import xtokens as t
import xlexer as xl

from utils import Backtrackable

def parse(char_stream):
    tokens = Backtrackable(xl.lex(char_stream))
    _list(tokens)
    _match(tokens, t.Eof)

def _test(tokens, f):
    with tokens.backtrack_always():
        try:
            f(tokens)
        except ValueError:
            return False
    return True

def _element(tokens):
    if _test(tokens, _assign):
        _assign(tokens)
    elif _test(tokens, _name):
        _name(tokens)
    elif _test(tokens, _assign_list):
        _assign_list(tokens)
    elif _test(tokens, _list):
        _list(tokens)
    else:
        raise ValueError(f"Invalid token {tokens[0]}")

def _list(tokens):
    _match(tokens, t.Lbrack)
    _elements(tokens)
    _match(tokens, t.Rbrack)

def _elements(tokens):
    _element(tokens)
    while tokens[0] == t.Comma():
        next(tokens)
        _element(tokens)

def _assign(tokens):
    _match(tokens, t.Ident)
    _match(tokens, t.Equal)
    _match(tokens, t.Ident)

def _assign_list(tokens):
    _list(tokens)
    _match(tokens, t.Equal)
    _list(tokens)

def _name(tokens):
    _match(tokens, t.Ident)

def _match(tokens, token_type):
    if isinstance(tokens[0], token_type):
        next(tokens)
    else:
        raise ValueError(f"Failed to match {tokens[0]} to type {token_type}")