"""Recursive-descent parser for arithmetic expressions.

AST nodes:
  Num(value)              — numeric literal
  BinOp(op, left, right)  — binary op; op in {'+', '-', '*', '/'}
  Unary(op, operand)      — unary op; op == '-'
"""
from dataclasses import dataclass
from typing import Union
from .lexer import Token


class ParseError(Exception):
    """Raised on malformed input."""


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

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


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

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


@dataclass
class Unary:
    op: str
    operand: object

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


_KIND_TO_OP = {
    'PLUS': '+',
    'MINUS': '-',
    'STAR': '*',
    'SLASH': '/',
}


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 = None) -> Token:
        tok = self._tokens[self._pos]
        if kind is not None and tok.kind != kind:
            raise ParseError(f"expected {kind!r}, got {tok.kind!r}")
        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':
            raise ParseError(f"unexpected token {self._peek().kind!r}")
        return node

    def _expr(self):
        node = self._term()
        while self._peek().kind in ('PLUS', 'MINUS'):
            op = _KIND_TO_OP[self._consume().kind]
            right = self._term()
            node = BinOp(op, node, right)
        return node

    def _term(self):
        node = self._unary()
        while self._peek().kind in ('STAR', 'SLASH'):
            op = _KIND_TO_OP[self._consume().kind]
            right = self._unary()
            node = BinOp(op, node, right)
        return node

    def _unary(self):
        if self._peek().kind == 'MINUS':
            self._consume()
            return Unary('-', self._unary())
        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()
            if self._peek().kind != 'RPAREN':
                raise ParseError("unclosed parenthesis, expected ')'")
            self._consume()
            return node
        if tok.kind == 'EOF':
            raise ParseError("unexpected end of input")
        raise ParseError(f"unexpected token {tok.kind!r}")


def parse(tokens: list):
    """Parse a token list into an AST.

    Returns:
        Num | BinOp | Unary — root node.

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