback. If the module on disk is older than the Elisp code expects, loading refuses with a message asking you to rebuild.

Interactive Use (buffer & region)

ekp-region.el turns the string API into buffer-level commands:

(require 'ekp-region)

• M-x ekp-justify-region — justify the region to the window text width (with a numeric prefix argument, to that many pixels).
• M-x ekp-unjustify-region — restore the original text exactly, including collapsed whitespace runs. Justification is lossless: every synthesized space, soft line break, and soft hyphen carries the original text it replaced, so restoring is a structural transform that also works after you edited the justified text.
• M-x ekp-auto-justify-mode — keep the whole buffer justified to the window width. Re-flows (debounced by ekp-auto-justify-resize-delay) when the window width changes, and after edits re-justifies only the touched paragraphs (ekp-auto-justify-edit-delay), so unchanged paragraphs hit the paragraph cache. Turning the mode off restores the buffer exactly.

ekp-region-margin-pixel (default 2) is subtracted from the window width as a rounding safety margin.

Configuration

Hyphenation language

(setq ekp-latin-lang "de_DE")   ; default "en_US"

Any dictionaries/hyph_<lang>.dic works; short codes like "de" resolve to the first matching dictionary.

Spacing parameters

Three glue classes control spacing (all values in pixels):

Group	Between
lws-*	two Latin words
mws-*	a Latin word and a CJK char
cws-*	two CJK characters

Each class has an ideal width, a maximum stretch and a maximum shrink:

(ekp-param-set lws-ideal lws-stretch lws-shrink
               mws-ideal mws-stretch mws-shrink
               cws-ideal cws-stretch cws-shrink)
;; e.g. (ekp-param-set 7 3 2  5 2 1  0 2 0)

• If you never call ekp-param-set, defaults are derived automatically from the font of each string.
• Explicit parameters persist until you call ekp-param-reset, which returns to automatic per-string defaults.

Algorithm parameters

Variable	Default	Meaning
ekp-line-penalty	10	Base cost per line; higher prefers fewer lines
ekp-hyphen-penalty	50	Cost of a hyphenated break (added as penalty²)
ekp-adjacent-fitness-penalty	100	Cost when adjacent lines differ in tightness by >1 class
ekp-consecutive-hyphen-penalty	100	Multiplier for runs of hyphenated lines (× count²)
ekp-last-line-min-ratio	0.5	Minimum fill ratio for the last line
ekp-last-line-short-penalty	50	Cost multiplier for a too-short last line
ekp-looseness	0	Target line count offset: +1 = one line more than optimal, −1 = one fewer

All parameters take effect with both engines: the Elisp side syncs them to the C module before every call. ekp-looseness is handled by a dedicated Elisp path (the C module is bypassed automatically while it is non-zero).

Caching

Tokenization, measurement, and DP results are cached per paragraph.

• ekp-para-cache-limit (default 256): max cached paragraphs; the cache is flushed when the limit is reached.
• M-x ekp-clear-caches clears everything (use after changing fonts or themes that affect glyph widths).

Performance

Measured on the bundled sample texts (tests/ekp-bench.el), batch Emacs 30.2, Apple Silicon; see DEVELOPER.md for methodology:

Case (text-zh.txt ≈ 3.6 KB)	Elisp (byte-compiled)	C module
justify, width 200px	96 ms	57 ms
optimal-width search 340–380	294 ms	75 ms
DP only, width 400px	15 ms	1.3 ms

Byte-compile the package — the Elisp engine is ~10× faster compiled. Both engines produce identical output; the C module pays off most for optimal-width search and long multi-paragraph texts.

Known Limitations

• Widths are computed from the string's own text properties. If the destination buffer remaps faces (different :height, themes), widths may differ; justify with the same properties you will display.
• One font is assumed per Latin/CJK script per paragraph when computing spacing defaults; mixed-font paragraphs work but spacing defaults come from the first font found.
• ekp-pixel-range-justify minimizes average demerits with a ternary search plus a local scan; cost is not perfectly unimodal in width, so the result is a very good, but not guaranteed global, optimum.
• In batch/tty Emacs, pixel widths degrade to character columns (the full pipeline still works; useful for testing).

Testing

tests/run-tests.sh /path/to/emacs     # 36 ERT tests, all batch-safe

Credits

• Core algorithm: "Breaking Paragraphs into Lines" by Donald E. Knuth and Michael F. Plass (1981)
• Hyphenation: Frank Liang's algorithm, adapted from Pyphen
• Dictionaries: Hunspell hyphenation patterns