# Copyright (c) 2011 Edward Langley
# All rights reserved.
# 
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions
# are met:
# 
# Redistributions of source code must retain the above copyright notice,
# this list of conditions and the following disclaimer.
# 
# Redistributions in binary form must reproduce the above copyright
# notice, this list of conditions and the following disclaimer in the
# documentation and/or other materials provided with the distribution.
# 
# Neither the name of the project's author nor the names of its
# contributors may be used to endorse or promote products derived from
# this software without specific prior written permission.
# 
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
# TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
# PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
# LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
# NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
# SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

import functools

def memoize(func):
    _cache = {}
    class NULL:pass
    NULL = NULL()
    @functools.wraps(func)
    def _inner(*args, **kwargs):
        _kwargs = tuple(
            (k, tuple(v) if hasattr(v, '__iter__') else v) for k,v in kwargs
        )
        value = _cache.get((args, _kwargs), NULL)
        if value is NULL:
            value = func(*args, **kwargs)
            _cache[(args,_kwargs)] = value
        return value
    _inner.__enter__ = lambda *_: _inner
    _inner.__exit__ = lambda *_: _cache.clear()
    _inner.reset = _cache.clear
    _inner.cache = _cache
    _inner.orig = func
    return _inner

class return_if(object):
    def __init__(self, map):
        self._map = dict(
            (tuple(k) if hasattr(k, '__iter__') else (k,),v) for k,v in map.iteritems()
        )
        self._func = lambda x:x
    def _inner(self, *args):
        if args in self._map: return self._map[args]
        else: return self._func(*args)
    def __call__(self, func):
        self._func = func
        return self._inner


@memoize
@return_if({1: 1})
def fact(n):
    return n*fact(n-1)


@return_if({(1,1): 2, (2,2): 3, (3,3):4})
def sum(a,b):
    return a+b


@memoize
@return_if({1: 1,2: 1})
def fib(n):
    if n < 1: raise ValueError, "not n >= 1"
    else:
        return fib(n-1) + fib(n-2)