"""Recursive-descent parser for calc expressions.

AST node shapes (all are dataclasses with __repr__):
  Num(value: int|float)
  BinOp(op: str, left: Node, right: Node)   -- op in {'+','-','*','/'}
  Unary(op: str, operand: Node)              -- op == '-'

Grammar:
  expr   ::= term   (( '+' | '-' ) term)*
  term   ::= unary  (( '*' | '/' ) unary)*
  unary  ::= '-' unary | primary
  primary::= NUMBER | '(' expr ')'
"""
from __future__ import annotations
from dataclasses import dataclass
from typing import Union

from calc.lexer import Token


class ParseError(Exception):
    pass


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

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

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


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

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


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

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


Node = Union[Num, BinOp, Unary]


# ---------- parser ----------

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

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

    def _consume(self) -> Token:
        tok = self._tokens[self._pos]
        self._pos += 1
        return tok

    def _expect(self, kind: str) -> Token:
        tok = self._peek()
        if tok.kind != kind:
            raise ParseError(
                f"expected {kind!r} but got {tok.kind!r} ({tok.value!r})"
            )
        return self._consume()

    def parse(self) -> Node:
        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 {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()
            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(tokens: list[Token]) -> Node:
    """Parse a list of tokens into an AST. Raises ParseError on malformed input."""
    return _Parser(tokens).parse()