# M2.5 — Port `parse_czk_amount` to `domain/money.ParseCZK`

> On execution, this plan should be moved to
> `docs/plans/2026-05-06-0928-go-m2-5-money-parse-czk.md` per project CLAUDE.md
> (`docs/plans/YYYY-MM-DD-HHMM-<slug>.md`). Plan mode forces it to live under
> `~/.claude/plans/` until then.

## Context

Continuing the Go backend rewrite tracked in
[2026-05-03-2349-go-backend-rewrite-progress.md](../../srv/personal/fuj-management/docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md).
M2.1–M2.4 are landed. Next leaf-level pure function is
`parse_czk_amount` from [scripts/infer_payments.py:17-45](../../srv/personal/fuj-management/scripts/infer_payments.py#L17-L45),
the Czech-locale amount parser used at [scripts/infer_payments.py:124](../../srv/personal/fuj-management/scripts/infer_payments.py#L124)
when reading the `Inferred Amount` column out of the payments sheet.

It's a small, isolated string→float helper, but its heuristic for
disambiguating `.` and `,` as decimal vs thousand separator is
non-obvious and needs to behave identically in Go to keep parity once
the Go infer pipeline lands in M4.8.

## Python behaviour (the spec)

```py
def parse_czk_amount(val) -> float:
    if val is None or val == "":
        return 0.0
    if isinstance(val, (int, float)):
        return float(val)

    val = str(val)
    val = val.replace("Kč", "").replace("CZK", "").strip()
    if "," in val:
        # 1.500,00 -> 1500.00 — comma is decimal sep
        val = val.replace(".", "").replace(" ", "").replace(",", ".")
    else:
        if val.count(".") > 1:
            # 1.500.000 -> 1500000 — multiple dots = thousand sep
            val = val.replace(".", "").replace(" ", "")
        else:
            # "1 500.00" -> "1500.00", "1.500" stays "1.500" (= 1.5)
            val = val.replace(" ", "")
    try:
        return float(val)
    except ValueError:
        return 0.0
```

Key behavioural notes for the Go port:

1. Empty / None → 0, no error.
2. `"1.500"` (single dot, no comma) is parsed as **1.5**, not 1500.
   The heuristic intentionally treats a lone dot as decimal.
3. `"1.500,00"` → 1500.0 (comma wins, dots are thousand seps).
4. `"1.500.000"` → 1500000.0 (multiple dots → all thousand seps).
5. `"1 500"` / `"1 500.00"` / `"500 Kč"` → spaces stripped.
6. Garbage → 0, no error in Python.
7. Strips literal substrings `"Kč"` and `"CZK"` (case-sensitive in Python).

## Approach

Create new package `internal/domain/money` mirroring the layout of
`internal/domain/fees` (single-file module + test file alongside).

### Signature

```go
// Package money ports Czech-locale currency parsing from
// scripts/infer_payments.py.
package money

// ParseCZK parses a Czech-locale amount string and returns the value
// in CZK as a float64.
//
// Mirrors scripts/infer_payments.py parse_czk_amount:
//   - empty input → (0, nil)
//   - "Kč"/"CZK" suffixes are stripped (case-sensitive, like Python)
//   - if input contains ",", comma is the decimal separator and
//     dots/spaces are thousand separators ("1.500,00" → 1500.0)
//   - else if input contains 2+ dots, all dots are thousand seps
//     ("1.500.000" → 1500000.0)
//   - else single dot stays as the decimal point ("1.500" → 1.5,
//     matching the Python heuristic)
//   - on parse failure, returns (0, ErrInvalidAmount). Callers wanting
//     Python-equivalent silent-zero behaviour can discard the error.
func ParseCZK(s string) (float64, error)
```

`ErrInvalidAmount` is a package-level sentinel:

```go
var ErrInvalidAmount = errors.New("money: invalid CZK amount")
```

Why `(float64, error)` instead of mirroring Python's silent zero:

- Go idiom prefers explicit errors.
- The single Python call site doesn't distinguish parse-fail from
  empty-input (both → 0), so if we want byte-equal behaviour at the
  Go infer site (M4.8), the caller can `v, _ := money.ParseCZK(s)`
  and get exactly the Python result.
- Future callers (e.g. user-facing import flows) may want to surface
  the error.

This matches the precedent set in M2.4 where we used
`Expected{Unknown bool}` rather than copying the Python `"?"` sentinel
verbatim — Go-idiomatic surface, parity-preserving semantics.

### Polymorphic input?

Python's `parse_czk_amount` also accepts raw int/float (passed through
unchanged) because Google Sheets API can return numeric cells as
`float64` rather than strings. **Skip this in Go.** The Sheets IO
adapter is M4.2, and that's where the `[]any` → string normalisation
will live. Keeping `ParseCZK` string-only keeps the leaf function tiny.

### Tests

`money_test.go` mirrors the existing `fees_test.go` table-driven style,
including the verification comment showing the Python command used to
confirm each expected value:

```sh
PYTHONPATH=scripts:. python -c '
from infer_payments import parse_czk_amount
for v in [None, "", "0", "500", "500 Kč", "500 CZK",
          "1 500", "1500.00", "1 500.00",
          "1.500,00", "1500,5", "1.500.000",
          "1.500", "abc", "  ", "100,5 Kč"]:
    print(repr(v), "->", parse_czk_amount(v))
'
```

Cases to cover (all numeric outputs verified against the Python output
of the snippet above):

| input | expected |
|---|---|
| `""` | 0 |
| `"0"` | 0 |
| `"500"` | 500 |
| `"500 Kč"` | 500 |
| `"500 CZK"` | 500 |
| `"1 500"` | 1500 |
| `"1500.00"` | 1500 |
| `"1 500.00"` | 1500 |
| `"1.500,00"` | 1500 |
| `"1500,5"` | 1500.5 |
| `"1.500.000"` | 1500000 |
| `"1.500"` | 1.5 *(heuristic — single dot = decimal)* |
| `"100,5 Kč"` | 100.5 |
| `"abc"` | 0, returns `ErrInvalidAmount` |
| `"  "` | 0, returns `ErrInvalidAmount` *(or 0 nil — confirm against Python; trim leaves `""`, then `float("")` raises → Python returns 0; Go test will assert whichever Python actually produces)* |

The `"  "` row is the only one that needs the Python verification step
to settle — once verified, lock the behaviour in.

Also add a "documentation example" assertion in the test that
`v, _ := ParseCZK(s)` recovers the Python silent-zero contract for
every garbage input, so we don't lose that property at the Go infer
call site.

## Files to create

- `go/internal/domain/money/money.go` — package + `ParseCZK` + `ErrInvalidAmount`
- `go/internal/domain/money/money_test.go` — table-driven tests

No existing Go files need editing.

## Verification

```sh
cd go && go test ./internal/domain/money/...
make go-lint
make go-build   # sanity: nothing else broke
```

Also run the Python snippet from the Tests section above and diff its
output against the test table to confirm parity.

## Out of scope (explicit non-goals)

- Polymorphic `any` input — leave for M4.2 IO adapter.
- Hooking into the Tier-1 parity runner — that comes with M3.5
  (`-tags=parity` build constraint). M2.5 just needs unit tests.
- Any callsite migration — `infer_payments.py` keeps using its own
  Python function until M4.8.

## Progress tracker + changelog

After the commit lands:

- Tick `M2.5` in [docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md](../../srv/personal/fuj-management/docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md)
  with the commit SHA, mirroring the M2.4 entry style.
- Add a CHANGELOG.md entry at top:
  `## YYYY-MM-DD HH:MM TZ — feat(go/M2.5): port domain/money.ParseCZK`.

Branch: `feat/m2-5-money-parse-czk` (per CLAUDE.md branch-per-feature
workflow). Push, open MR via `tea pr create`, leave merge to the user.