# Developer Documentation for Emacs-KP

This document describes the internal architecture, algorithms and APIs of
`emacs-kp`, as implemented.  It is intended for contributors and advanced
users.

Current repository health and prioritized follow-up work are recorded in
the [2026-07-28 system audit](./Docs/REPOSITORY_AUDIT_20260728.md).

## 1. Pipeline Overview

A justification call flows through five stages:

```
 string
   │
   ▼
 ① Tokenize          ekp-split-to-boxes           (ekp-utils.el)
   │                 Latin words / CJK chars / space runs → boxes,
   │                 kinsoku attachment of CJK punctuation
   ▼
 ② Hyphenate         ekp--split-with-hyphen        (ekp.el + ekp-hyphen.el)
   │                 Latin word boxes → syllable boxes (Liang patterns)
   ▼
 ③ Measure & index   ekp--make-para                (ekp.el)
   │                 pixel widths, glue types, prefix-sum arrays
   │                 → cached `ekp-para` struct
   ▼
 ④ Break (DP)        ekp--dp-run-1d / C module     (ekp.el / ekp_c/)
   │                 Knuth-Plass dynamic program → break positions
   ▼
 ⑤ Render            ekp-line-glues, ekp--pixel-justify
                     distribute glue pixels, strip edge space boxes,
                     attach hyphens → lines joined with "\n"
```

`ekp-pixel-justify` splits its input on `"\n"` and runs each non-blank
segment through this pipeline as an independent paragraph (in parallel
via the C batch API when available).

## 2. Data Structures

### `ekp-para` (the paragraph cache entry)

Everything the DP and renderer need, computed once per paragraph:

| Field | Contents |
|:------|:---------|
| `string`, `latin-font`, `cjk-font` | source text and detected fonts |
| `boxes` | vector of box strings |
| `boxes-widths` | pixel width per box (measured with deduplication) |
| `boxes-types` | `(START-TYPE . END-TYPE)` per box: `latin`/`cjk`/`cjk-punct`/`space` |
| `glues-types` | glue class *before* each box: `lws`/`mws`/`cws`/`nws` |
| `hyphen-pixel`, `hyphen-positions` | hyphen width; sorted vector of box indices after which a hyphen may be inserted |
| `ideal/min/max-prefixs` | prefix sums of box+glue widths at ideal / max-shrunk / max-stretched (n+1 elements) |
| `glue-ideals/shrinks/stretches` | leading-glue values per box index (n elements) — also passed verbatim to C |
| `lws/mws/cws-prefixs` | prefix **counts** of each stretchable glue class → O(1) gap counting per candidate line |
| `lead-spaces` | `lead-spaces[i]` = width of the space-box run starting at box i; index 0 forced to 0 (first-line indentation is kept) |
| `trail-spaces` | `trail-spaces[k]` = width of the space-box run ending at box k−1 |
| `glue-params` | plist snapshot of the nine spacing values at creation time |
| `dp-cache` | equal-keyed hash: complete DP signature → dp-result plist |

The paragraph cache (`ekp--para-cache`) is keyed with `equal` on a
structured key — string content, printed text-property intervals,
detected fonts, the hyphenation language (`ekp-latin-lang`), and
either the nine explicit spacing values or the automatic CJK stretch
default. The other automatic values are derived from font measurement,
which is already represented by the font and display-context fields.
Structured keys make hash collisions harmless (they were possible with
the previous `sxhash`-integer scheme).  The cache is flushed when it
exceeds `ekp-para-cache-limit`.  A one-entry fast path
(`ekp--last-para`) bypasses only the hash lookup: it compares the same
complete structural key before reuse. In-place changes to layout-relevant
text properties therefore miss both paths and match a fresh paragraph.

The DP signature is separate from the paragraph key.  It contains line
width, looseness, and all six runtime cost parameters; therefore a
parameter change selects a new result without flushing width-independent
paragraph data.  Structural `equal` comparison also lets non-zero
looseness signatures hit the cache.

### dp-result

`(:rests R :gaps G :breaks B :cost C :line-count N)` where `breaks` are
exclusive end indices per line, `rests[i]` = line-width − line-ideal
(the pixels the glue must absorb), `gaps[i]` = `(lws-count mws-count
cws-count)` for glue distribution (nil for single-box and last lines).

## 3. Line Metrics

For a candidate line spanning boxes `[i, k)`:

```
raw       = prefix[k] − prefix[i] − leading-glue(i)
space-w   = min(raw, lead-spaces[i] + trail-spaces[k])
width     = raw − space-w  (+ hyphen-pixel if box k−1 hyphenates)
```

computed for ideal, min and max in O(1).  Space-box runs at the line
edges are excluded because the renderer strips them; the DP and the
renderer therefore agree exactly, and every justified line renders at
precisely the target width (`ekp-test-justify-line-width-invariant`).
`ekp--line-stripped-space-pixel` owns this exclusion rule for the 1D/loose
DP, C-result reconstruction, and renderer.

## 4. The Knuth-Plass DP

`ekp--dp-run-1d` relaxes positions left to right.  For each reachable
start `i` it scans end positions `k` until the line's minimum width
exceeds the target.  A break at `k` is valid when
`min ≤ target ≤ max`, or for the last line when `ideal ≤ target`.

**Demerits** (per line, matching `ekp_c/ekp_kp.c` exactly):

```
demerits = (line-penalty + badness)²
         + penalty²                       ; hyphen-penalty at hyphen breaks
         + adjacent-fitness-penalty       ; if |fitness − prev-fitness| > 1
         + consecutive-hyphen-penalty × run²
badness  = min(10000, 100·|adjustment/flexibility|³)
```

Fitness classes (tight/decent/loose/very-loose) follow the TeX ratio
thresholds.  Special cases: single-box lines use flexibility 1 and
fitness decent; the last line pays `(line-penalty + short-badness)²`
where `short-badness = last-line-short-penalty × (1 − fill)` when the
fill ratio is below `ekp-last-line-min-ratio`.

Deviations from the 1981 paper, by design: penalties are always added
as `+p²` (no negative/flagged penalties), there is no `q`/looseness in
the main pass (see §6), and adjacent-fitness is a flat constant.

### Two-pass emergency strategy

Some inputs admit no valid layout: an unbreakable box wider than the
line, or a rigid (all-`nws`) region that cannot stretch to the target.
A strict pass runs first; if the paragraph end is unreachable, a second
pass additionally allows **emergency breaks** — single-box lines with
demerits `(line-penalty + 10000)² + rest²`, at least as bad as any
regular line.  This guarantees, by induction over positions, that every
input produces output (regression: narrow CJK used to return an empty
string), while the common case pays nothing and keeps pure K-P
optimality.  Both engines implement the identical strategy.

## 5. Rendering

`ekp-line-glues` turns each line's `rest` into per-glue pixel values:

- rest > 0 → stretch, distributed latin → mixed → CJK; CJK gaps absorb
  any leftover beyond nominal capacity (emergency spreading).
- rest < 0 → shrink, same priority order, never below the per-class
  shrink limit; glue widths are clamped at ≥ 0.
- Last lines are ragged-right (ideal glues + trailing filler);
  single-box lines get a trailing filler clamped at ≥ 0.

`ekp--pixel-justify` then strips leading space boxes (except on the
first line — indentation) and trailing space boxes, and appends a
hyphen — propertized like the word it breaks — where a line ends at a
hyphenation point.  Stripped widths are *not* redistributed: the DP
already excluded them (§3).

Glues become `(space :width (N))` display properties, so justification
is pixel-exact in GUI Emacs and column-exact in batch/tty.

The output is **lossless**: boxes are located in the source string
(`ekp--box-offsets`), and every synthesized or hidden piece records the
original text it stands for —

| property          | on                    | value / meaning                |
|-------------------|-----------------------|--------------------------------|
| `ekp-glue`        | synthesized glue space| original text it replaced      |
| `ekp-soft-break`  | inserted `\n`         | whitespace swallowed at break  |
| `ekp-soft-hyphen` | inserted hyphen       | marker only                    |
| `ekp-hidden`      | paragraph-edge text   | kept verbatim, `display ""`    |

Zero-width glue with a non-empty original renders as the hidden
original itself, so no character is ever dropped.  `ekp-region.el`
inverts these four structurally (`ekp-unjustify-region`) — exact even
after the justified text was edited — and builds
`ekp-justify-region` / `ekp-auto-justify-mode` on top.
`ekp--layout-marker-properties` owns the complete renderer/region marker
vocabulary and its non-inheritance contract.

Saving is a non-mutating serialization boundary.
`ekp-region--write-logical-buffer` runs first in the buffer-local
`write-region-annotate-functions`, switches whole-buffer writes to a hidden
logical copy, and leaves the display buffer untouched. Subsequent annotation
functions and coding conversion operate on that copy. A successful write
disposes it immediately; a failed write keeps at most one copy, which the
next write or integration teardown replaces. Region-only `write-region`
calls intentionally retain Emacs's physical-buffer semantics; the logical
serialization boundary is the whole-buffer save path.

Copy filtering has explicit single-slot ownership. EKP records whether the
previous `filter-buffer-substring-function` was local, temporarily restores
that value, and invokes the public `filter-buffer-substring` dispatcher to
preserve its transform and DELETE behavior. EKP then structurally removes
layout markers from the returned string. DELETE lifecycle cleanup runs after
the temporary binding is unwound, so removing the final justified span
outside auto mode restores that exact local value or reveals the inherited
value, and removes the save/search/change hooks.

### 5.1 Break permissions, alignment, protrusion, shapes

- **Break permissions**: every CJK char (punctuation included) is its
  own box; `ekp-para-breaks-allowed` forbids gaps per kinsoku (full-
  and halfwidth), `ekp-no-break' spans and NBSP-family joiners.
  Forbidden gaps carry no glue.  The DP skips them as candidates while
  the line keeps extending; the emergency fallback treats a run with
  no permitted inner break as atomic.  C receives the sparse
  `forbidden-positions` vector.
- **Alignment** (`ekp-alignment`): non-justify modes zero the glue
  stretch/shrink arrays and class params; the DP widens `max_w` by an
  extra per-line stretch R (`ekp-c-set-penalties` arg 7), so badness =
  100·(shortfall/R)³.  The renderer places each line's leftover per
  mode (trailing / split / leading).
- **Protrusion** (`ekp-protrusion`): per-gap `tail-protrudes[k]` (the
  last non-space box's allowance, looked through trailing spaces) and
  a `hyphen-protrude` scalar widen each candidate's effective target
  (`lw = width + release`) in the DP, in `ekp-line-glues', and in the
  C-result reconstruction — all three must stay in lockstep.
- **Per-line widths** (`ekp-parshape' / `ekp-first-line-indent'):
  resolved by `ekp--line-spec' (line-index → INDENT . WIDTH).  A
  plain first-line indent only changes line 0, and a line begins at
  box 0 exactly when the DP start i = 0, so the 1D pass (and the C
  engine, via `FIRST-LINE-WIDTH') handle it with no extra state.
  Only full `ekp-parshape' and `ekp-looseness' need the
  (position × line-count) DP and bypass C.  Indents render as leading
  `ekp-glue' spacers.

C module 1.6: `ekp-c-break-with-arrays' takes 15 args
(…, forbidden-positions, tail-protrudes, hyphen-protrude,
first-line-width); batch vectors have 15 elements;
`ekp-c-set-penalties' takes 4–7.

Performance after the feature wave (byte-compiled + C, Apple
Silicon, batch): justify zh w=200 ≈ 54 ms, range zh ≈ 117 ms —
justify at parity with the pre-feature numbers, range ≈ +55 % from
the larger box count.  Hot-path caches: per-char `ekp--str-type'
memo, interned glue strings, per-(paragraph, width) rendered-output
cache in the dp-cache (capped at 64 widths).  Continuous-reflow
reality check (60-paragraph, 26 k-char article, region layer
included): ≈ 73 ms per width change cold, less on revisit;
incremental single-paragraph re-justify after an edit ≈ 17 ms.

Adversarial source-mode builders are measured separately by
`ekp-bench-adversarial-builders`.  From 1,000 to 8,000 characters, the
fragment-based tokenizer grew 6.3× and dense insertion 7.7× (near the
expected linear 8× input growth).  At 8,000 characters they took 1.100 s and
0.013 s, versus 3.133 s and 0.945 s before the change.  The hyphen-position
cache uses an explicit miss sentinel so a legitimate nil result is reusable.

## 6. Looseness

`ekp-looseness` ≠ 0 switches to `ekp--dp-run-loose`, a full
(position × line-count) DP that keeps the best path *per line count*,
then picks the final count closest to (optimal + looseness), breaking
ties by demerits.  This is heavier than the 1D pass and is Elisp-only;
`ekp--c-available-p` returns nil while looseness is active so both
engines never disagree.

## 7. C Module Integration

The C module (`ekp_c/`, version 1.6) runs only stage ④.  Elisp remains
the source of truth for all font-dependent data.

- `ekp-c-break-with-arrays` (15 args): the para's prefix arrays, glue
  arrays, hyphen data, line width, the two space-run arrays, the
  forbidden/protrusion arrays and the first-line width.  Returns
  `(breaks . cost)`.
- `ekp-c-break-batch`: a vector of 15-element vectors, processed in
  parallel by a pthread pool — one task per paragraph (that is the
  correct granularity; the DP itself is sequential by nature).  The
  pool is created lazily on the first multi-paragraph batch and sized
  to the machine's cores; a full queue blocks the submitter rather
  than dropping the task.
- `ekp-c-set-penalties` (4–7 args): called by `ekp--c-sync-params`
  before *every* C entry, so `ekp-line-penalty` & friends always take
  effect (regression: they were never synced before).
- `ekp-c-module-load` refuses modules older than
  `ekp-c-module-required-version` and falls back to Elisp, preventing
  arity mismatches after upgrades.

An unavailable module, an allocation/no-result nil, or an incompatible
module version falls back to the Elisp engine.  Invalid direct API input
signals `ekp-c-invalid-input`, and any signal from an enabled backend
propagates through the public formatter; the dispatcher does not catch and
hide it.  The module never silently produces a different layout on partial
failure.  The two engines are verified byte-identical by
`ekp-test-c-parity-simple` / `ekp-test-c-parity-files` and the 300-case
property fuzz.

### Future direction: a paragraph-handle API

Each `ekp-c-break-with-arrays` call re-marshals the paragraph's
width-independent arrays (≈18·n `env` extractions).  This is invisible
for a single justify but dominates `ekp-pixel-range-justify`, which
re-marshals the same arrays once per candidate width: with a warm
paragraph cache the C path still costs ≈12 ms per width, most of it
marshal, not DP (the DP is ≈2.5 ms for the whole sample).

The fix is a `make_user_ptr` handle: `ekp-c-para-upload` copies the
arrays into a C struct once and returns a handle with a GC finalizer;
`ekp-c-break (handle, width)` then passes only the two width-dependent
scalars.  It is deliberately **not** part of this release — it is a
breaking (2.0) ABI change introducing C-side object lifetime, and the
common interactive paths (single justify, `ekp-auto-justify-mode`)
already avoid the repeated marshal because they hit the dp-cache.  It
is the clear next step whenever range search or very large batches
become a bottleneck.

## 8. Hyphenation (ekp-hyphen.el)

Liang's ordinary pattern algorithm:

- `dictionaries/hyph_*.dic` are compiled to a pattern hash on first
  use and cached per path.  Files may be UTF-8 or ISO-8859 (Emacs
  auto-detects; verified by `ekp-test-hyphen-de-iso8859-dict`).
- `ekp-hyphen-create LANG` resolves exact codes, then progressively
  shorter prefixes (`"de_CH" → "de"`).
- Margins default to 2 characters on each side of a break.
- A slash in a non-comment pattern is fail-closed.  Libhyphen replacement
  rules change the visible text and width only when their break wins; EKP's
  current fixed-width boxes cannot represent that honestly.  The compiler
  counts the rules and signals `ekp-hyphen-unsupported-pattern`, and the
  public formatter propagates it.
- `dictionaries/MANIFEST.tsv` pins 49 inventory entries to one LibreOffice
  commit (plus one explicitly identified legacy Basque byte), their SHA-256,
  syntax flag, and license evidence.  `tests/check-dictionaries.sh` is the
  offline gate; `dictionaries/update.sh check` verifies normalized bytes
  against the upstream commit on macOS and Linux.

Word boxes are matched against
`^[left-punct]* (latin-word) [right-punct]*$` so that punctuation-
wrapped words (`(word)`, `word!`, `»word«`) still hyphenate; the
punctuation stays glued to the first/last syllable box.

## 9. Testing & Benchmarks

```bash
tests/run-tests.sh [emacs]        # batch-safe ERT suite
tests/run-tests.sh [emacs] --random-order
tests/run-tests-isolated.sh [emacs] # each ERT in a fresh process
tests/check-dictionaries.sh           # offline inventory/checksum gate
dictionaries/update.sh check          # verify pinned upstream bytes
make -C ekp_c PROFILE=portable      # release-portable default
make -C ekp_c PROFILE=native        # local benchmark only
make -C ekp_c PROFILE=debug         # symbols, no optimization
make -C ekp_c PROFILE=sanitize      # ASan + UBSan
emacs -Q --batch -L . --eval '(setq ekp-use-c-module nil)' -l tests/ekp-bench.el
emacs -Q --batch -L . --eval '(progn (require (quote ekp)) (ekp-c-module-load))' \
      -l tests/ekp-bench.el
```

Set `EKP_TEST_SEED` to reproduce or vary the permuted-order run. Test
fixtures dynamically restore all EKP configuration they isolate; tests that
exercise dispatch must use the public formatter rather than only an internal
eligibility predicate.

The GUI matrix is loaded explicitly from `tests/ekp-gui-verify.el`. It
returns status 1 after printing the table when any row fails; the ERT suite
contains a forced-failure control for this boundary.

`M-x ekp-c-module-build` uses the same four profile names and invokes make
as an argv process in `ekp_c/`; it never constructs a shell `cd` command.
Release/CI artifacts use `portable`. Use `native` only for measurements on
the machine that will run the module.

Key invariants under test: rendered line width == target (pixel-exact
justification), no content loss at any width, brute-force cross-checks
of the O(1) prefix machinery, Elisp/C parity on the bundled texts, and
parameter persistence/sync regressions.

Benchmark results (batch Emacs 30.2, Apple Silicon M-series,
`tests/text-zh.txt` ≈ 3.6 KB Chinese + samples; min of 3 cold-cache
runs) — before is the pre-rewrite implementation, interpreted:

| Case                     | Before (Elisp) | After (Elisp, interpreted) | After (Elisp, compiled) | After (C) |
|:-------------------------|---------------:|---------------------------:|------------------------:|----------:|
| justify zh w=200         |        7547 ms |                    1780 ms |                   96 ms |     57 ms |
| justify zh w=400         |        2928 ms |                     815 ms |                   71 ms |     57 ms |
| justify mixed w=300      |        5540 ms |                    1275 ms |                   53 ms |     23 ms |
| range-justify zh 340–380 |       29696 ms |                    8937 ms |                  294 ms |     75 ms |
| range-justify mix 280–320|       68534 ms |                   14552 ms |                  480 ms |     34 ms |
| DP only, zh w=400        |        2382 ms |                     591 ms |                   15 ms |    1.3 ms |

("After (C)" columns measured with byte-compiled Elisp around the C
calls.  For reference, the pre-rewrite C module measured 197 ms /
430 ms / 25 ms on justify-zh-200 / range-zh / DP-only — the rewrite
also sped up the C path 3–19× via prebuilt per-para glue arrays, an
`eq' fast path in the para cache, and O(1) rest/gap reconstruction.)

The dominant wins: O(1) line metrics via prefix arrays (the old inner
loop allocated O(n) subsequences per candidate, O(n³) total), the
two-pass emergency strategy (keeps the DP sparse), box-measurement
deduplication, and per-para glue arrays reused across C calls.

## 10. File Map

```
ekp.el            Core: para struct, caching, DP (1D + looseness),
                  glue distribution, rendering, public API
ekp-utils.el      Tokenizer (boxes, kinsoku), font detection with
                  batch/tty fallbacks, C module loading
ekp-hyphen.el     Liang hyphenation + dictionary registry
ekp-region.el     Buffer/region commands, ekp-auto-justify-mode, and
                  editor integration (save, isearch, kill-ring, undo)
ekp_c/            C dynamic module (see ekp_c/README.md)
dictionaries/     Hunspell hyphenation patterns (from LibreOffice)
tests/            ekp-tests.el, ekp-region-tests.el (ERT),
                  ekp-fuzz.el (parity fuzz), ekp-bench.el,
                  ekp-demo.el, ekp-showcase.el, sample texts,
                  run-tests.sh
```