/*
 * Copyright (c) 2005, Jon Seymour
 *
 * For more information about epoch theory on which this module is based,
 * refer to http://blackcubes.dyndns.org/epoch/. That web page defines
 * terms such as "epoch" and "minimal, non-linear epoch" and provides rationales
 * for some of the algorithms used here.
 *
 */
#include <stdlib.h>
#include <openssl/bn.h>         // provides arbitrary precision integers
                                // required to accurately represent fractional 
                                //mass

#include "cache.h"
#include "commit.h"
#include "epoch.h"

struct fraction {
        BIGNUM numerator;
        BIGNUM denominator;
};

#define HAS_EXACTLY_ONE_PARENT(n) ((n)->parents && !(n)->parents->next)

static BN_CTX *context = NULL;
static struct fraction *one = NULL;
static struct fraction *zero = NULL;

static BN_CTX *get_BN_CTX()
{
        if (!context) {
                context = BN_CTX_new();
        }
        return context;
}

static struct fraction *new_zero()
{
        struct fraction *result = xmalloc(sizeof(*result));
        BN_init(&result->numerator);
        BN_init(&result->denominator);
        BN_zero(&result->numerator);
        BN_one(&result->denominator);
        return result;
}

static void clear_fraction(struct fraction *fraction)
{
        BN_clear(&fraction->numerator);
        BN_clear(&fraction->denominator);
}

static struct fraction *divide(struct fraction *result, struct fraction *fraction, int divisor)
{
        BIGNUM bn_divisor;

        BN_init(&bn_divisor);
        BN_set_word(&bn_divisor, divisor);

        BN_copy(&result->numerator, &fraction->numerator);
        BN_mul(&result->denominator, &fraction->denominator, &bn_divisor, get_BN_CTX());

        BN_clear(&bn_divisor);
        return result;
}

static struct fraction *init_fraction(struct fraction *fraction)
{
        BN_init(&fraction->numerator);
        BN_init(&fraction->denominator);
        BN_zero(&fraction->numerator);
        BN_one(&fraction->denominator);
        return fraction;
}

static struct fraction *get_one()
{
        if (!one) {
                one = new_zero();
                BN_one(&one->numerator);
        }
        return one;
}

static struct fraction *get_zero()
{
        if (!zero) {
                zero = new_zero();
        }
        return zero;
}

static struct fraction *copy(struct fraction *to, struct fraction *from)
{
        BN_copy(&to->numerator, &from->numerator);
        BN_copy(&to->denominator, &from->denominator);
        return to;
}

static struct fraction *add(struct fraction *result, struct fraction *left, struct fraction *right)
{
        BIGNUM a, b, gcd;

        BN_init(&a);
        BN_init(&b);
        BN_init(&gcd);

        BN_mul(&a, &left->numerator, &right->denominator, get_BN_CTX());
        BN_mul(&b, &left->denominator, &right->numerator, get_BN_CTX());
        BN_mul(&result->denominator, &left->denominator, &right->denominator, get_BN_CTX());
        BN_add(&result->numerator, &a, &b);

        BN_gcd(&gcd, &result->denominator, &result->numerator, get_BN_CTX());
        BN_div(&result->denominator, NULL, &result->denominator, &gcd, get_BN_CTX());
        BN_div(&result->numerator, NULL, &result->numerator, &gcd, get_BN_CTX());

        BN_clear(&a);
        BN_clear(&b);
        BN_clear(&gcd);

        return result;
}

static int compare(struct fraction *left, struct fraction *right)
{
        BIGNUM a, b;

        int result;

        BN_init(&a);
        BN_init(&b);

        BN_mul(&a, &left->numerator, &right->denominator, get_BN_CTX());
        BN_mul(&b, &left->denominator, &right->numerator, get_BN_CTX());

        result = BN_cmp(&a, &b);

        BN_clear(&a);
        BN_clear(&b);

        return result;
}

struct mass_counter {
        struct fraction seen;
        struct fraction pending;
};

static struct mass_counter *new_mass_counter(struct commit *commit, struct fraction *pending)
{
        struct mass_counter *mass_counter = xmalloc(sizeof(*mass_counter));
        memset(mass_counter, 0, sizeof(*mass_counter));

        init_fraction(&mass_counter->seen);
        init_fraction(&mass_counter->pending);

        copy(&mass_counter->pending, pending);
        copy(&mass_counter->seen, get_zero());

        if (commit->object.util) {
                die("multiple attempts to initialize mass counter for %s\n", sha1_to_hex(commit->object.sha1));
        }

        commit->object.util = mass_counter;

        return mass_counter;
}

static void free_mass_counter(struct mass_counter *counter)
{
        clear_fraction(&counter->seen);
        clear_fraction(&counter->pending);
        free(counter);
}

//
// Finds the base commit of a list of commits.
//
// One property of the commit being searched for is that every commit reachable 
// from the base commit is reachable from the commits in the starting list only 
// via paths that include the base commit.
//
// This algorithm uses a conservation of mass approach to find the base commit.
//
// We start by injecting one unit of mass into the graph at each
// of the commits in the starting list. Injecting mass into a commit
// is achieved by adding to its pending mass counter and, if it is not already
// enqueued, enqueuing the commit in a list of pending commits, in latest 
// commit date first order.
//
// The algorithm then preceeds to visit each commit in the pending queue.
// Upon each visit, the pending mass is added to the mass already seen for that 
// commit and then divided into N equal portions, where N is the number of 
// parents of the commit being visited. The divided portions are then injected 
// into each of the parents.
//
// The algorithm continues until we discover a commit which has seen all the
// mass originally injected or until we run out of things to do. 
//
// If we find a commit that has seen all the original mass, we have found
// the common base of all the commits in the starting list. 
//
// The algorithm does _not_ depend on accurate timestamps for correct operation.
// However, reasonably sane (e.g. non-random) timestamps are required in order 
// to prevent an exponential performance characteristic. The occasional 
// timestamp inaccuracy will not dramatically affect performance but may 
// result in more nodes being processed than strictly necessary.
//
// This procedure sets *boundary to the address of the base commit. It returns 
// non-zero if, and only if, there was a problem parsing one of the 
// commits discovered during the traversal.
//
static int find_base_for_list(struct commit_list *list, struct commit **boundary)
{

        int ret = 0;

        struct commit_list *cleaner = NULL;
        struct commit_list *pending = NULL;

        *boundary = NULL;

        struct fraction injected;

        init_fraction(&injected);

        for (; list; list = list->next) {

                struct commit *item = list->item;

                if (item->object.util || (item->object.flags & UNINTERESTING)) {
                        die("%s:%d:%s: logic error: this should not have happened - commit %s\n",
                            __FILE__, __LINE__, __FUNCTION__, sha1_to_hex(item->object.sha1));
                }

                new_mass_counter(list->item, get_one());
                add(&injected, &injected, get_one());

                commit_list_insert(list->item, &cleaner);
                commit_list_insert(list->item, &pending);
        }

        while (!*boundary && pending && !ret) {

                struct commit *latest = pop_commit(&pending);

                struct mass_counter *latest_node = (struct mass_counter *) latest->object.util;

                if ((ret = parse_commit(latest)))
                        continue;

                add(&latest_node->seen, &latest_node->seen, &latest_node->pending);

                int num_parents = count_parents(latest);

                if (num_parents) {

                        struct fraction distribution;
                        struct commit_list *parents;

                        divide(init_fraction(&distribution), &latest_node->pending, num_parents);

                        for (parents = latest->parents; parents; parents = parents->next) {

                                struct commit *parent = parents->item;
                                struct mass_counter *parent_node = (struct mass_counter *) parent->object.util;

                                if (!parent_node) {

                                        parent_node = new_mass_counter(parent, &distribution);

                                        insert_by_date(&pending, parent);
                                        commit_list_insert(parent, &cleaner);

                                } else {

                                        if (!compare(&parent_node->pending, get_zero())) {
                                                insert_by_date(&pending, parent);
                                        }
                                        add(&parent_node->pending, &parent_node->pending, &distribution);

                                }
                        }

                        clear_fraction(&distribution);

                }

                if (!compare(&latest_node->seen, &injected)) {
                        *boundary = latest;
                }

                copy(&latest_node->pending, get_zero());

        }

        while (cleaner) {

                struct commit *next = pop_commit(&cleaner);
                free_mass_counter((struct mass_counter *) next->object.util);
                next->object.util = NULL;

        }

        if (pending)
                free_commit_list(pending);

        clear_fraction(&injected);

        return ret;

}


//
// Finds the base of an minimal, non-linear epoch, headed at head, by
// applying the find_base_for_list to a list consisting of the parents
//
static int find_base(struct commit *head, struct commit **boundary)
{
        int ret = 0;
        struct commit_list *pending = NULL;
        struct commit_list *next;

        commit_list_insert(head, &pending);
        for (next = head->parents; next; next = next->next) {
                commit_list_insert(next->item, &pending);
        }
        ret = find_base_for_list(pending, boundary);
        free_commit_list(pending);

        return ret;
}

//
// This procedure traverses to the boundary of the first epoch in the epoch
// sequence of the epoch headed at head_of_epoch. This is either the end of
// the maximal linear epoch or the base of a minimal non-linear epoch.
//
// The queue of pending nodes is sorted in reverse date order and each node
// is currently in the queue at most once.
//
static int find_next_epoch_boundary(struct commit *head_of_epoch, struct commit **boundary)
{
        int ret;
        struct commit *item = head_of_epoch;

        ret = parse_commit(item);
        if (ret)
                return ret;

        if (HAS_EXACTLY_ONE_PARENT(item)) {

                // we are at the start of a maximimal linear epoch .. traverse to the end

                // traverse to the end of a maximal linear epoch
                while (HAS_EXACTLY_ONE_PARENT(item) && !ret) {
                        item = item->parents->item;
                        ret = parse_commit(item);
                }
                *boundary = item;

        } else {

                // otherwise, we are at the start of a minimal, non-linear
                // epoch - find the common base of all parents.

                ret = find_base(item, boundary);

        }

        return ret;
}

//
// Returns non-zero if parent is known to be a parent of child.
//
static int is_parent_of(struct commit *parent, struct commit *child)
{
        struct commit_list *parents;
        for (parents = child->parents; parents; parents = parents->next) {
                if (!memcmp(parent->object.sha1, parents->item->object.sha1, sizeof(parents->item->object.sha1)))
                        return 1;
        }
        return 0;
}

//
// Pushes an item onto the merge order stack. If the top of the stack is
// marked as being a possible "break", we check to see whether it actually
// is a break.
//
static void push_onto_merge_order_stack(struct commit_list **stack, struct commit *item)
{
        struct commit_list *top = *stack;
        if (top && (top->item->object.flags & DISCONTINUITY)) {
                if (is_parent_of(top->item, item)) {
                        top->item->object.flags &= ~DISCONTINUITY;
                }
        }
        commit_list_insert(item, stack);
}

//
// Marks all interesting, visited commits reachable from this commit
// as uninteresting. We stop recursing when we reach the epoch boundary,
// an unvisited node or a node that has already been marking uninteresting.
// This doesn't actually mark all ancestors between the start node and the
// epoch boundary uninteresting, but does ensure that they will 
// eventually be marked uninteresting when the main sort_first_epoch 
// traversal eventually reaches them. 
//
static void mark_ancestors_uninteresting(struct commit *commit)
{
        unsigned int flags = commit->object.flags;
        int visited = flags & VISITED;
        int boundary = flags & BOUNDARY;
        int uninteresting = flags & UNINTERESTING;

        if (uninteresting || boundary || !visited) {
                commit->object.flags |= UNINTERESTING;
                return;

                // we only need to recurse if
                //      we are not on the boundary, and,
                //      we have not already been marked uninteresting, and,
                //      we have already been visited.

                //
                // the main sort_first_epoch traverse will 
                // mark unreachable all uninteresting, unvisited parents 
                // as they are visited so there is no need to duplicate
                // that traversal here.
                //
                // similarly, if we are already marked uninteresting
                // then either all ancestors have already been marked
                // uninteresting or will be once the sort_first_epoch
                // traverse reaches them.
                //
        }

        struct commit_list *next;

        for (next = commit->parents; next; next = next->next)
                mark_ancestors_uninteresting(next->item);
}

//
// Sorts the nodes of the first epoch of the epoch sequence of the epoch headed at head
// into merge order.
//
static void sort_first_epoch(struct commit *head, struct commit_list **stack)
{
        struct commit_list *parents;
        struct commit_list *reversed_parents = NULL;

        head->object.flags |= VISITED;

        //
        // parse_commit builds the parent list in reverse order with respect to the order of
        // the git-commit-tree arguments.
        //
        // so we need to reverse this list to output the oldest (or most "local") commits last.
        //

        for (parents = head->parents; parents; parents = parents->next)
                commit_list_insert(parents->item, &reversed_parents);

        //
        // todo: by sorting the parents in a different order, we can alter the 
        // merge order to show contemporaneous changes in parallel branches
        // occurring after "local" changes. This is useful for a developer
        // when a developer wants to see all changes that were incorporated
        // into the same merge as her own changes occur after her own
        // changes.
        //

        while (reversed_parents) {

                struct commit *parent = pop_commit(&reversed_parents);

                if (head->object.flags & UNINTERESTING) {
                        // propagates the uninteresting bit to
                        // all parents. if we have already visited
                        // this parent, then the uninteresting bit
                        // will be propagated to each reachable 
                        // commit that is still not marked uninteresting
                        // and won't otherwise be reached.
                        mark_ancestors_uninteresting(parent);
                }

                if (!(parent->object.flags & VISITED)) {
                        if (parent->object.flags & BOUNDARY) {

                                if (*stack) {
                                        die("something else is on the stack - %s\n", sha1_to_hex((*stack)->item->object.sha1));
                                }

                                push_onto_merge_order_stack(stack, parent);
                                parent->object.flags |= VISITED;

                        } else {

                                sort_first_epoch(parent, stack);

                                if (reversed_parents) {
                                        //
                                        // this indicates a possible discontinuity
                                        // it may not be be actual discontinuity if
                                        // the head of parent N happens to be the tail
                                        // of parent N+1
                                        //
                                        // the next push onto the stack will resolve the 
                                        // question
                                        //
                                        (*stack)->item->object.flags |= DISCONTINUITY;
                                }
                        }
                }
        }

        push_onto_merge_order_stack(stack, head);
}

//
// Emit the contents of the stack. 
//
// The stack is freed and replaced by NULL.
//
// Sets the return value to STOP if no further output should be generated.
//
static int emit_stack(struct commit_list **stack, emitter_func emitter)
{
        unsigned int seen = 0;
        int action = CONTINUE;

        while (*stack && (action != STOP)) {

                struct commit *next = pop_commit(stack);

                seen |= next->object.flags;

                if (*stack) {
                        action = (*emitter) (next);
                }
        }

        if (*stack) {
                free_commit_list(*stack);
                *stack = NULL;
        }

        return (action == STOP || (seen & UNINTERESTING)) ? STOP : CONTINUE;
}

//
// Sorts an arbitrary epoch into merge order by sorting each epoch
// of its epoch sequence into order.
//
// Note: this algorithm currently leaves traces of its execution in the
// object flags of nodes it discovers. This should probably be fixed.
//
static int sort_in_merge_order(struct commit *head_of_epoch, emitter_func emitter)
{
        struct commit *next = head_of_epoch;
        int ret = 0;
        int action = CONTINUE;

        ret = parse_commit(head_of_epoch);

        while (next && next->parents && !ret && (action != STOP)) {

                struct commit *base = NULL;

                if ((ret = find_next_epoch_boundary(next, &base)))
                        return ret;

                next->object.flags |= BOUNDARY;
                if (base) {
                        base->object.flags |= BOUNDARY;
                }

                if (HAS_EXACTLY_ONE_PARENT(next)) {

                        while (HAS_EXACTLY_ONE_PARENT(next)
                               && (action != STOP)
                               && !ret) {

                                if (next->object.flags & UNINTERESTING) {
                                        action = STOP;
                                } else {
                                        action = (*emitter) (next);
                                }

                                if (action != STOP) {
                                        next = next->parents->item;
                                        ret = parse_commit(next);
                                }
                        }

                } else {

                        struct commit_list *stack = NULL;
                        sort_first_epoch(next, &stack);
                        action = emit_stack(&stack, emitter);
                        next = base;

                }

        }

        if (next && (action != STOP) && !ret) {
                (*emitter) (next);
        }

        return ret;
}

//
// Sorts the nodes reachable from a starting list in merge order, we 
// first find the base for the starting list and then sort all nodes in this 
// subgraph using the sort_first_epoch algorithm. Once we have reached the base
// we can continue sorting using sort_in_merge_order.
//
int sort_list_in_merge_order(struct commit_list *list, emitter_func emitter)
{
        struct commit_list *stack = NULL;
        struct commit *base;

        int ret = 0;
        int action = CONTINUE;

        struct commit_list *reversed = NULL;

        for (; list; list = list->next) {

                struct commit *next = list->item;

                if (!(next->object.flags & UNINTERESTING)) {
                        if (next->object.flags & DUPCHECK) {
                                fprintf(stderr, "%s: duplicate commit %s ignored\n", __FUNCTION__, sha1_to_hex(next->object.sha1));
                        } else {
                                next->object.flags |= DUPCHECK;
                                commit_list_insert(list->item, &reversed);
                        }
                }
        }

        if (!reversed->next) {

                // if there is only one element in the list, we can sort it using 
                // sort_in_merge_order.

                base = reversed->item;

        } else {

                // otherwise, we search for the base of the list

                if ((ret = find_base_for_list(reversed, &base)))
                        return ret;

                if (base) {
                        base->object.flags |= BOUNDARY;
                }

                while (reversed) {
                        sort_first_epoch(pop_commit(&reversed), &stack);
                        if (reversed) {
                                //
                                // if we have more commits to push, then the
                                // first push for the next parent may (or may not)
                                // represent a discontinuity with respect to the
                                // parent currently on the top of the stack.
                                //
                                // mark it for checking here, and check it
                                // with the next push...see sort_first_epoch for
                                // more details.
                                //
                                stack->item->object.flags |= DISCONTINUITY;
                        }
                }

                action = emit_stack(&stack, emitter);
        }

        if (base && (action != STOP)) {
                ret = sort_in_merge_order(base, emitter);
        }

        return ret;
}