"""
Recursive-descent parser for arithmetic expressions.

AST node shapes:
    Num(value)              — a numeric literal; value is int or float
    BinOp(op, left, right)  — binary operation; op is '+', '-', '*', or '/'
    Unary(op, operand)      — unary minus; op is '-'

Grammar (precedence low → high):
    expr    := term   (('+' | '-') term)*
    term    := unary  (('*' | '/') unary)*
    unary   := '-' unary | primary
    primary := NUMBER | '(' expr ')'
"""
from __future__ import annotations
from dataclasses import dataclass
from typing import Union


class ParseError(Exception):
    pass


@dataclass
class Num:
    value: Union[int, float]

    def __repr__(self) -> str:
        return f"Num({self.value!r})"


@dataclass
class BinOp:
    op: str
    left: "Node"
    right: "Node"

    def __repr__(self) -> str:
        return f"BinOp({self.op!r}, {self.left!r}, {self.right!r})"


@dataclass
class Unary:
    op: str
    operand: "Node"

    def __repr__(self) -> str:
        return f"Unary({self.op!r}, {self.operand!r})"


Node = Union[Num, BinOp, Unary]


class _Parser:
    def __init__(self, tokens: list) -> None:
        self._tokens = tokens
        self._pos = 0

    def _peek(self):
        return self._tokens[self._pos]

    def _consume(self, kind: str = None):
        tok = self._tokens[self._pos]
        if kind is not None and tok.kind != kind:
            raise ParseError(
                f"expected {kind!r}, got {tok.kind!r} ({tok.value!r})"
            )
        self._pos += 1
        return tok

    def parse(self) -> Node:
        if self._peek().kind == "EOF":
            raise ParseError("empty input")
        node = self._expr()
        if self._peek().kind != "EOF":
            tok = self._peek()
            raise ParseError(
                f"unexpected token {tok.kind!r} ({tok.value!r}) after expression"
            )
        return node

    def _expr(self) -> Node:
        node = self._term()
        while self._peek().kind in ("PLUS", "MINUS"):
            op = self._consume().value
            right = self._term()
            node = BinOp(op, node, right)
        return node

    def _term(self) -> Node:
        node = self._unary()
        while self._peek().kind in ("STAR", "SLASH"):
            op = self._consume().value
            right = self._unary()
            node = BinOp(op, node, right)
        return node

    def _unary(self) -> Node:
        if self._peek().kind == "MINUS":
            op = self._consume().value
            operand = self._unary()
            return Unary(op, operand)
        return self._primary()

    def _primary(self) -> Node:
        tok = self._peek()
        if tok.kind == "NUMBER":
            self._consume()
            return Num(tok.value)
        if tok.kind == "LPAREN":
            self._consume()
            node = self._expr()
            if self._peek().kind != "RPAREN":
                raise ParseError(
                    f"expected ')' but got {self._peek().kind!r}"
                )
            self._consume("RPAREN")
            return node
        raise ParseError(
            f"unexpected token {tok.kind!r} ({tok.value!r})"
        )


def parse(tokens: list) -> Node:
    """Parse a token list (from lexer.tokenize) into an AST Node."""
    return _Parser(tokens).parse()