"""Lexer for arithmetic expressions."""

from dataclasses import dataclass
from typing import Union


class LexError(Exception):
    pass


@dataclass
class Token:
    kind: str
    value: Union[int, float, None]

    def __repr__(self):
        return f"{self.kind}({self.value!r})"


def tokenize(src: str) -> list:
    tokens = []
    i = 0
    while i < len(src):
        ch = src[i]

        if ch in ' \t':
            i += 1
            continue

        if ch.isdigit() or (ch == '.' and i + 1 < len(src) and src[i + 1].isdigit()):
            j = i
            while j < len(src) and src[j].isdigit():
                j += 1
            if j < len(src) and src[j] == '.':
                j += 1
                while j < len(src) and src[j].isdigit():
                    j += 1
                tokens.append(Token('NUMBER', float(src[i:j])))
            else:
                tokens.append(Token('NUMBER', int(src[i:j])))
            i = j
            continue

        if ch == '+':
            tokens.append(Token('PLUS', None))
        elif ch == '-':
            tokens.append(Token('MINUS', None))
        elif ch == '*':
            tokens.append(Token('STAR', None))
        elif ch == '/':
            tokens.append(Token('SLASH', None))
        elif ch == '(':
            tokens.append(Token('LPAREN', None))
        elif ch == ')':
            tokens.append(Token('RPAREN', None))
        else:
            raise LexError(f"Invalid character {ch!r} at position {i}")

        i += 1

    tokens.append(Token('EOF', None))
    return tokens