"""Lexer for arithmetic expressions."""

from __future__ import annotations
import re


class LexError(Exception):
    pass


class Token:
    __slots__ = ("kind", "value")

    def __init__(self, kind: str, value):
        self.kind = kind
        self.value = value

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

    def __eq__(self, other):
        return isinstance(other, Token) and self.kind == other.kind and self.value == other.value


_NUMBER_RE = re.compile(r"\d+\.?\d*|\.\d+")

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


def tokenize(src: str) -> list[Token]:
    tokens: list[Token] = []
    i = 0
    while i < len(src):
        ch = src[i]
        if ch in " \t":
            i += 1
            continue
        if ch in _SINGLE:
            tokens.append(Token(_SINGLE[ch], ch))
            i += 1
            continue
        m = _NUMBER_RE.match(src, i)
        if m:
            raw = m.group()
            value = float(raw) if "." in raw else int(raw)
            tokens.append(Token("NUMBER", value))
            i = m.end()
            continue
        raise LexError(f"unexpected character {ch!r} at position {i}")
    tokens.append(Token("EOF", None))
    return tokens