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+# Diffing and merging notebooks
+
+## Problem
+
+Diffing and merging notebooks is not properly handled by standard linebased diff and merge tools.
+
+## Proposed Enhancement
+
+* Make a package containing tools for diff and merge of notebooks
+* Include a command line api
+* Pretty-printing of diffs for command line display
+* Command line tools for interactive resolution of merge conflicts
+* Make the merge tool git compatible
+* Make a web gui for displaying notebook diffs
+* Make a web gui for interactive resolution of merge conflicts
+* Plugin framework for mime type specific diffing
+
+
+## Detailed Explanation
+
+Preliminary work resides in [nbdime](https://github.com/martinal/nbdime).
+
+Fundamentally, we envision use cases mainly in the categories
+of a merge command for version control integration, and
+diff command for inspecting changes and automated regression
+testing. At the core of it all is the diff algorithms, which
+must handle not only text in source cells but also a number of
+data formats based on mime types in output cells.
+
+
+### Basic diffing use cases
+
+* View difference between versions of a file
+* View diff of sources only
+* View diff of output cells (basic text diff of output cells, image diff with external tool)
+
+
+### Version control use cases
+
+Most commonly, cell source is the primary content,
+and output can presumably be regenerated. Indeed, it
+is not possible to guarantee that merged sources and
+merged output is consistent or makes any kind of sense.
+
+The main use case for the merge tool will be a git-compatible commandline merge tool:
+
+    nbmerge base.ipynb local.ipynb remote.ipynb merged.ipynb
+
+and a web gui for conflict resolution. Ideally the web gui can
+reuse as much as possible from jupyter notebook. An initial
+version of conflict resolution can be to output a notebook with
+conflicts marked within cells, to be manually edited as a regular
+jupyter notebook.
+
+Goals:
+
+* Trouble free automatic merge when no merge conflicts occur
+* Optional behaviour to drop conflicting output
+* Easy to use interactive conflict resolution
+
+Not planning (for now):
+
+* Merge of arbitrary output cell content
+
+Open questions:
+
+* Is it important to track source lines moving between cells?
+
+Should make a collection of tricky corner cases, and
+run merge tools on test cases from e.g. git if possible.
+
+
+### Regression testing use cases
+
+* View difference of output cells after re-running cells
+
+
+### Diff format
+
+A preliminary diff format has been defined, where the diff result is itself a json object.
+The details of this format is being refined. For examples of concrete diff
+objects, see e.g. the test suite for patch.
+
+
+#### Diff format for dicts (current)
+
+A diff of two dicts is a list of diff entries:
+
+    key = string
+    entry = [action, key] | [action, key, argument]
+    diff = [entry0, entry1, ...]
+
+A dict diff entry is a list of action and argument (except for deletion):
+
+* ["-", key]: delete value at key
+* ["+", key, newvalue]: insert newvalue at key
+* ["!", key, diff]: patch value at key with diff
+* [":", key, newvalue]: replace value at key with newvalue
+
+
+#### Diff format for dicts (alternative)
+
+A diff of two dicts is itself a dict mapping string keys to diff entries:
+
+    key = string
+    entry = [action] | [action, argument]
+    diff = {key0: entry0, key1: entry1, ...}
+
+A dict diff entry is a list of action and argument (except for deletion):
+
+* ["-"]: delete value at key
+* ["+", newvalue]: insert newvalue at key
+* ["!", diff]: patch value at key with diff
+* [":", newvalue]: replace value at key with newvalue
+
+
+#### Diff format for sequences (list and string)
+
+A diff of two sequences is an ordered list of diff entries:
+
+index = integer
+entry = [action, index] | [action, index, argument]
+diff = [entry0, entry1, ...]
+
+A sequence diff entry is a list of action, index and argument (except for deletion):
+
+* ["-", index]: delete entry at index
+* ["+", index, newvalue]: insert single newvalue before index
+* ["--", index, n]: delete n entries starting at index
+* ["++", index, newvalues]: insert sequence newvalues before index
+* ["!", index, diff]: patch value at index with diff
+
+Possible simplifications:
+
+* Remove single-item "-", "+" and rename "--" and "++" to single-letter.
+* OR remove "--" and "++" and stick with just single-item versions.
+
+
+Note: The currently implemented sequence diff algorithm is
+based on a brute force O(N^2) longest common subsequence (LCS)
+algorithm, this will be rewritten in terms of a faster algorithm
+such as Myers O(ND) LCS based diff algorithm, optionally
+using Pythons difflib for use cases it can handle.
+In particular difflib does not handle custom compare predicate,
+which we need to e.g. identify almost equal cells within sequences
+of cells in a notebook.
+
+
+### Merge format
+
+The merge process should return two things: The merge result and the conflicts.
+
+A format for representing merge conflicts is work in progress.
+
+Each transformation in the base->local and base->remote diffs must either
+end up in the merge result or be recorded in the conflicts representation.
+
+
+## Pros and Cons
+
+Pros associated with this implementation include:
+* Improved workflows when placing notebooks in version control systems
+* Possibility to use notebooks for self-documenting regression tests
+
+Cons associated with this implementation include:
+* Vanilla git installs will not receive the improved behaviour
+
+
+## Interested Contributors
+@martinal @minrk