The zh pair's other direction, and a rule pack that mostly says no

`pair_lang` had always been answering a second question nobody asked: it
says which two languages, and every surface built on it assumed English
was the one being learned. That is why hanzi is never tokenized, never
spell-checked, never glossed — correct for a Mandarin native practising
English, backwards for an English native practising Mandarin.
`users.direction` (migration 0016) separates the two questions; a
`zh-learner` pair code would have been cheaper and would have made two
directions of one pair look like two unrelated languages to every query.

Segmentation is what replaces `wordAt` where there are no spaces: a
shortest-path walk over log-probabilities, 232 ms and 14 MB for 188,522
words. The browser gets the word list because segmentation runs on hover;
the server keeps the whole dictionary. Their coverage gates come out
opposite on purpose — the client list is frequency-gated because the
segmentation is measurably identical without the tail, and the dictionary
is gated by nothing, because its only power is to explain and the word a
learner stops on is the rare one.

The 错别字 pack is 24 confusable pairs behind two mechanical gates. One
admits a pair only if the wrong form is not a dictionary word and the
right form is, which is why it refuses 自已 for 自己 — a real error whose
wrong form is a headword. The other asks the segmenter whether the two
characters already belong to two different words, without which 自己经常,
睡觉的时候 and 不知到底 would all be corrupted silently into text still
made of real characters.

Not deployed (this carries a migration), not seen in a browser, and no
account has ever been in the learner direction. The IME composition
guards were in scope and are not done — see BUILD_PLAN Phase 26.

web/src/components/Companion/hanzi.test.ts

@@ -0,0 +1,141 @@
+import { readFileSync } from 'node:fs'
+import { gunzipSync } from 'node:zlib'
+import { describe, expect, it } from 'vitest'
+
+import { CONFUSION_PAIRS, hanziFindings } from './hanzi'
+import { buildSegmenter } from '../../lib/segment'
+
+// The 错别字 pack, held to the bar Phase 22 set for the English rule pack: every
+// rule pinned in *two* directions — the mistake it must catch, and the correct
+// writing next to it that it must leave alone.
+//
+// Here the second direction is the one that matters, and it is unusually easy to
+// get wrong. Chinese has no spaces, so every one of these rules is a substring
+// match on running text, and for most of them there exists an ordinary correct
+// sentence that contains the substring across a word boundary. Those sentences
+// are the real test.
+
+const raw = gunzipSync(readFileSync(new URL('../../../public/dictionaries/zh/words.txt.gz', import.meta.url)))
+const seg = buildSegmenter(raw.toString('utf8'))
+
+const flagged = (text: string) => hanziFindings(text, seg).map((f) => `${f.original}→${f.replacement}`)
+
+describe('the gate that admits a rule', () => {
+  // The pack's own claim about itself, checked against the shipped dictionary
+  // rather than asserted in a comment. A pair whose wrong form is a real word
+  // cannot be decided mechanically and does not belong here.
+  it('every wrong form is not a word, and every right form is', () => {
+    for (const { wrong, right } of CONFUSION_PAIRS) {
+      expect(seg.has(wrong), `${wrong} is a dictionary word and must not be flagged`).toBe(false)
+      expect(seg.has(right), `${right} is not a dictionary word`).toBe(true)
+    }
+  })
+
+  // The errors this pack deliberately refuses, and why — each is a genuine
+  // mistake by a modern standard whose wrong form is itself a headword. If a
+  // dictionary rebuild ever drops one of these, this test fails and the pair
+  // becomes admissible; that is the intended way to find out.
+  it('refuses the well-known errors it cannot decide', () => {
+    for (const undecidable of ['自已', '好象', '倒底', '帐号', '部份']) {
+      expect(seg.has(undecidable), `${undecidable} is no longer a word — reconsider the rule`).toBe(true)
+      expect(flagged(`这是${undecidable}的例子`)).toEqual([])
+    }
+  })
+})
+
+describe('the mistakes it catches', () => {
+  it('已 / 己 / 以', () => {
+    expect(flagged('我己经写完了作业')).toEqual(['己经→已经'])
+    expect(flagged('我以经吃过饭了')).toEqual(['以经→已经'])
+    expect(flagged('下课已后我们去公园')).toEqual(['已后→以后'])
+  })
+
+  it('在 / 再', () => {
+    expect(flagged('明天在见')).toEqual(['在见→再见'])
+    expect(flagged('他正再看书')).toEqual(['正再→正在'])
+    expect(flagged('现再几点了')).toEqual(['现再→现在'])
+  })
+
+  it('做 / 作', () => {
+    expect(flagged('我的工做很忙')).toEqual(['工做→工作'])
+    expect(flagged('老师给我们很多做业')).toEqual(['做业→作业'])
+    expect(flagged('这本书的做者是谁')).toEqual(['做者→作者'])
+  })
+
+  it('the rest', () => {
+    expect(flagged('我觉的这个很好')).toEqual(['觉的→觉得'])
+    expect(flagged('你因该早点睡')).toEqual(['因该→应该'])
+    expect(flagged('即然你来了就坐下吧')).toEqual(['即然→既然'])
+    expect(flagged('你知到吗')).toEqual(['知到→知道'])
+    expect(flagged('请输入你的蜜码')).toEqual(['蜜码→密码'])
+  })
+
+  it('reports an exact span, so the card replaces the right characters', () => {
+    const text = '我己经到了'
+    const [f] = hanziFindings(text, seg)
+    expect(text.slice(f.from, f.to)).toBe('己经')
+    expect(text.slice(0, f.from) + f.replacement + text.slice(f.to)).toBe('我已经到了')
+  })
+
+  it('finds every occurrence, in document order', () => {
+    expect(flagged('我己经吃了，他也己经吃了')).toEqual(['己经→已经', '己经→已经'])
+    expect(flagged('我的工做很忙，所以我觉的很累')).toEqual(['工做→工作', '觉的→觉得'])
+  })
+})
+
+// ── the direction that matters ──────────────────────────────────────────────
+
+describe('the correct writing it must not touch', () => {
+  // Each of these is an ordinary sentence containing a flagged substring across
+  // a word boundary. Without the boundary gate, every one would be corrupted —
+  // and corrupted silently, into text that is still made of real characters.
+  it('leaves two real words alone where they happen to abut', () => {
+    // 自己 + 经常. The substring is 己经.
+    expect(flagged('他自己经常做饭')).toEqual([])
+    // 睡觉 + 的. The substring is 觉的.
+    expect(flagged('睡觉的时候不要看手机')).toEqual([])
+    // 感觉 + 的.
+    expect(flagged('这是我感觉的方向')).toEqual([])
+    // 不知 + 到底.
+    expect(flagged('我不知到底该怎么办')).toEqual([])
+    // 因 + 位置.
+    expect(flagged('因位置不好我们换了座位')).toEqual([])
+    // 已 + 后悔.
+    expect(flagged('他已后悔了')).toEqual([])
+  })
+
+  it('leaves ordinary correct prose entirely alone', () => {
+    for (const good of [
+      '我今天早上去公园跑步了',
+      '他的中文说得很好',
+      '我已经完成了我的作业',
+      '现在几点了，我们再见面吧',
+      '我觉得这个工作很有意思',
+      '既然你已经知道了，就按照计划做',
+    ]) {
+      expect(flagged(good), good).toEqual([])
+    }
+  })
+
+  // Where the gate costs the pack a real catch, and the trade it is making.
+  // 不知 is itself a word, so 我不知到他在哪里 — which really is 知到 for 知道 —
+  // reads to the segmenter as 不知 + 到 and is left alone. That is the gate
+  // preferring a missed error to a corrupted sentence, which is the whole
+  // premise: 我不知到底该怎么办 is the same three characters and is correct.
+  it('declines a real error rather than risk the sentence beside it', () => {
+    expect(flagged('我不知到他在哪里')).toEqual([])
+    expect(flagged('你知到吗')).toEqual(['知到→知道'])
+  })
+
+  it('says nothing about English, or about nothing', () => {
+    expect(flagged('I already finished my homework')).toEqual([])
+    expect(flagged('')).toEqual([])
+  })
+
+  // The direction gate. The word list is loaded only for an account learning
+  // Chinese, so without one this pack is silent — a writer practising English
+  // must never be told her own quoted Chinese is wrong.
+  it('is silent without a segmenter, which is how the direction gate works', () => {
+    expect(hanziFindings('我己经写完了', null)).toEqual([])
+  })
+})