Ediff supports three-way comparison via the functions
ediff-buffers3. The interface is the same as for two-way comparison.
In three-way comparison and merging, Ediff reports if any two difference
regions are identical. For instance, if the current region in buffer A
is the same as the region in buffer C, then the mode line of buffer A will
display ‘[=diff(C)]’ and the mode line of buffer C will display
Merging is done according to the following algorithm.
If a difference region in one of the buffers, say B, differs from the ancestor file while the region in the other buffer, A, doesn’t, then the merge buffer, C, gets B’s region. Similarly when buffer A’s region differs from the ancestor and B’s doesn’t, A’s region is used.
If both regions in buffers A and B differ from the ancestor file, Ediff
chooses the region according to the value of the variable
ediff-default-variant. If its value is
default-A then A’s
region is chosen. If it is
default-B then B’s region is chosen.
If it is
combined then the region in buffer C will look like
<<<<<<< variant A the difference region from buffer A >>>>>>> variant B the difference region from buffer B ####### Ancestor the difference region from the ancestor buffer, if available ======= end
The above is the default template for the combined region. The user can
customize this template using the variable
ediff-combination-pattern specifies the template that
determines how the combined merged region looks like. The template is
represented as a list of the form
(STRING1 Symbol1 STRING2 Symbol2
STRING3 Symbol3 STRING4). The symbols here must be atoms of the form
Ancestor. They determine the order in which
the corresponding difference regions (from buffers A, B, and the ancestor
buffer) are displayed in the merged region of buffer C. The strings in the
template determine the text that separates the aforesaid regions. The
default template is
("<<<<<<< variant A" A ">>>>>>> variant B" B "####### Ancestor" Ancestor "======= end")
(this is one long line) and the corresponding combined region is shown above. The order in which the regions are shown (and the separator strings) can be changed by changing the above template. It is even possible to add or delete region specifiers in this template (although the only possibly useful such modification seems to be the deletion of the ancestor).
In addition to the state of the difference, Ediff displays the state of the
merge for each region. If a difference came from buffer A by default
(because both regions A and B were different from the ancestor and
ediff-default-variant was set to
‘[=diff(A) default-A]’ is displayed in the mode line. If the
difference in buffer C came, say, from buffer B because the difference
region in that buffer differs from the ancestor, but the region in buffer A
does not (if merging with an ancestor) then ‘[=diff(B) prefer-B]’ is
displayed. The indicators default-A/B and prefer-A/B are inspired by
Emerge and have the same meaning.
Another indicator of the state of merge is ‘combined’. It appears with any difference region in buffer C that was obtained by combining the difference regions in buffers A and B as explained above.
In addition to the state of merge and state of difference indicators, while merging with an ancestor file or buffer, Ediff informs the user when the current difference region in the (normally invisible) ancestor buffer is empty via the AncestorEmpty indicator. This helps determine if the changes made to the original in variants A and B represent pure insertion or deletion of text: if the mode line shows AncestorEmpty and the corresponding region in buffers A or B is not empty, this means that new text was inserted. If this indicator is not present and the difference regions in buffers A or B are non-empty, this means that text was modified. Otherwise, the original text was deleted.
Although the ancestor buffer is normally invisible, Ediff maintains difference regions there and advances the current difference region accordingly. All highlighting of difference regions is provided in the ancestor buffer, except for the fine differences. Therefore, if desired, the user can put the ancestor buffer in a separate frame and watch it there. However, on a TTY, only one frame can be visible at any given time, and Ediff doesn’t support any single-frame window configuration where all buffers, including the ancestor buffer, would be visible. However, the ancestor buffer can be displayed by typing / to the control window. (Type C-l to hide it again.)
Note that the state-of-difference indicators ‘=diff(A)’ and ‘=diff(B)’ above are not redundant, even in the presence of a state-of-merge indicator. In fact, the two serve different purposes.
For instance, if the mode line displays ‘=diff(B) prefer(B)’ and you copy a difference region from buffer A to buffer C then ‘=diff(B)’ will change to ‘diff-A’ and the mode line will display ‘=diff(A) prefer-B’. This indicates that the difference region in buffer C is identical to that in buffer A, but originally buffer C’s region came from buffer B. This is useful to know because you can recover the original difference region in buffer C by typing r.
Ediff never changes the state-of-merge indicator, except in response to the ! command (see below), in which case the indicator is lost. On the other hand, the state-of-difference indicator is changed automatically by the copying/recovery commands, a, b, r, +.
The ! command loses the information about origins of the regions
in the merge buffer (default-A, prefer-B, or combined). This is because
recomputing differences in this case means running
buffers A, B, and the merge buffer, not on the ancestor buffer. (It
makes no sense to recompute differences using the ancestor file, since
in the merging mode Ediff assumes that you have not edited buffers A and
B, but that you may have edited buffer C, and these changes are to be
preserved.) Since some difference regions may disappear as a result of
editing buffer C and others may arise, there is generally no simple way
to tell where the various regions in the merge buffer came from.
In three-way comparison, Ediff tries to disregard regions that consist entirely of white space. For instance, if, say, the current region in buffer A consists of the white space only (or if it is empty), Ediff will not take it into account for the purpose of computing fine differences. The result is that Ediff can provide a better visual information regarding the actual fine differences in the non-white regions in buffers B and C. Moreover, if the regions in buffers B and C differ in the white space only, then a message to this effect will be displayed.
In the merge mode, the share of the split between window C (the window
displaying the merge-buffer) and the windows displaying buffers A and B
is controlled by the variable
default value is 0.5. To make the merge-buffer window smaller, reduce
We don’t recommend increasing the size of the merge-window to more than
half the frame (i.e., to increase the value of
ediff-merge-window-share) to more than 0.5, since it would be
hard to see the contents of buffers A and B.
You can temporarily shrink the merge window to just one line by typing s. This change is temporary, until Ediff finds a reason to redraw the screen. Typing s again restores the original window size.
With a positive prefix argument, the s command will make the merge window slightly taller. This change is persistent. With ‘-’ or with a negative prefix argument, the command s makes the merge window slightly shorter. This change also persistent.
Ediff lets you automatically ignore the regions where only one of the
buffers A and B disagrees with the ancestor. To do this, set the
ediff-show-clashes-only to non-
You can toggle this feature interactively by typing $$.
Note that this variable affects only the show next/previous difference commands. You can still jump directly to any difference region directly using the command j (with a prefix argument specifying the difference number).
ediff-autostore-merges controls what happens to the
merge buffer when Ediff quits. If the value is
nil, nothing is done
to the merge buffer—it will be the user’s responsibility to save it.
If the value is
t, the user will be asked where to save the buffer
and whether to delete it afterwards. It the value is neither
t, the merge buffer is saved only if this merge session was
invoked from a group of related Ediff session, such as those that result
See Session Groups. This behavior is implemented in the function
ediff-maybe-save-and-delete-merge, which is a hook in
ediff-quit-merge-hook. The user can supply a different hook, if
ediff-autostore-merges is buffer-local, so it can be
set in a per-buffer manner. Therefore, use
setq-default to globally
change this variable.
When merge buffers are saved automatically as directed by
ediff-autostore-merges, Ediff attaches a prefix to each file, as
specified by the variable
ediff-merge-filename-prefix. The default
merge_, but this can be changed by the user.