"""Recursive-descent parser for arithmetic expressions.

AST node types:
  Num(value)              -- a numeric literal (int or float)
  BinOp(op, left, right)  -- binary operation; op in ('+','-','*','/')
  Unary(op, operand)      -- unary operation; op == '-'

Grammar (precedence low → high):
  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, tokenize


class ParseError(Exception):
    """Raised on syntactically invalid input."""


# ---------------------------------------------------------------------------
# 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, kind: str | None = None) -> Token:
        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 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_tok = self._consume()
            op = op_tok.value  # '+' or '-'
            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_tok = self._consume()
            op = op_tok.value  # '*' or '/'
            right = self._unary()
            node = BinOp(op, node, right)
        return node

    def _unary(self) -> Node:
        if self._peek().kind == 'MINUS':
            self._consume('MINUS')
            operand = self._unary()
            return Unary('-', operand)
        return self._primary()

    def _primary(self) -> Node:
        tok = self._peek()
        if tok.kind == 'NUMBER':
            self._consume('NUMBER')
            return Num(tok.value)
        if tok.kind == 'LPAREN':
            self._consume('LPAREN')
            node = self._expr()
            if self._peek().kind != 'RPAREN':
                raise ParseError(
                    f"Expected ')' but got {self._peek().kind!r}"
                )
            self._consume('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 *tokens* (from lexer.tokenize) into an AST Node.

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