"""Recursive-descent parser for arithmetic expressions.

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

Grammar (precedence encoded by structure):
  expr   = term   ( ('+' | '-') term   )*
  term   = unary  ( ('*' | '/') unary  )*
  unary  = '-' unary | primary
  primary= NUMBER | '(' expr ')'
"""
from __future__ import annotations
from dataclasses import dataclass
from typing import List, Union
from calc.lexer import Token


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[Token]) -> None:
        self._tokens = tokens
        self._pos = 0

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

    def _advance(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._advance()

    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})")
        return node

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

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

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


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