""" Generic SymPy-Independent Strategies """

identity = lambda x: x

def exhaust(rule):
    """ Apply a rule repeatedly until it has no effect """
    def exhaustive_rl(expr):
        new, old = rule(expr), expr
        while new != old:
            new, old = rule(new), new
        return new
    return exhaustive_rl

def memoize(rule):
    """ Memoized version of a rule """
    cache = {}
    def memoized_rl(expr):
        if expr in cache:
            return cache[expr]
        else:
            result = rule(expr)
            cache[expr] = result
            return result
    return memoized_rl

def condition(cond, rule):
    """ Only apply rule if condition is true """
    def conditioned_rl(expr):
        if cond(expr):
            return rule(expr)
        else:
            return expr
    return conditioned_rl

def chain(*rules):
    """
    Compose a sequence of rules so that they apply to the expr sequentially
    """
    def chain_rl(expr):
        for rule in rules:
            expr = rule(expr)
        return expr
    return chain_rl

def debug(rule, file=None):
    """ Print out before and after expressions each time rule is used """
    if file is None:
        from sys import stdout
        file = stdout
    def debug_rl(*args, **kwargs):
        expr = args[0]
        result = rule(*args, **kwargs)
        if result != expr:
            file.write("Rule: %s\n" % rule.__name__)
            file.write("In:   %s\nOut:  %s\n\n"%(expr, result))
        return result
    return debug_rl

def null_safe(rule):
    """ Return original expr if rule returns None """
    def null_safe_rl(expr):
        result = rule(expr)
        if result is None:
            return expr
        else:
            return result
    return null_safe_rl

def tryit(rule, exception):
    """ Return original expr if rule raises exception """
    def try_rl(expr):
        try:
            return rule(expr)
        except exception:
            return expr
    return try_rl

def do_one(*rules):
    """ Try each of the rules until one works. Then stop. """
    def do_one_rl(expr):
        for rl in rules:
            result = rl(expr)
            if result != expr:
                return result
        return expr
    return do_one_rl

def switch(key, ruledict):
    """ Select a rule based on the result of key called on the function """
    def switch_rl(expr):
        rl = ruledict.get(key(expr), identity)
        return rl(expr)
    return switch_rl

def minimize(*rules, objective=identity):
    """ Select result of rules that minimizes objective

    >>> from sympy.strategies import minimize
    >>> inc = lambda x: x + 1
    >>> dec = lambda x: x - 1
    >>> rl = minimize(inc, dec)
    >>> rl(4)
    3

    >>> rl = minimize(inc, dec, objective=lambda x: -x)  # maximize
    >>> rl(4)
    5
    """

    def minrule(expr):
        return min([rule(expr) for rule in rules], key=objective)
    return minrule
