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Diff is broken into four phases:

  1. Building a list of things that have changed. These changes are called deltas (git_diff_delta objects) and are grouped into a git_diff_list.
  2. Applying file similarity measurement for rename and copy detection (and to potentially split files that have changed radically). This step is optional.
  3. Computing the textual diff for each delta. Not all deltas have a meaningful textual diff. For those that do, the textual diff can either be generated on the fly and passed to output callbacks or can be turned into a git_diff_patch object.
  4. Formatting the diff and/or patch into standard text formats (such as patches, raw lists, etc).

In the source code, step 1 is implemented in src/diff.c, step 2 in src/diff_tform.c, step 3 in src/diff_patch.c, and step 4 in src/diff_print.c. Additionally, when it comes to accessing file content, everything goes through diff drivers that are implemented in src/diff_driver.c.

External Objects

  • git_diff_options represents user choices about how a diff should be performed and is passed to most diff generating functions.
  • git_diff_file represents an item on one side of a possible delta
  • git_diff_delta represents a pair of items that have changed in some way - it contains two git_diff_file plus a status and other stuff.
  • git_diff_list is a list of deltas along with information about how those particular deltas were found.
  • git_diff_patch represents the actual diff between a pair of items. In some cases, a delta may not have a corresponding patch, if the objects are binary, for example. The content of a patch will be a set of hunks and lines.
  • A hunk is range of lines described by a git_diff_range (i.e. "lines 10-20 in the old file became lines 12-23 in the new"). It will have a header that compactly represents that information, and it will have a number of lines of context surrounding added and deleted lines.
  • A line is simple a line of data along with a git_diff_line_t value that tells how the data should be interpreted (e.g. context or added).

Internal Objects

  • git_diff_file_content is an internal structure that represents the data on one side of an item to be diffed; it is an augmented git_diff_file with more flags and the actual file data.

    • it is created from a repository plus a) a git_diff_file, b) a git_blob, or c) raw data and size
    • there are three main operations on git_diff_file_content:

      • initialization sets up the data structure and does what it can up to, but not including loading and looking at the actual data
      • loading loads the data, preprocesses it (i.e. applies filters) and potentially analyzes it (to decide if binary)
      • free releases loaded data and frees any allocated memory
  • The internal structure of a git_diff_patch stores the actual diff between a pair of git_diff_file_content items

    • it may be "unset" if the items are not diffable
    • "empty" if the items are the same
    • otherwise it will consist of a set of hunks each of which covers some number of lines of context, additions and deletions
    • a patch is created from two git_diff_file_content items
    • a patch is fully instantiated in three phases:

      • initial creation and initialization
      • loading of data and preliminary data examination
      • diffing of data and optional storage of diffs
        • (TBD) if a patch is asked to store the diffs and the size of the diff is significantly smaller than the raw data of the two sides, then the patch may be flattened using a pool of string data
  • git_diff_output is an internal structure that represents an output target for a git_diff_patch

    • It consists of file, hunk, and line callbacks, plus a payload
    • There is a standard flattened output that can be used for plain text output
    • Typically we use a git_xdiff_output which drives the callbacks via the xdiff code taken from core Git.
  • git_diff_driver is an internal structure that encapsulates the logic for a given type of file

    • a driver is looked up based on the name and mode of a file.
    • the driver can then be used to:
      • determine if a file is binary (by attributes, by git_diff_options settings, or by examining the content)
      • give you a function pointer that is used to evaluate function context for hunk headers
    • At some point, the logic for getting a filtered version of file content or calculating the OID of a file may be moved into the driver.