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


class ParseError(Exception):
    pass


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

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


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

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


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

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


Node = Union[Num, BinOp, Unary]


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, kind: str = None) -> Token:
        tok = self.tokens[self.pos]
        if kind 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':
            raise ParseError(
                f"unexpected token {self._peek().kind!r} ({self._peek().value!r})"
            )
        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
            return Unary(op, self._unary())
        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 ')', got {self._peek().kind!r} ({self._peek().value!r})"
                )
            self._consume()
            return node
        raise ParseError(f"unexpected token {tok.kind!r} ({tok.value!r})")


def parse(tokens: List[Token]) -> Node:
    """Parse a flat token list into an AST Node.

    AST shape
    ---------
    Num(value)              — numeric literal (int or float)
    BinOp(op, left, right)  — binary op; op in {'+', '-', '*', '/'}
    Unary(op, operand)      — unary op; op == '-'

    Precedence (high to low): unary-minus > * / > + -
    Associativity: left for all binary operators.

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