# obsutil.py - utility functions for obsolescence
#
# Copyright 2017 Boris Feld <boris.feld@octobus.net>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

from __future__ import absolute_import

def closestpredecessors(repo, nodeid):
    """yield the list of next predecessors pointing on visible changectx nodes

    This function respect the repoview filtering, filtered revision will be
    considered missing.
    """

    precursors = repo.obsstore.precursors
    stack = [nodeid]
    seen = set(stack)

    while stack:
        current = stack.pop()
        currentpreccs = precursors.get(current, ())

        for prec in currentpreccs:
            precnodeid = prec[0]

            # Basic cycle protection
            if precnodeid in seen:
                continue
            seen.add(precnodeid)

            if precnodeid in repo:
                yield precnodeid
            else:
                stack.append(precnodeid)

def successorssets(repo, initialnode, cache=None):
    """Return set of all latest successors of initial nodes

    The successors set of a changeset A are the group of revisions that succeed
    A. It succeeds A as a consistent whole, each revision being only a partial
    replacement. The successors set contains non-obsolete changesets only.

    This function returns the full list of successor sets which is why it
    returns a list of tuples and not just a single tuple. Each tuple is a valid
    successors set. Note that (A,) may be a valid successors set for changeset A
    (see below).

    In most cases, a changeset A will have a single element (e.g. the changeset
    A is replaced by A') in its successors set. Though, it is also common for a
    changeset A to have no elements in its successor set (e.g. the changeset
    has been pruned). Therefore, the returned list of successors sets will be
    [(A',)] or [], respectively.

    When a changeset A is split into A' and B', however, it will result in a
    successors set containing more than a single element, i.e. [(A',B')].
    Divergent changesets will result in multiple successors sets, i.e. [(A',),
    (A'')].

    If a changeset A is not obsolete, then it will conceptually have no
    successors set. To distinguish this from a pruned changeset, the successor
    set will contain itself only, i.e. [(A,)].

    Finally, successors unknown locally are considered to be pruned (obsoleted
    without any successors).

    The optional `cache` parameter is a dictionary that may contain precomputed
    successors sets. It is meant to reuse the computation of a previous call to
    `successorssets` when multiple calls are made at the same time. The cache
    dictionary is updated in place. The caller is responsible for its life
    span. Code that makes multiple calls to `successorssets` *must* use this
    cache mechanism or suffer terrible performance.
    """

    succmarkers = repo.obsstore.successors

    # Stack of nodes we search successors sets for
    toproceed = [initialnode]
    # set version of above list for fast loop detection
    # element added to "toproceed" must be added here
    stackedset = set(toproceed)
    if cache is None:
        cache = {}

    # This while loop is the flattened version of a recursive search for
    # successors sets
    #
    # def successorssets(x):
    #    successors = directsuccessors(x)
    #    ss = [[]]
    #    for succ in directsuccessors(x):
    #        # product as in itertools cartesian product
    #        ss = product(ss, successorssets(succ))
    #    return ss
    #
    # But we can not use plain recursive calls here:
    # - that would blow the python call stack
    # - obsolescence markers may have cycles, we need to handle them.
    #
    # The `toproceed` list act as our call stack. Every node we search
    # successors set for are stacked there.
    #
    # The `stackedset` is set version of this stack used to check if a node is
    # already stacked. This check is used to detect cycles and prevent infinite
    # loop.
    #
    # successors set of all nodes are stored in the `cache` dictionary.
    #
    # After this while loop ends we use the cache to return the successors sets
    # for the node requested by the caller.
    while toproceed:
        # Every iteration tries to compute the successors sets of the topmost
        # node of the stack: CURRENT.
        #
        # There are four possible outcomes:
        #
        # 1) We already know the successors sets of CURRENT:
        #    -> mission accomplished, pop it from the stack.
        # 2) Node is not obsolete:
        #    -> the node is its own successors sets. Add it to the cache.
        # 3) We do not know successors set of direct successors of CURRENT:
        #    -> We add those successors to the stack.
        # 4) We know successors sets of all direct successors of CURRENT:
        #    -> We can compute CURRENT successors set and add it to the
        #       cache.
        #
        current = toproceed[-1]
        if current in cache:
            # case (1): We already know the successors sets
            stackedset.remove(toproceed.pop())
        elif current not in succmarkers:
            # case (2): The node is not obsolete.
            if current in repo:
                # We have a valid last successors.
                cache[current] = [(current,)]
            else:
                # Final obsolete version is unknown locally.
                # Do not count that as a valid successors
                cache[current] = []
        else:
            # cases (3) and (4)
            #
            # We proceed in two phases. Phase 1 aims to distinguish case (3)
            # from case (4):
            #
            #     For each direct successors of CURRENT, we check whether its
            #     successors sets are known. If they are not, we stack the
            #     unknown node and proceed to the next iteration of the while
            #     loop. (case 3)
            #
            #     During this step, we may detect obsolescence cycles: a node
            #     with unknown successors sets but already in the call stack.
            #     In such a situation, we arbitrary set the successors sets of
            #     the node to nothing (node pruned) to break the cycle.
            #
            #     If no break was encountered we proceed to phase 2.
            #
            # Phase 2 computes successors sets of CURRENT (case 4); see details
            # in phase 2 itself.
            #
            # Note the two levels of iteration in each phase.
            # - The first one handles obsolescence markers using CURRENT as
            #   precursor (successors markers of CURRENT).
            #
            #   Having multiple entry here means divergence.
            #
            # - The second one handles successors defined in each marker.
            #
            #   Having none means pruned node, multiple successors means split,
            #   single successors are standard replacement.
            #
            for mark in sorted(succmarkers[current]):
                for suc in mark[1]:
                    if suc not in cache:
                        if suc in stackedset:
                            # cycle breaking
                            cache[suc] = []
                        else:
                            # case (3) If we have not computed successors sets
                            # of one of those successors we add it to the
                            # `toproceed` stack and stop all work for this
                            # iteration.
                            toproceed.append(suc)
                            stackedset.add(suc)
                            break
                else:
                    continue
                break
            else:
                # case (4): we know all successors sets of all direct
                # successors
                #
                # Successors set contributed by each marker depends on the
                # successors sets of all its "successors" node.
                #
                # Each different marker is a divergence in the obsolescence
                # history. It contributes successors sets distinct from other
                # markers.
                #
                # Within a marker, a successor may have divergent successors
                # sets. In such a case, the marker will contribute multiple
                # divergent successors sets. If multiple successors have
                # divergent successors sets, a Cartesian product is used.
                #
                # At the end we post-process successors sets to remove
                # duplicated entry and successors set that are strict subset of
                # another one.
                succssets = []
                for mark in sorted(succmarkers[current]):
                    # successors sets contributed by this marker
                    markss = [[]]
                    for suc in mark[1]:
                        # cardinal product with previous successors
                        productresult = []
                        for prefix in markss:
                            for suffix in cache[suc]:
                                newss = list(prefix)
                                for part in suffix:
                                    # do not duplicated entry in successors set
                                    # first entry wins.
                                    if part not in newss:
                                        newss.append(part)
                                productresult.append(newss)
                        markss = productresult
                    succssets.extend(markss)
                # remove duplicated and subset
                seen = []
                final = []
                candidate = sorted(((set(s), s) for s in succssets if s),
                                   key=lambda x: len(x[1]), reverse=True)
                for setversion, listversion in candidate:
                    for seenset in seen:
                        if setversion.issubset(seenset):
                            break
                    else:
                        final.append(listversion)
                        seen.append(setversion)
                final.reverse() # put small successors set first
                cache[current] = final
    return cache[initialnode]