[PATCH] Modify git-rev-list to linearise the commit history in merge order.

This patch linearises the GIT commit history graph into merge order
which is defined by invariants specified in Documentation/git-rev-list.txt.

The linearisation produced by this patch is superior in an objective sense
to that produced by the existing git-rev-list implementation in that
the linearisation produced is guaranteed to have the minimum number of
discontinuities, where a discontinuity is defined as an adjacent pair of
commits in the output list which are not related in a direct child-parent
relationship.

With this patch a graph like this:

	a4 ---
	| \   \
	|  b4 |
	|/ |  |
	a3 |  |
	|  |  |
	a2 |  |
	|  |  c3
	|  |  |
	|  |  c2
	|  b3 |
	|  | /|
	|  b2 |
	|  |  c1
	|  | /
	|  b1
	a1 |
	|  |
	a0 |
	| /
	root

Sorts like this:

	= a4
	| c3
	| c2
	| c1
	^ b4
	| b3
	| b2
	| b1
	^ a3
	| a2
	| a1
	| a0
	= root

Instead of this:

	= a4
	| c3
	^ b4
	| a3
	^ c2
	^ b3
	^ a2
	^ b2
	^ c1
	^ a1
	^ b1
	^ a0
	= root

A test script, t/t6000-rev-list.sh, includes a test which demonstrates
that the linearisation produced by --merge-order has less discontinuities
than the linearisation produced by git-rev-list without the --merge-order
flag specified. To see this, do the following:

	cd t
	./t6000-rev-list.sh
	cd trash
	cat actual-default-order
	cat actual-merge-order

The existing behaviour of git-rev-list is preserved, by default. To obtain
the modified behaviour, specify --merge-order or --merge-order --show-breaks
on the command line.

This version of the patch has been tested on the git repository and also on the linux-2.6
repository and has reasonable performance on both - ~50-100% slower than the original algorithm.

This version of the patch has incorporated a functional equivalent of the Linus' output limiting
algorithm into the merge-order algorithm itself. This operates per the notes associated
with Linus' commit 337cb3fb8da45f10fe9a0c3cf571600f55ead2ce.

This version has incorporated Linus' feedback regarding proposed changes to rev-list.c.
(see: [PATCH] Factor out filtering in rev-list.c)

This version has improved the way sort_first_epoch marks commits as uninteresting.

For more details about this change, refer to Documentation/git-rev-list.txt
and http://blackcubes.dyndns.org/epoch/.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 693 insertions, 0 deletions

diff --git a/epoch.c b/epoch.c
new file mode 100644
index 0000000000..ba63eac68e
--- /dev/null
+++ b/epoch.c
@@ -0,0 +1,693 @@
+/*
+ * 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;
+}