"""Lexer for arithmetic expressions."""

from typing import NamedTuple, Union


class LexError(Exception):
    pass


class Token(NamedTuple):
    kind: str
    value: Union[int, float, str, None]


_SINGLE = {
    '+': 'PLUS',
    '-': 'MINUS',
    '*': 'STAR',
    '/': 'SLASH',
    '(': 'LPAREN',
    ')': 'RPAREN',
}


def tokenize(src: str) -> list:
    tokens = []
    i = 0
    while i < len(src):
        ch = src[i]
        if ch in ' \t':
            i += 1
        elif ch in _SINGLE:
            tokens.append(Token(_SINGLE[ch], ch))
            i += 1
        elif ch.isdigit() or ch == '.':
            j = i
            has_dot = False
            while j < len(src) and (src[j].isdigit() or (src[j] == '.' and not has_dot)):
                if src[j] == '.':
                    has_dot = True
                j += 1
            raw = src[i:j]
            value = float(raw) if has_dot else int(raw)
            tokens.append(Token('NUMBER', value))
            i = j
        else:
            raise LexError(f"unexpected character {ch!r} at position {i}")
    tokens.append(Token('EOF', None))
    return tokens