"""Recursive-descent parser for arithmetic expressions.

Grammar (precedence low→high):
  expr   ::= term   ( ('+' | '-') term   )*
  term   ::= factor ( ('*' | '/') factor )*
  factor ::= NUMBER | '(' expr ')' | '-' factor

AST node shapes (all dataclasses):
  Num(value)              — numeric leaf; value is int or float
  BinOp(op, left, right)  — op in ('+', '-', '*', '/'), children are nodes
  Unary(op, operand)      — op is '-', operand is a node
"""

from dataclasses import dataclass
from typing import Any

from calc.lexer import Token, tokenize


class ParseError(Exception):
    pass


@dataclass
class Num:
    value: Any

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


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

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


@dataclass
class Unary:
    op: str
    operand: Any

    def __repr__(self):
        return f"Unary(op={self.op!r}, operand={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) -> 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} but got {tok.kind!r}")
        return self._consume()

    def parse(self):
        node = self._expr()
        if self._peek().kind != 'EOF':
            raise ParseError(f"Unexpected token {self._peek()!r}")
        return node

    def _expr(self):
        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 = self._factor()
        while self._peek().kind in ('STAR', 'SLASH'):
            op = self._consume().value
            right = self._factor()
            node = BinOp(op, node, right)
        return node

    def _factor(self):
        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 == 'MINUS':
            self._consume()
            operand = self._factor()
            return Unary('-', operand)

        if tok.kind == 'EOF':
            raise ParseError("Unexpected end of input")

        raise ParseError(f"Unexpected token {tok!r}")


def parse(tokens: list):
    """Parse a token list (from tokenize()) into an AST. Raises ParseError on bad input."""
    return _Parser(tokens).parse()