@c This is part of the Emacs manual.
@c Copyright (C) 2004-2018 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@c
@c This file is included either in emacs-xtra.texi (when producing the
@c printed version) or in the main Emacs manual (for the on-line version).
@node Emerge
@section Merging Files with Emerge
@cindex Emerge
@cindex merging files

  It's not unusual for programmers to get their signals crossed and
modify the same program in two different directions.  To recover from
this confusion, you need to merge the two versions.  Emerge makes this
easier.  For other ways to compare files, see
@iftex
@ref{Comparing Files,,, emacs, the Emacs Manual},
@end iftex
@ifnottex
@ref{Comparing Files},
@end ifnottex
and @ref{Top,, Ediff, ediff, The Ediff Manual}.

@menu
* Overview of Emerge::     How to start Emerge.  Basic concepts.
* Submodes of Emerge::     Fast mode vs. Edit mode.
                             Skip Prefers mode and Auto Advance mode.
* State of Difference::    You do the merge by specifying state A or B
                             for each difference.
* Merge Commands::         Commands for selecting a difference,
                             changing states of differences, etc.
* Exiting Emerge::         What to do when you've finished the merge.
* Combining in Emerge::    How to keep both alternatives for a difference.
* Fine Points of Emerge::  Miscellaneous issues.
@end menu

@node Overview of Emerge
@subsection Overview of Emerge

  To start Emerge, run one of these four commands:

@table @kbd
@item M-x emerge-files
@findex emerge-files
Merge two specified files.

@item M-x emerge-files-with-ancestor
@findex emerge-files-with-ancestor
Merge two specified files, with reference to a common ancestor.

@item M-x emerge-buffers
@findex emerge-buffers
Merge two buffers.

@item M-x emerge-buffers-with-ancestor
@findex emerge-buffers-with-ancestor
Merge two buffers with reference to a common ancestor in a third
buffer.
@end table

@cindex merge buffer (Emerge)
@cindex A and B buffers (Emerge)
  The Emerge commands compare two files or buffers, and display the
comparison in three buffers: one for each input text (the @dfn{A buffer}
and the @dfn{B buffer}), and one (the @dfn{merge buffer}) where merging
takes place.  The merge buffer shows the full merged text, not just the
differences.  Wherever the two input texts differ, you can choose which
one of them to include in the merge buffer.

  The Emerge commands that take input from existing buffers use only
the accessible portions of those buffers, if they are narrowed.
@iftex
@xref{Narrowing,,, emacs, the Emacs Manual}.
@end iftex
@ifnottex
@xref{Narrowing}.
@end ifnottex


  If a common ancestor version is available, from which the two texts to
be merged were both derived, Emerge can use it to guess which
alternative is right.  Wherever one current version agrees with the
ancestor, Emerge presumes that the other current version is a deliberate
change which should be kept in the merged version.  Use the
@samp{with-ancestor} commands if you want to specify a common ancestor
text.  These commands read three file or buffer names---variant A,
variant B, and the common ancestor.

  After the comparison is done and the buffers are prepared, the
interactive merging starts.  You control the merging by typing special
@dfn{merge commands} in the merge buffer (@pxref{Merge Commands}).
For each run of differences between the input texts, you can choose
which one of them to keep, or edit them both together.

  The merge buffer uses a special major mode, Emerge mode, with commands
for making these choices.  But you can also edit the buffer with
ordinary Emacs commands.

  At any given time, the attention of Emerge is focused on one
particular difference, called the @dfn{selected} difference.  This
difference is marked off in the three buffers like this:

@example
vvvvvvvvvvvvvvvvvvvv
@var{text that differs}
^^^^^^^^^^^^^^^^^^^^
@end example

@noindent
Emerge numbers all the differences sequentially and the mode
line always shows the number of the selected difference.

  Normally, the merge buffer starts out with the A version of the text.
But when the A version of a difference agrees with the common ancestor,
then the B version is initially preferred for that difference.

  Emerge leaves the merged text in the merge buffer when you exit.  At
that point, you can save it in a file with @kbd{C-x C-w}.  If you give a
numeric argument to @code{emerge-files} or
@code{emerge-files-with-ancestor}, it reads the name of the output file
using the minibuffer.  (This is the last file name those commands read.)
Then exiting from Emerge saves the merged text in the output file.

  Normally, Emerge commands save the output buffer in its file when you
exit.  If you abort Emerge with @kbd{C-]}, the Emerge command does not
save the output buffer, but you can save it yourself if you wish.

@node Submodes of Emerge
@subsection Submodes of Emerge

  You can choose between two modes for giving merge commands: Fast mode
and Edit mode.  In Fast mode, basic merge commands are single
characters, but ordinary Emacs commands are disabled.  This is
convenient if you use only merge commands.  In Edit mode, all merge
commands start with the prefix key @kbd{C-c C-c}, and the normal Emacs
commands are also available.  This allows editing the merge buffer, but
slows down Emerge operations.

  Use @kbd{e} to switch to Edit mode, and @kbd{C-c C-c f} to switch to
Fast mode.  The mode line indicates Edit and Fast modes with @samp{E}
and @samp{F}.

  Emerge has two additional submodes that affect how particular merge
commands work: Auto Advance mode and Skip Prefers mode.

  If Auto Advance mode is in effect, the @kbd{a} and @kbd{b} commands
advance to the next difference.  This lets you go through the merge
faster as long as you simply choose one of the alternatives from the
input.  The mode line indicates Auto Advance mode with @samp{A}.

  If Skip Prefers mode is in effect, the @kbd{n} and @kbd{p} commands
skip over differences in states ``prefer-A'' and ``prefer-B''
(@pxref{State of Difference}).  Thus you see only differences for
which neither version is presumed correct.  The mode line
indicates Skip Prefers mode with @samp{S}.  This mode is only relevant
when there is an ancestor.

@findex emerge-auto-advance
@findex emerge-skip-prefers
  Use the command @kbd{s a} (@code{emerge-auto-advance}) to set or clear
Auto Advance mode.  Use @kbd{s s} (@code{emerge-skip-prefers}) to set or
clear Skip Prefers mode.  These commands turn on the mode with a
positive argument, turn it off with a negative or zero argument, and
toggle the mode with no argument.

@node State of Difference
@subsection State of a Difference

  In the merge buffer, a difference is marked with lines of @samp{v} and
@samp{^} characters.  Each difference has one of these seven states:

@table @asis
@item A
The difference is showing the A version.  The @kbd{a} command always
produces this state; the mode line indicates it with @samp{A}.

@item B
The difference is showing the B version.  The @kbd{b} command always
produces this state; the mode line indicates it with @samp{B}.

@item default-A
@itemx default-B
The difference is showing the A or the B state by default, because you
haven't made a choice.  All differences start in the default-A state
(and thus the merge buffer is a copy of the A buffer), except those for
which one alternative is preferred (see below).

When you select a difference, its state changes from default-A or
default-B to plain A or B@.  Thus, the selected difference never has
state default-A or default-B, and these states are never displayed in
the mode line.

The command @kbd{d a} chooses default-A as the default state, and @kbd{d
b} chooses default-B@.  This chosen default applies to all differences
that you have never selected and for which no alternative is preferred.
If you are moving through the merge sequentially, the differences you
haven't selected are those following the selected one.  Thus, while
moving sequentially, you can effectively make the A version the default
for some sections of the merge buffer and the B version the default for
others by using @kbd{d a} and @kbd{d b} between sections.

@item prefer-A
@itemx prefer-B
The difference is showing the A or B state because it is
@dfn{preferred}.  This means that you haven't made an explicit choice,
but one alternative seems likely to be right because the other
alternative agrees with the common ancestor.  Thus, where the A buffer
agrees with the common ancestor, the B version is preferred, because
chances are it is the one that was actually changed.

These two states are displayed in the mode line as @samp{A*} and @samp{B*}.

@item combined
The difference is showing a combination of the A and B states, as a
result of the @kbd{x c} or @kbd{x C} commands.

Once a difference is in this state, the @kbd{a} and @kbd{b} commands
don't do anything to it unless you give them a numeric argument.

The mode line displays this state as @samp{comb}.
@end table

@node Merge Commands
@subsection Merge Commands

  Here are the Merge commands for Fast mode; in Edit mode, precede them
with @kbd{C-c C-c}:

@table @kbd
@item p
Select the previous difference.

@item n
Select the next difference.

@item a
Choose the A version of this difference.

@item b
Choose the B version of this difference.

@item C-u @var{n} j
Select difference number @var{n}.

@item .
Select the difference containing point.
@c [Does not work in the A or B buffer?]
@c You can use this command in the merge buffer or in the A or B buffer.

@item q
Quit---finish the merge.

@item C-]
Abort---exit merging and do not save the output.

@item f
Go into Fast mode.  (In Edit mode, this is actually @kbd{C-c C-c f}.)

@item e
Go into Edit mode.

@item l
Recenter (like @kbd{C-l}) all three windows.  With an argument,
reestablish the default three-window display.

@item -
Specify part of a prefix numeric argument.

@item @var{digit}
Also specify part of a prefix numeric argument.

@item d a
Choose the A version as the default from here down in
the merge buffer.

@item d b
Choose the B version as the default from here down in
the merge buffer.

@item c a
Copy the A version of this difference into the kill ring.

@item c b
Copy the B version of this difference into the kill ring.

@item i a
Insert the A version of this difference at point.

@item i b
Insert the B version of this difference at point.

@item m
Put point and mark around the difference.

@item ^
Scroll all three windows down (like @kbd{M-v}).

@item v
Scroll all three windows up (like @kbd{C-v}).

@item <
Scroll all three windows left (like @kbd{C-x <}).

@item >
Scroll all three windows right (like @kbd{C-x >}).

@item |
Reset horizontal scroll on all three windows.

@item x 1
Shrink the merge window to one line.  (Use @kbd{C-u l} to restore it
to full size.)

@item x c
Combine the two versions of this difference (@pxref{Combining in
Emerge}).

@item x f
Show the names of the files/buffers Emerge is operating on, in a Help
window.  (Use @kbd{C-u l} to restore windows.)

@item x j
Join this difference with the following one.
(@kbd{C-u x j} joins this difference with the previous one.)

@item x s
Split this difference into two differences.  Before you use this
command, position point in each of the three buffers at the place where
you want to split the difference.

@item x t
Trim identical lines off the top and bottom of the difference.
Such lines occur when the A and B versions are
identical but differ from the ancestor version.
@end table

@node Exiting Emerge
@subsection Exiting Emerge

  The @kbd{q} command (@code{emerge-quit}) finishes the merge, storing
the results into the output file if you specified one.  It restores the
A and B buffers to their proper contents, or kills them if they were
created by Emerge and you haven't changed them.  It also disables the
Emerge commands in the merge buffer, since executing them later could
damage the contents of the various buffers.

  @kbd{C-]} aborts the merge.  This means exiting without writing the
output file.  If you didn't specify an output file, then there is no
real difference between aborting and finishing the merge.

  If the Emerge command was called from another Lisp program, then its
return value is @code{t} for successful completion, or @code{nil} if you
abort.

@node Combining in Emerge
@subsection Combining the Two Versions

  Sometimes you want to keep @emph{both} alternatives for a particular
difference.  To do this, use @kbd{x c}, which edits the merge buffer
like this:

@example
@group
#ifdef NEW
@var{version from B buffer}
#else /* not NEW */
@var{version from A buffer}
#endif /* not NEW */
@end group
@end example

@noindent
@vindex emerge-combine-versions-template
While this example shows C preprocessor conditionals delimiting the two
alternative versions, you can specify the strings to use by setting
the variable @code{emerge-combine-versions-template} to a string of your
choice.  In the string, @samp{%a} says where to put version A, and
@samp{%b} says where to put version B@.  The default setting, which
produces the results shown above, looks like this:

@example
@group
"#ifdef NEW\n%b#else /* not NEW */\n%a#endif /* not NEW */\n"
@end group
@end example

@node Fine Points of Emerge
@subsection Fine Points of Emerge

  During the merge, you mustn't try to edit the A and B buffers yourself.
Emerge modifies them temporarily, but ultimately puts them back the way
they were.

  You can have any number of merges going at once---just don't use any one
buffer as input to more than one merge at once, since the temporary
changes made in these buffers would get in each other's way.

  Starting Emerge can take a long time because it needs to compare the
files fully.  Emacs can't do anything else until @code{diff} finishes.
Perhaps in the future someone will change Emerge to do the comparison in
the background when the input files are large---then you could keep on
doing other things with Emacs until Emerge is ready to accept
commands.

@vindex emerge-startup-hook
  After setting up the merge, Emerge runs the hook
@code{emerge-startup-hook}.
@iftex
@xref{Hooks,,, emacs, the Emacs Manual}.
@end iftex
@ifnottex
@xref{Hooks}.
@end ifnottex
