from dataclasses import dataclass
from typing import Union
from calc.lexer import Token


class ParseError(Exception):
    pass


@dataclass
class Num:
    """Leaf node: a numeric literal."""
    value: Union[int, float]

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


@dataclass
class Unary:
    """Unary operator node: op is '-', operand is the inner Node."""
    op: str
    operand: object

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


@dataclass
class BinOp:
    """Binary operator node. op is one of '+', '-', '*', '/'.
    Precedence and associativity are encoded in the tree structure, not here.
    """
    op: str
    left: object
    right: object

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


Node = Union[Num, Unary, BinOp]


class _Parser:
    def __init__(self, tokens: list):
        self._tokens = tokens
        self._pos = 0

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

    def _consume(self, kind: str) -> Token:
        tok = self._peek()
        if tok.kind != kind:
            raise ParseError(
                f"expected {kind}, got {tok.kind!r} ({tok.value!r})"
            )
        self._pos += 1
        return tok

    def _advance(self) -> Token:
        tok = self._tokens[self._pos]
        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

    # Grammar (lowest to highest precedence):
    #   expr   -> term  (('+' | '-') term)*
    #   term   -> unary (('*' | '/') unary)*
    #   unary  -> '-' unary | primary
    #   primary-> NUMBER | '(' expr ')'

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

    def _unary(self) -> Node:
        if self._peek().kind == 'MINUS':
            self._advance()
            operand = self._unary()
            return Unary('-', 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()
            if self._peek().kind != 'RPAREN':
                raise ParseError(
                    f"expected ')' but got {self._peek().kind!r}"
                )
            self._advance()
            return node
        if tok.kind == 'EOF':
            raise ParseError("unexpected end of input")
        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.

    AST node shapes:
      Num(value)              — numeric literal
      Unary('-', operand)     — unary negation
      BinOp(op, left, right)  — binary operation; op in {'+','-','*','/'}

    Raises ParseError on malformed input.
    """
    return _Parser(tokens).parse()