# JOURNAL — Phase `lex` (Builder)

## Implementation

Built `calc/lexer.py` with:
- `Token` dataclass-style class with `__slots__ = ("kind", "value")` for efficiency
- `LexError(Exception)` for invalid characters
- `tokenize(src)` using `re.compile(r"\d+\.?\d*|\.\d+")` for number matching
  - Integer if no `.` in raw string; float otherwise
  - Single-char dispatch table `_SINGLE` for operators/parens
  - Raises `LexError` with char + position for unknown characters
  - Appends `EOF` token at end

## Test run

```
$ python -m unittest -q
Ran 21 tests in 0.000s
OK
```

## Cold-verify outputs

```
$ python -c "from calc.lexer import tokenize; print([(t.kind,t.value) for t in tokenize('3.5*(1-2)')])"
[('NUMBER', 3.5), ('STAR', '*'), ('LPAREN', '('), ('NUMBER', 1), ('MINUS', '-'), ('NUMBER', 2), ('RPAREN', ')'), ('EOF', None)]

$ python -c "from calc.lexer import tokenize; tokenize('1 @ 2')"
Traceback (most recent call last):
  ...
calc.lexer.LexError: unexpected character '@' at position 2
```