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

From: Jon Seymour <[email-masked]>
Date: Fri, 3 Jun 2005 01:16:38 +1000

[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.

A test script, t/t5000-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
        ./t5000-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.

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

Signed-off-by: Jon Seymour <[email-masked]>
---
diff --git a/Makefile b/Makefile
--- a/Makefile
+++ b/Makefile
@@ -40,9 +40,9 @@ install: $(PROG) $(SCRIPTS)
        $(INSTALL) $(PROG) $(SCRIPTS) $(dest)$(bin)

 LIB_OBJS=read-cache.o sha1_file.o usage.o object.o commit.o tree.o blob.o \
-        tag.o delta.o date.o index.o diff-delta.o patch-delta.o
+        tag.o delta.o date.o index.o diff-delta.o patch-delta.o epoch.o
 LIB_FILE=libgit.a
-LIB_H=cache.h object.h blob.h tree.h commit.h tag.h delta.h
+LIB_H=cache.h object.h blob.h tree.h commit.h tag.h delta.h epoch.h

 LIB_H += strbuf.h
 LIB_OBJS += strbuf.o
@@ -134,6 +134,7 @@ diffcore-pathspec.o : $(LIB_H) diffcore.
 diffcore-pickaxe.o : $(LIB_H) diffcore.h
 diffcore-break.o : $(LIB_H) diffcore.h
 diffcore-order.o : $(LIB_H) diffcore.h
+epoch.o: $(LIB_H)

 test: all
        $(MAKE) -C t/ all
diff --git a/commit.c b/commit.c
--- a/commit.c
+++ b/commit.c
@@ -129,7 +129,7 @@ void free_commit_list(struct commit_list
        }
 }

-static void insert_by_date(struct commit_list **list, struct commit *item)
+void insert_by_date(struct commit_list **list, struct commit *item)
 {
        struct commit_list **pp = list;
        struct commit_list *p;
@@ -254,3 +254,25 @@ unsigned long pretty_print_commit(const
        buf[offset] = '\0';
        return offset;
 }
+
+struct commit *pop_commit(struct commit_list **stack)
+{
+       struct commit_list *top = *stack;
+       struct commit *item = top ? top->item : NULL;
+
+       if (top) {
+               *stack = top->next;
+               free(top);
+       }
+       return item;
+}
+
+int count_parents(struct commit * commit)
+{
+        int count = 0;
+        struct commit_list * parents = commit->parents;
+        for (count=0;parents; parents=parents->next,count++)
+          ;
+        return count;
+}
+
diff --git a/commit.h b/commit.h
--- a/commit.h
+++ b/commit.h
@@ -32,6 +32,8 @@ void free_commit_list(struct commit_list

 void sort_by_date(struct commit_list **list);

+void insert_by_date(struct commit_list **list, struct commit *item);
+
 extern unsigned long pretty_print_commit(const char *msg, unsigned
long len, char *buf, unsigned long space);

@@ -41,4 +43,7 @@ extern unsigned long pretty_print_commit
 struct commit *pop_most_recent_commit(struct commit_list **list,
                                      unsigned int mark);

+struct commit *pop_commit(struct commit_list **stack);
+
+int count_parents(struct commit * commit);
 #endif /* COMMIT_H */
diff --git a/epoch.c b/epoch.c
new file mode 100644
--- /dev/null
+++ b/epoch.c
@@ -0,0 +1,454 @@
+/*
+ * 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());
+
+       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 node of an epoch.
+//
+// This algorithm uses a conservation of mass approach.
+//
+// The head starts with a weight of 1 and distributes equal portions
+// along each edge to each of its parents. The process continues until
+// one node discovers that it has acquired a total mass of 1 from its
+// descendents. This node is the base node.
+//
+// Importantly the queue of nodes to be processed is sorted in
+// reverse date order and each node occupies at most one
+// position in the queue simultaneously. These two properties
+// do not effect the algorithm's correctness. However assuming most, but
+// not all, timestamps are sane they do prevent the algorithm becoming
+// exponential by ensuring that most nodes are only visited once.
+// This is true because mass is only propagated through the network
+// when we are pretty sure we are not going to see any more coming
+// our way. The occasional bad timestamp will mean that we are
+// wrong, occasionally, but this does not matter too much.
+//
+static int find_base(struct commit *head, struct commit **boundary)
+{
+       int ret = 0;
+
+       struct commit_list *cleaner = NULL;
+       struct commit_list *pending = NULL;
+
+       *boundary = NULL;
+
+       // we inject a mass of 1 unit here
+
+       new_mass_counter(head, get_one());
+
+       commit_list_insert(head, &pending);
+       commit_list_insert(head, &cleaner);
+
+       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;
+
+                       divide(init_fraction(&distribution),
&latest_node->pending, num_parents);
+
+                       struct commit_list *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);
+                                       commit_list_insert(parent, &cleaner);
+                                       insert_by_date(&pending, parent);
+                               } else {
+                                       if
(!compare(&parent_node->pending, get_zero())) {
+                                              
copy(&parent_node->pending, &distribution);
+                                              
insert_by_date(&pending, parent);
+                                       } else {
+                                              
add(&parent_node->pending, &parent_node->pending, &distribution);
+                                       }
+                               }
+                       }
+               }
+
+               if (!compare(&latest_node->seen, get_one()) && (latest
!= head)) {
+                       // if this node has seen one unit of mass, it
must be the base, so mark
+                       // the boundary which will cause the loop to stop.
+                       *boundary = latest;
+               }
+
+               copy(&latest_node->pending, get_zero());
+
+       }
+
+       for (; 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);
+
+       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;
+}
+
+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 & MERGE_ORDER_BREAK_FLAG)) {
+               if (is_parent_of(top->item, item)) {
+                       top->item->object.flags &= ~MERGE_ORDER_BREAK_FLAG;
+               }
+       }
+       commit_list_insert(item, stack);
+}
+
+//
+// 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)
+{
+       parse_commit(head);
+
+       head->object.flags |= MERGE_ORDER_VISIT_FLAG;
+
+       struct commit_list *parents;
+
+       struct commit_list *reversed_parents = NULL;
+
+       //
+       // 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.
+       //
+
+       for (; reversed_parents;) {
+               struct commit *parent = pop_commit(&reversed_parents);
+               parse_commit(parent);
+               if (!(parent->object.flags & MERGE_ORDER_VISIT_FLAG)) {
+                       if (parent->object.flags & MERGE_ORDER_EPOCH_FLAG) {
+                               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 |= MERGE_ORDER_VISIT_FLAG;
+                       } 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
|= MERGE_ORDER_BREAK_FLAG;
+                               }
+                       }
+               }
+       }
+
+       push_onto_merge_order_stack(stack, head);
+}
+
+//
+// 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.
+//
+int sort_in_merge_order(struct commit *head_of_epoch, emitter_func emitter)
+{
+       struct commit *next;
+       int ret = 0;
+       int stop = 0;
+
+       parse_commit(head_of_epoch);
+
+       for (next = head_of_epoch; next && next->parents && !ret && !stop;) {
+
+               struct commit *base = NULL;
+
+               ret = find_next_epoch_boundary(next, &base);
+               if (ret)
+                       return ret;
+
+               next->object.flags |= MERGE_ORDER_EPOCH_FLAG;
+               if (base) {
+                       base->object.flags |= MERGE_ORDER_EPOCH_FLAG;
+               }
+
+               if (HAS_EXACTLY_ONE_PARENT(next)) {
+
+                       while (HAS_EXACTLY_ONE_PARENT(next) && !stop && !ret) {
+                               stop = (*emitter) (next);
+                               next = next->parents->item;
+                               ret = parse_commit(next);
+                       }
+
+               } else {
+
+                       struct commit_list *stack = NULL;
+                       sort_first_epoch(next, &stack);
+                       for (; stack && !stop;) {
+                               next = pop_commit(&stack);
+                               if (stack) {
+                                       stop = (*emitter) (next);
+                               }
+                       }
+                       if (stack) {
+                               free_commit_list(stack);
+                       }
+               }
+
+       }
+
+       if (next && !stop) {
+               stop = (*emitter) (next);
+       }
+
+       return ret;
+}
diff --git a/epoch.h b/epoch.h
new file mode 100644
--- /dev/null
+++ b/epoch.h
@@ -0,0 +1,12 @@
+#ifndef EPOCH_H
+#define EPOCH_H
+
+typedef int (*emitter_func) (struct commit *);
+
+int sort_in_merge_order(struct commit *head_of_epoch, emitter_func emitter);
+
+#define MERGE_ORDER_EPOCH_FLAG (1u<<3)
+#define MERGE_ORDER_VISIT_FLAG (1u<<4)
+#define MERGE_ORDER_BREAK_FLAG (1u<<5)
+
+#endif                         /* EPOCH_H */
diff --git a/object.h b/object.h
--- a/object.h
+++ b/object.h
@@ -15,6 +15,7 @@ struct object {
        const char *type;
        struct object_list *refs;
        struct object_list *attached_deltas;
+       void *util;
 };

 extern int nr_objs;
diff --git a/rev-list.c b/rev-list.c
--- a/rev-list.c
+++ b/rev-list.c
@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "commit.h"
+#include "epoch.h"

 #define SEEN           (1u << 0)
 #define INTERESTING    (1u << 1)
@@ -11,7 +12,8 @@ static const char rev_list_usage[] =
                      "  --max-age=epoch\n"
                      "  --min-age=epoch\n"
                      "  --header\n"
-                     "  --pretty";
+                     "  --pretty\n"
+                     "  --merge-order [ --show-breaks ]";

 static void mark_parents_uninteresting(struct commit *commit)
 {
@@ -36,18 +38,63 @@ static int everybody_uninteresting(struc
        return 1;
 }

+static unsigned long max_age = -1;
+static unsigned long min_age = -1;
+static int max_count = -1;
+static int verbose_header = 0, show_parents = 0, pretty_print = 0,
merge_order = 0, show_breaks = 0;
+static const char *prefix = "";
+static int hdr_termination = 0;
+
+static int process_commit(struct commit * commit)
+{
+       if (commit->object.flags & UNINTERESTING)
+               return 0;
+       if (min_age != -1 && (commit->date > min_age))
+               return 0;
+       if (max_age != -1 && (commit->date < max_age))
+               return 1;
+       if (max_count != -1 && !max_count--)
+               return 1;
+
+        if (!show_breaks) {
+               printf("%s%s", prefix, sha1_to_hex(commit->object.sha1));
+               if (show_parents) {
+                       struct commit_list *parents = commit->parents;
+                       while (parents) {
+                               printf(" %s",
sha1_to_hex(parents->item->object.sha1));
+                               parents = parents->next;
+                       }
+               }
+               putchar('\n');
+               if (verbose_header) {
+                       const char *buf = commit->buffer;
+                       if (pretty_print) {
+                               static char pretty_header[16384];
+                               pretty_print_commit(commit->buffer,
~0, pretty_header, sizeof(pretty_header));
+                               buf = pretty_header;
+                       }
+                       printf("%s%c", buf, hdr_termination);
+               }
+        } else {
+                char c='|';
+                if (commit->object.flags & MERGE_ORDER_BREAK_FLAG) {
+                  c = '^';
+                } else if (commit->object.flags & MERGE_ORDER_EPOCH_FLAG) {
+                  c = '=';
+                }
+                printf("%c %s\n", c, sha1_to_hex(commit->object.sha1));
+        }
+
+        return 0;
+}
+
 int main(int argc, char **argv)
 {
        int nr_sha;
        unsigned char sha1[2][20];
        struct commit_list *list = NULL;
        struct commit *commit, *end;
-       int i, verbose_header = 0, show_parents = 0, pretty_print = 0;
-       int hdr_termination = 0;
-       const char *prefix = "";
-       unsigned long max_age = -1;
-       unsigned long min_age = -1;
-       int max_count = -1;
+       int i;

        nr_sha = 0;
        for (i = 1 ; i < argc; i++) {
@@ -80,8 +127,16 @@ int main(int argc, char **argv)
                        show_parents = 1;
                        continue;
                }
+               if (!strncmp(arg, "--merge-order", 13)) {
+                       merge_order = 1;
+                       continue;
+               }
+               if (!strncmp(arg, "--show-breaks", 13)) {
+                       show_breaks = 1;
+                       continue;
+               }

-               if (nr_sha > 2 || get_sha1(arg, sha1[nr_sha]))
+               if (nr_sha > 2 || get_sha1(arg, sha1[nr_sha])||
(show_breaks && !merge_order))
                        usage(rev_list_usage);
                nr_sha++;
        }
@@ -118,36 +173,19 @@ int main(int argc, char **argv)
                } while (list);
                list = newlist;
        }
+
+       if (!merge_order) {
+               while (list) {
+                       struct commit *commit =
pop_most_recent_commit(&list, SEEN);
+                       if (process_commit(commit)) {
+                              break;
+                        }
+               }
+        } else {
+                if (sort_in_merge_order(commit, &process_commit)) {
+                        die("merge order sort failed\n");
+                }
+        }

-       while (list) {
-               struct commit *commit = pop_most_recent_commit(&list, SEEN);
-
-               if (commit->object.flags & UNINTERESTING)
-                       continue;
-               if (min_age != -1 && (commit->date > min_age))
-                       continue;
-               if (max_age != -1 && (commit->date < max_age))
-                       break;
-               if (max_count != -1 && !max_count--)
-                       break;
-               printf("%s%s", prefix, sha1_to_hex(commit->object.sha1));
-               if (show_parents) {
-                       struct commit_list *parents = commit->parents;
-                       while (parents) {
-                               printf(" %s",
sha1_to_hex(parents->item->object.sha1));
-                               parents = parents->next;
-                       }
-               }
-               putchar('\n');
-               if (verbose_header) {
-                       const char *buf = commit->buffer;
-                       if (pretty_print) {
-                               static char pretty_header[16384];
-                               pretty_print_commit(commit->buffer,
~0, pretty_header, sizeof(pretty_header));
-                               buf = pretty_header;
-                       }
-                       printf("%s%c", buf, hdr_termination);
-               }
-       }
        return 0;
 }
diff --git a/t/t5000-rev-list.sh b/t/t5000-rev-list.sh
new file mode 100755
--- /dev/null
+++ b/t/t5000-rev-list.sh
@@ -0,0 +1,85 @@
+#!/bin/sh
+#
+# Copyright (c) 2005 Jon Seymour
+#
+
+test_description='Test rev-list --merge-order
+'
+. ./test-lib.sh
+
+function do_commit
+{
+    git-commit-tree "$@" </dev/null
+}
+
+function check_adjacency
+{
+    read previous
+    echo "= $previous"
+    while read next
+    do
+        if ! (git-cat-file commit $previous | grep "^parent $next" >/dev/null)
+        then
+            echo "^ $next"
+        else
+            echo "| $next"
+        fi
+        previous=$next
+    done
+}
+
+function sed_script
+{
+   for c in root a0 a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3
+   do
+       echo -n "s/${!c}/$c/;"
+   done
+}
+
+date >path0
+git-update-cache --add path0
+tree=$(git-write-tree)
+root=$(do_commit $tree)
+export GIT_COMMITTER_NAME=foobar  # to guarantee that the commit is different
+a0=$(do_commit $tree -p $root)
+a1=$(do_commit $tree -p $a0)
+export GIT_COMMITTER_NAME=foobar2 # to guarantee that the commit is different
+b1=$(do_commit $tree -p $root)
+c1=$(do_commit $tree -p $b1)
+export GIT_COMMITTER_NAME=foobar3 # to guarantee that the commit is different
+b2=$(do_commit $tree -p $b1)
+b3=$(do_commit $tree -p $b2)
+c2=$(do_commit $tree -p $c1 -p $b2)
+c3=$(do_commit $tree -p $c2)
+a2=$(do_commit $tree -p $a1)
+a3=$(do_commit $tree -p $a2)
+b4=$(do_commit $tree -p $b3 -p $a3)
+a4=$(do_commit $tree -p $a3 -p $b4 -p $c3)
+echo $a4 > .git/HEAD
+
+git-rev-list --merge-order --show-breaks HEAD | sed "$(sed_script)" >
actual-merge-order
+cat > expected-merge-order <<EOF
+= a4
+| c3
+| c2
+| c1
+^ b4
+| b3
+| b2
+| b1
+^ a3
+| a2
+| a1
+| a0
+= root
+EOF
+
+git-rev-list HEAD | check_adjacency | sed "$(sed_script)" >
actual-default-order
+normal_adjacency_count=$(git-rev-list HEAD | check_adjacency | grep
-c "\^" | tr -d ' ')
+merge_order_adjacency_count=$(git-rev-list --merge-order HEAD |
check_adjacency | grep -c "\^" | tr -d ' ')
+
+test_expect_success 'Testing that the rev-list has correct number of
entries' '[ $(git-rev-list HEAD | wc -l) -eq 13 ]'
+test_expect_success 'Testing that --merge-order produces the correct
result' 'diff expected-merge-order actual-merge-order'
+test_expect_success 'Testing that --merge-order produces as many or
fewer discontinuities' '[ $merge_order_adjacency_count -le
$normal_adjacency_count ]'
+
+test_done

-- 
homepage: http://www.zeta.org.au/~jon/
blog: http://orwelliantremors.blogspot.com/