# Plan: Go rewrite — M2.1 `domain/czech.Normalize`

## Context

The Go rewrite finished M1 (skeleton, tooling, hello server) in commit
`cf0f176` on 2026-05-04. The next milestone, **M2 — Pure-domain helpers**,
is current per [progress tracker](2026-05-03-2349-go-backend-rewrite-progress.md)
but has no work landed yet (all 12 sub-tasks unchecked).

This plan covers only the **first** M2 task: porting Python's
`normalize` from [scripts/czech_utils.py](../../scripts/czech_utils.py)
to Go as `internal/domain/czech.Normalize`. It is the lowest-level helper
in the domain — `parse_month_references`, `_build_name_variants`,
`match_members`, exception keys, and `reconcile` all transitively depend
on it. Getting it byte-equivalent first removes a class of "why does my
match not fire" failures from every later M2 task.

**Decision (confirmed in plan-mode Q):** start with hand-written Go unit
tests for fresh Czech edge cases. Defer parity-fixture wiring until
M3.1/M3.2 land (separate task); add the parity test for `Normalize`
retroactively at that point.

## Scope

- New package `go/internal/domain/czech/` with `Normalize` and unit tests.
- Add `golang.org/x/text` dependency to `go/go.mod` (currently zero deps).
- **Out of scope:** `ParseMonthReferences` (M2.2), fixture tooling
  (M3.1/M3.2), CLI subcommand wiring (M2.11/M2.12), parity test runner.

## Recommended approach

### Python contract to match

```python
def normalize(text: str) -> str:
    nfkd = unicodedata.normalize("NFKD", text)
    return "".join(c for c in nfkd if not unicodedata.combining(c)).lower()
```

Three semantic operations:
1. NFKD decompose
2. Drop characters where `unicodedata.combining(c)` is non-zero
3. Lowercase

### Go implementation

`go/internal/domain/czech/normalize.go`:

```go
package czech

import (
    "strings"
    "unicode"
    "golang.org/x/text/unicode/norm"
)

func Normalize(s string) string {
    decomposed := norm.NFKD.String(s)
    var b strings.Builder
    b.Grow(len(decomposed))
    for _, r := range decomposed {
        if unicode.In(r, unicode.Mn) {
            continue
        }
        b.WriteRune(r)
    }
    return strings.ToLower(b.String())
}
```

**Two precision points worth flagging:**

1. **`unicode.Mn` not `unicode.IsMark`.** The plan's library-choices
   table mentions `unicode.IsMark`, but that covers Mn + Mc + Me. Python
   `unicodedata.combining()` returns 0 for Mc/Me (their canonical
   combining class is 0), so it effectively filters only Mn. Use
   `unicode.In(r, unicode.Mn)` for byte-equivalence with Python. Cite
   this in a one-line code comment; it's the kind of thing a future
   reader will second-guess.
2. **`strings.ToLower` vs Go's locale-aware tools.** Python's `.lower()`
   on already-decomposed Latin is straight ASCII lowercase for Czech.
   Stdlib `strings.ToLower` matches; do not pull in `golang.org/x/text/cases`.

### Tests

`go/internal/domain/czech/normalize_test.go` — table-driven, covers:

- ASCII passthrough: `"Honza" → "honza"`
- Czech lowercase diacritics: `"žluťoučký" → "zlutoucky"`
- Mixed case + diacritics: `"Příliš" → "prilis"`
- Czech caron + ring: `"Dvořák" → "dvorak"`, `"Růžena" → "ruzena"`
- Hard letters: `"Čeněk" → "cenek"`, `"Kačer" → "kacer"`
- Empty string: `"" → ""`
- Already-normalized: `"prilis" → "prilis"` (idempotence)
- Pre-composed vs decomposed input both produce the same output (NFC
  `"é"` and `"é"` both → `"e"`)
- Whitespace preserved: `"Jan Novák" → "jan novak"`

Run a one-shot cross-check against the live Python implementation for
each test input before locking the table:
```
PYTHONPATH=scripts:. python -c \
  'from czech_utils import normalize; print(repr(normalize("Dvořák")))'
```
This is the manual stand-in for the M3 parity fixtures.

### Wire-up

- `go get golang.org/x/text@latest` (run from `go/`); `go mod tidy`.
- No CLI changes — `cmd/fuj` already stubs `fees`/`reconcile` with
  exit code 2; no need to touch dispatcher for this task. `Normalize`
  is consumed by other domain code, not by users directly.

## Critical files

- New: [go/internal/domain/czech/normalize.go](../../go/internal/domain/czech/normalize.go)
- New: [go/internal/domain/czech/normalize_test.go](../../go/internal/domain/czech/normalize_test.go)
- Modified: [go/go.mod](../../go/go.mod), `go/go.sum` (new)
- Reference (read-only): [scripts/czech_utils.py](../../scripts/czech_utils.py) — the porting source
- Reference (read-only): [docs/plans/2026-05-03-2349-go-backend-rewrite.md](2026-05-03-2349-go-backend-rewrite.md) — risk #3 (NFKD edge cases)

## Verification

End-to-end checks before marking M2.1 done:

1. `cd go && go build ./...` — clean compile.
2. `cd go && go test ./internal/domain/czech/...` — all table cases green.
3. `cd go && go test -race ./...` — race-clean.
4. `cd go && golangci-lint run` (or `make go-lint` from repo root) — clean.
5. **Spot parity** (manual, will be automated in M3): for each Go test
   input, run the Python `normalize` via `PYTHONPATH=scripts:. python -c
   '...'` and confirm bytes match. Capture the diff in the commit
   message if anything surprises.
6. `make go-build && make go-test && make go-lint` from repo root — proves
   the existing M1 gate still passes.

## Branching & follow-up

Per [CLAUDE.md](../../CLAUDE.md), this is feature work → branch + Gitea MR:

- Branch: `feat/m2-1-czech-normalize` off `main`.
- Single commit, Co-Authored-By trailer.
- Push with `-u`, print compare URL
  `https://gitea.home.hrajfrisbee.cz/kacerr/fuj-management/compare/main...feat/m2-1-czech-normalize`
- User opens/merges the MR.
- After merge: tick `M2.1` in the progress tracker with the commit SHA;
  add a one-line CHANGELOG entry; record any porting surprise in the
  tracker's "Notes & decisions" section (e.g. the `Mn`-vs-`IsMark`
  precision point if it bears noting).

Next task after this lands is **M2.2 `ParseMonthReferences`** — the
larger, edge-case-heavier sibling. Whether to start it before or after
M3.1/M3.2 is a separate decision the user can make then.