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7.9: Merging and diff3

Ediff supports three-way comparison via the functions ediff-files3 and 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 `[=diff(A)]'.

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 this:

#ifdef NEW  /* variant A */
difference region from buffer A
#else  /* variant B */
difference region from buffer B
#endif  /* NEW */

The actual strings that separate the regions copied from buffer A and B are controlled by the variable ediff-combination-pattern. Its value should be a list of three strings. The first is inserted before the difference region of buffer A; the second string goes between the regions; the third goes after region B, as shown in the above example.

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 default-A) then `[=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 diff3 on 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 ediff-merge-window-share. Its default value is 0.5. To make the merge-buffer window smaller, reduce this amount.

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 variable ediff-show-clashes-only to non-nil.

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).

The variable 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 nil nor 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 from ediff-merge-directories, ediff-merge-directory-revisions, etc. 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 necessary.

The variable 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.

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