"""Recursive-descent parser for arithmetic expressions.

Grammar (precedence low → high):
    expr   = term   ( ('+' | '-') term   )*
    term   = unary  ( ('*' | '/') unary  )*
    unary  = '-' unary | primary
    primary = NUMBER | '(' expr ')'

AST nodes (stable shape for the evaluator):
    Num(value)               – numeric literal; value is int or float
    BinOp(op, left, right)   – op is one of '+', '-', '*', '/'
    Unary(op, operand)       – op is '-'
"""

from __future__ import annotations
from typing import List
from .lexer import Token


class ParseError(Exception):
    pass


class Num:
    __slots__ = ("value",)

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

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

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


class BinOp:
    __slots__ = ("op", "left", "right")

    def __init__(self, op: str, left, right):
        self.op = op
        self.left = left
        self.right = right

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

    def __eq__(self, other):
        return (
            isinstance(other, BinOp)
            and self.op == other.op
            and self.left == other.left
            and self.right == other.right
        )


class Unary:
    __slots__ = ("op", "operand")

    def __init__(self, op: str, operand):
        self.op = op
        self.operand = operand

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

    def __eq__(self, other):
        return (
            isinstance(other, Unary)
            and self.op == other.op
            and self.operand == other.operand
        )


class _Parser:
    def __init__(self, tokens: List[Token]):
        self._tokens = tokens
        self._pos = 0

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

    def _consume(self) -> Token:
        tok = self._tokens[self._pos]
        self._pos += 1
        return tok

    def _expect(self, kind: str) -> Token:
        tok = self._peek()
        if tok.kind != kind:
            raise ParseError(f"expected {kind}, got {tok.kind!r} ({tok.value!r})")
        return self._consume()

    def parse(self):
        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})")
        return node

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

    def _primary(self):
        tok = self._peek()
        if tok.kind == "NUMBER":
            self._consume()
            return Num(tok.value)
        if tok.kind == "LPAREN":
            self._consume()
            node = self._expr()
            self._expect("RPAREN")
            return node
        raise ParseError(f"unexpected token {tok.kind!r} ({tok.value!r})")


def parse(tokens: List[Token]):
    """Parse a token list produced by calc.lexer.tokenize and return an AST root node."""
    return _Parser(tokens).parse()