# 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)