/src/tail_recursion.py

'''
Support for unlimited tail recursion for Python.

@author: Pedro Matiello <pmatiello@gmail.com>
'''

from sys import _getframe as getframe
from dis import opmap

OPCODES = [chr(opmap['RETURN_VALUE']), chr(opmap['POP_TOP'])]

class tail_recursive(object):
    """
    Tail recursion decorator.
    """

    def __init__(self, function):
        self.function = function
        
    def _is_tailcall(self, frame):
        """
        Analyze bytecode to verify whether the previous call is a tail call.
        
        References:
            1. http://docs.python.org/release/2.7/library/dis.html#bytecodes
            2. http://lambda-the-ultimate.org/node/1331#comment-15183
        """
        caller_frame = frame.f_back
        code = caller_frame.f_code.co_code
        ip = caller_frame.f_lasti
        return code[ip + 3] in OPCODES


    def __call__(self, *args, **kwargs):
        
        frame = getframe()
        
        if frame.f_back and frame.f_back.f_back and frame.f_back.f_back.f_code == frame.f_code and self._is_tailcall(frame):
            return _continue(self.function, *args, **kwargs)
        
        retval = self.function(*args, **kwargs)
        while (True):
            if (type(retval) == _continue):
                retval = retval.func(*retval.args, **retval.kwargs)
            else:
                return retval


class _continue(object):
    
    def __init__(self, func, *args, **kwargs):
        self.func = func
        self.args = args
        self.kwargs = kwargs