from dataclasses import dataclass
from typing import Any

from calc.lexer import Token


class ParseError(Exception):
    pass


@dataclass
class Num:
    value: Any

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


@dataclass
class BinOp:
    op: str
    left: Any
    right: Any

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


@dataclass
class Unary:
    op: str
    operand: Any

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


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):
        if self._peek().kind == 'EOF':
            raise ParseError("empty expression")
        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):
        # addition / subtraction — left-associative, lowest precedence
        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):
        # multiplication / division — left-associative, higher precedence
        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):
        if self._peek().kind == 'MINUS':
            op = self._advance().value
            operand = self._unary()
            return Unary(op, operand)
        return self._primary()

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


def parse(tokens: list):
    """Parse a token list produced by calc.lexer.tokenize into an AST.

    AST node types:
      Num(value)              — a numeric literal
      BinOp(op, left, right)  — binary +, -, *, /
      Unary(op, operand)      — unary -

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