"""
Recursive-descent parser for the calc expression grammar.

Grammar:
    expr    = term   ( ('+' | '-') term   )*
    term    = unary  ( ('*' | '/') unary  )*
    unary   = '-' unary | primary
    primary = NUMBER | '(' expr ')'

AST node shapes (stable contract for the evaluator):
    Num(value)             — numeric literal; .value is int or float
    BinOp(op, left, right) — binary operation; .op is '+', '-', '*', or '/'
    Unary(op, operand)     — unary prefix; .op is '-'
"""


class ParseError(Exception):
    pass


# ---------------------------------------------------------------------------
# AST nodes
# ---------------------------------------------------------------------------

class Num:
    __slots__ = ('value',)

    def __init__(self, value):
        self.value = value

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

    def __eq__(self, other):
        return isinstance(other, Num) and self.value == other.value


class BinOp:
    __slots__ = ('op', 'left', 'right')

    def __init__(self, op: str, left, right):
        self.op = op
        self.left = left
        self.right = right

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

    def __eq__(self, other):
        return (isinstance(other, BinOp)
                and self.op == other.op
                and self.left == other.left
                and self.right == other.right)


class Unary:
    __slots__ = ('op', 'operand')

    def __init__(self, op: str, operand):
        self.op = op
        self.operand = operand

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

    def __eq__(self, other):
        return (isinstance(other, Unary)
                and self.op == other.op
                and self.operand == other.operand)


# ---------------------------------------------------------------------------
# Parser
# ---------------------------------------------------------------------------

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

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

    def _advance(self):
        tok = self._tokens[self._pos]
        self._pos += 1
        return tok

    def _expect(self, kind):
        tok = self._peek()
        if tok.kind != kind:
            raise ParseError(
                f"Expected {kind}, got {tok.kind!r} ({tok.value!r})"
            )
        return self._advance()

    # expr = term ( ('+' | '-') term )*
    def _expr(self):
        node = self._term()
        while self._peek().kind in ('PLUS', 'MINUS'):
            op = self._advance().value
            node = BinOp(op, node, self._term())
        return node

    # term = unary ( ('*' | '/') unary )*
    def _term(self):
        node = self._unary()
        while self._peek().kind in ('STAR', 'SLASH'):
            op = self._advance().value
            node = BinOp(op, node, self._unary())
        return node

    # unary = '-' unary | primary
    def _unary(self):
        if self._peek().kind == 'MINUS':
            self._advance()
            return Unary('-', self._unary())
        return self._primary()

    # primary = NUMBER | '(' expr ')'
    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._expect('RPAREN')
            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(self):
        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 after expression: {tok.kind!r} ({tok.value!r})"
            )
        return node


def parse(tokens) -> object:
    """Parse a token list produced by `calc.lexer.tokenize` into an AST."""
    return _Parser(tokens).parse()