| # Copyright 2016 The Chromium Authors. All rights reserved. |
| # Use of this source code is governed by a BSD-style license that can be |
| # found in the LICENSE file. |
| |
| |
| class Function(object): |
| """Base class for mathematical functions. |
| |
| The ``callable`` interface is sufficient for when you only ever need to |
| invoke a function. But many times we want to have more information about |
| the function, such as getting its domain or range or knowing whether |
| it's sparse. In addition, we often want to adjust the computational |
| representation of functions (e.g., adding memoization). So this class |
| provides a base class for functions supporting all these sorts of |
| operations in addition to being callable. |
| """ |
| |
| def __init__(self, f): |
| self._f = f |
| |
| def __call__(self, x): |
| return self._f(x) |
| |
| def map(self, g): |
| """Returns a new function that applies ``g`` after ``self``. |
| |
| Args: |
| g (callable): the function to post-compose. |
| |
| Returns: |
| An object of the same type as ``self`` which computes ``lambda x: |
| g(self(x))``. N.B., although mathematically we have the equivalence: |
| ``SomeFunction(f).map(g) == SomeFunction(lambda x: g(f(x)))``; |
| operationally the left- and right-hand sides may differ. For |
| example, with the ``MemoizedFunction`` class, the left-hand side |
| will memoize the intermediate ``f(x)`` values whereas the right-hand |
| side will not. |
| """ |
| return self.__class__(lambda x: g(self(x))) |
| |
| |
| class MemoizedFunction(Function): |
| """A function which memoizes its value for all arguments.""" |
| |
| def __init__(self, f): |
| super(MemoizedFunction, self).__init__(f) |
| self._memos = {} |
| |
| def ClearMemos(self): |
| """Discard all memoized results of this function.""" |
| self._memos = {} |
| |
| def __call__(self, x): |
| try: |
| return self._memos[x] |
| except KeyError: |
| fx = self._f(x) |
| self._memos[x] = fx |
| return fx |
| except TypeError: |
| # Not to memorize when x is not hashable. |
| return self._f(x) |