# Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
# Licensed under the Apache License, Version 2.0 (see LICENSE).

import logging
from abc import ABC, abstractmethod

logger = logging.getLogger(__name__)


class DefaultFingerprintHashingMixin:
    """Default definitions for __hash__ and __eq__.

    Warning: Don't use this when the mixed in class has instance attributes mixed into its
    fingerprints.  This will cause subtle bugs because fingerprints are cached on the Target
    base class, and the cache key is the instance of the FingerprintStrategy.
    """

    def __hash__(self):
        return hash(type(self))

    def __eq__(self, other):
        return type(self) == type(other)


class FingerprintStrategy(ABC):
    """A helper object for doing per-task, finer grained invalidation of Targets."""

    @abstractmethod
    def compute_fingerprint(self, target):
        """Subclasses override this method to actually compute the Task specific fingerprint."""

    def fingerprint_target(self, target):
        """Consumers of subclass instances call this to get a fingerprint labeled with the name."""
        fingerprint = self.compute_fingerprint(target)
        if fingerprint:
            return "{fingerprint}-{name}".format(fingerprint=fingerprint, name=type(self).__name__)
        else:
            return None

    def direct(self, target):
        return False

    def dependencies(self, target):
        return target.dependencies

    @abstractmethod
    def __hash__(self):
        """Subclasses must implement a hash so computed fingerprints can be safely memoized."""

    @abstractmethod
    def __eq__(self, other):
        """Subclasses must implement an equality check so computed fingerprints can be safely
        memoized."""


class DefaultFingerprintStrategy(DefaultFingerprintHashingMixin, FingerprintStrategy):
    """The default FingerprintStrategy, which delegates to target.payload.fingerprint().

    :API: public
    """

    def compute_fingerprint(self, target):
        """
        :API: public
        """
        return target.payload.fingerprint()