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merge API
=========

The merge API helps a program to reconcile two competing sets of
improvements to some files (e.g., unregistered changes from the work
tree versus changes involved in switching to a new branch), reporting
conflicts if found.  The library called through this API is
responsible for a few things.

 * determining which trees to merge (recursive ancestor consolidation);

 * lining up corresponding files in the trees to be merged (rename
   detection, subtree shifting), reporting edge cases like add/add
   and rename/rename conflicts to the user;

 * performing a three-way merge of corresponding files, taking
   path-specific merge drivers (specified in `.gitattributes`)
   into account.

Data structures
---------------

* `mmbuffer_t`, `mmfile_t`

These store data usable for use by the xdiff backend, for writing and
for reading, respectively.  See `xdiff/xdiff.h` for the definitions
and `diff.c` for examples.

* `struct ll_merge_options`

This describes the set of options the calling program wants to affect
the operation of a low-level (single file) merge.  Some options:

`virtual_ancestor`::
	Behave as though this were part of a merge between common
	ancestors in a recursive merge.
	If a helper program is specified by the
	`[merge "<driver>"] recursive` configuration, it will
	be used (see linkgit:gitattributes[5]).

`variant`::
	Resolve local conflicts automatically in favor
	of one side or the other (as in 'git merge-file'
	`--ours`/`--theirs`/`--union`).  Can be `0`,
	`XDL_MERGE_FAVOR_OURS`, `XDL_MERGE_FAVOR_THEIRS`, or
	`XDL_MERGE_FAVOR_UNION`.

`renormalize`::
	Resmudge and clean the "base", "theirs" and "ours" files
	before merging.  Use this when the merge is likely to have
	overlapped with a change in smudge/clean or end-of-line
	normalization rules.

Low-level (single file) merge
-----------------------------

`ll_merge`::

	Perform a three-way single-file merge in core.  This is
	a thin wrapper around `xdl_merge` that takes the path and
	any merge backend specified in `.gitattributes` or
	`.git/info/attributes` into account.  Returns 0 for a
	clean merge.

Calling sequence:

* Prepare a `struct ll_merge_options` to record options.
  If you have no special requests, skip this and pass `NULL`
  as the `opts` parameter to use the default options.

* Allocate an mmbuffer_t variable for the result.

* Allocate and fill variables with the file's original content
  and two modified versions (using `read_mmfile`, for example).

* Call `ll_merge()`.

* Read the merged content from `result_buf.ptr` and `result_buf.size`.

* Release buffers when finished.  A simple
  `free(ancestor.ptr); free(ours.ptr); free(theirs.ptr);
  free(result_buf.ptr);` will do.

If the modifications do not merge cleanly, `ll_merge` will return a
nonzero value and `result_buf` will generally include a description of
the conflict bracketed by markers such as the traditional `<<<<<<<`
and `>>>>>>>`.

The `ancestor_label`, `our_label`, and `their_label` parameters are
used to label the different sides of a conflict if the merge driver
supports this.

Everything else
---------------

Talk about <merge-recursive.h> and merge_file():

 - merge_trees() to merge with rename detection
 - merge_recursive() for ancestor consolidation
 - try_merge_command() for other strategies
 - conflict format
 - merge options

(Daniel, Miklos, Stephan, JC)