"""Recursive-descent parser for arithmetic expressions.

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

Grammar (encodes precedence and left-associativity):
  expr    → term   (('+' | '-') term)*
  term    → unary  (('*' | '/') unary)*
  unary   → '-' unary | primary
  primary → NUMBER | '(' expr ')'
"""

from __future__ import annotations
from typing import Union


class ParseError(Exception):
    pass


class Num:
    __slots__ = ("value",)

    def __init__(self, value: Union[int, float]) -> None:
        self.value = value

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

    def __eq__(self, other: object) -> bool:
        return isinstance(other, Num) and self.value == other.value


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

    def __init__(self, op: str, left: "Node", right: "Node") -> None:
        self.op = op
        self.left = left
        self.right = right

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

    def __eq__(self, other: object) -> bool:
        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: "Node") -> None:
        self.op = op
        self.operand = operand

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

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


Node = Union[Num, BinOp, Unary]


class _Parser:
    def __init__(self, tokens: list) -> None:
        self._tokens = tokens
        self._pos = 0

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

    def _consume(self, kind: str = None):
        tok = self._tokens[self._pos]
        if kind is not None and tok.kind != kind:
            raise ParseError(
                f"expected {kind}, got {tok.kind!r} ({tok.value!r})"
            )
        self._pos += 1
        return tok

    def parse(self) -> Node:
        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) -> 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
            operand = self._unary()
            return Unary(op, operand)
        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("LPAREN")
            node = self._expr()
            if self._peek().kind != "RPAREN":
                raise ParseError("unclosed parenthesis — expected ')'")
            self._consume("RPAREN")
            return node
        if tok.kind == "EOF":
            raise ParseError("unexpected end of expression")
        raise ParseError(f"unexpected token {tok.kind!r} ({tok.value!r})")


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