"""Recursive-descent parser for arithmetic expressions.

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

AST node shapes:
    Num(value)                 – leaf; value is int or float
    BinOp(op, left, right)     – op is '+', '-', '*', or '/'
    Unary(op, operand)         – op is '-'
"""
from __future__ import annotations
from dataclasses import dataclass
from typing import List, 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, kind: str) -> Token:
        tok = self._peek()
        if tok.kind != kind:
            raise ParseError(
                f"expected {kind!r} but got {tok.kind!r} (value={tok.value!r})"
            )
        self._pos += 1
        return tok

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

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

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

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

    # primary ::= NUMBER | '(' expr ')'
    def _primary(self) -> Node:
        tok = self._peek()
        if tok.kind == 'NUMBER':
            self._advance()
            return Num(tok.value)
        if tok.kind == 'LPAREN':
            self._advance()
            node = self._expr()
            self._consume('RPAREN')
            return node
        if tok.kind == 'EOF':
            raise ParseError("unexpected end of input")
        raise ParseError(f"unexpected token {tok.kind!r} (value={tok.value!r})")

    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 after expression: {tok.kind!r} (value={tok.value!r})"
            )
        return node


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