[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, 29 insertions, 1 deletions

diff --git a/Documentation/git-rev-list.txt b/Documentation/git-rev-list.txt
index f2c5fa9f4c..3cfa58b5bd 100644
--- a/Documentation/git-rev-list.txt
+++ b/Documentation/git-rev-list.txt
@@ -9,7 +9,7 @@ git-rev-list - Lists commit objects in reverse chronological order
 
 SYNOPSIS
 --------
-'git-rev-list' <commit>
+'git-rev-list' [ *--max-count*=number ] [ *--max-age*=timestamp ] [ *--min-age*=timestamp ] [ *--merge-order* [ *--show-breaks* ] ] <commit>
 
 DESCRIPTION
 -----------
@@ -17,11 +17,39 @@ Lists commit objects in reverse chronological order starting at the
 given commit, taking ancestry relationship into account.  This is
 useful to produce human-readable log output.
 
+If *--merge-order* is specified, the commit history is decomposed into a unique sequence of minimal, non-linear
+epochs and maximal, linear epochs. Non-linear epochs are then linearised by sorting them into merge order, which 
+is described below.
+
+Maximal, linear epochs correspond to periods of sequential development. Minimal, non-linear epochs 
+correspond to periods of divergent development followed by a converging merge. The theory of epochs is described
+in more detail at link:http://blackcubes.dyndns.org/epoch/[http://blackcubes.dyndns.org/epoch/].
+
+The merge order for a non-linear epoch is defined as a linearisation for which the following invariants are true:
+
+    1. if a commit P is reachable from commit N, commit P sorts after commit N in the linearised list.
+    2. if Pi and Pj are any two parents of a merge M (with i < j), then any commit N, such that N is reachable from Pj
+       but not reachable from Pi, sorts before all commits reachable from Pi.
+   
+Invariant 1 states that later commits appear before earlier commits they are derived from.
+
+Invariant 2 states that commits unique to "later" parents in a merge, appear before all commits from "earlier" parents of
+a merge.
+
+If *--show-breaks* is specified, each item of the list is output with a 2-character prefix consisting of one of: 
+ (|), (^), (=) followed by a space. 
+Commits marked with (=) represent the boundaries of minimal, non-linear epochs and correspond either to the start of a period of divergent development or to the end of such a period. 
+Commits marked with (|) are direct parents of commits immediately preceding the marked commit in the list.
+Commits marked with (^) are not parents of the immediately preceding commit. These "breaks" represent necessary discontinuities implied by trying to represent an arbtirary DAG in a linear form.
+
+*--show-breaks* is only valid if *--merge-order* is also specified.
 
 Author
 ------
 Written by Linus Torvalds <torvalds@osdl.org>
 
+Original *--merge-order* logic by Jon Seymour <jon.seymour@gmail.com>
+
 Documentation
 --------------
 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.