lumiera_/doc/technical/library/DiffFramework.txt

Diff Handling Framework
=======================

Within the support library, in the namespace `lib::diff`, there is a collection of loosely coupled of tools
known as »the diff framework«. It revolves around generic representation and handling of structural differences.
Beyond some rather general assumptions, to avoid stipulating the usage of specific data elements or containers,
the framework is kept _generic_, cast in terms of *elements*, *sequences* and *strategies*
for access, indexing and traversal.

.motivation
**********************
Diff handling is multi purpose within Lumiera:
Diff representation is seen as a meta language and abstraction mechanism;
it enables tight collaboration without the need to tie and tangle the involved implementation data structures.
Used this way, diff representation reduces coupling and helps to cut down overall complexity — so to justify
the considerable amount of complexity seen within the diff framework implementation.
**********************

Definitions
-----------
element::
  the atomic unit treated in diff detection, representation and application. +
  Elements are considered to be

  - lightweight copyable values
  - equality comparable
  - bearing distinct identity
  - unique _as far as concerned_

sequence::
  data is delivered in the form of a sequence, which might or might not be _ordered,_
  but in any case will be traversed once only.

diff::
  the changes necessary to transform an input sequence (``old sequence'') into a target sequence (``new sequence'')

diff language::
  differences are spelled out in linearised form: as a sequence of constant-size diff actions, called »diff verbs«

diff verb::
  a single element within a diff. Diff verbs are conceived as operations, which,
  when applied consuming the input sequence, will produce the target sequence of the diff.

diff application::
  the process of consuming a diff (sequence of diff verbs), with the goal to produce some effect at the 
  _target_ of diff application. Typically we want to apply a diff to a data sequence, to mutate it
  into a new shape, conforming with the shape of the diff's ``target sequence''

diff generator::
  a facility producing a diff, which is a sequence of diff verbs.
  Typically, a diff generator works lazily, demand driven.

diff detector::
  special kind of diff generator, which takes two data sequences as input:
  an ``old sequence'' and a ``new sequence''. The diff detector traverses and compares
  these sequences to produce a diff, which describes the steps necessary to transform
  the ``old'' shape into the ``new'' shape of the data.


List Diff Algorithm
-------------------
While in general this is a well studied subject, in Lumiera we'll confine ourselves to a very
specific flavour of diff handling: we rely on _elementary atomic units_ with well established
object identity. And in addition, within the scope of one coherent diff handling task,
we require those elements to be 'unique'. The purpose of this design decision is to segregate
the notorious matching problem and treat diff handling in isolation.

Effectively this means that, for any given element, there can be at most _one_ matching
counterpart in the other sequence, and the presence of such can be detected by using an *index*.
In fact, we retrieve an index for every sequence involved into the diff detection task;
this is our trade-off for simplicity in the diff detection algorithm.footnote:[traditionally,
diff detection schemes, especially those geared at text diff detection, engage into great lengths
of producing an ``optimal'' diff, which effectively means to build specifically tuned pattern
or decision tables, from which the final diff can then be pulled or interpreted.
We acknowledge that in our case building a lookup table index can be O(n log n); we might
well be able to do better, but certainly for the price of an algorithm more mentally challenging.]
In case this turns out as a performance problem, we might consider integrating the index
maintenance into the data structure to be diffed, which shifts the additional impact of
indexing onto the data population phase.footnote:[in the general tree diff case this is far
from trivial, since we need an self-contained element index for every node, and we need the
ability to take a snapshot of the ``old'' state before mutating a node into ``new'' shape]

Element classification
~~~~~~~~~~~~~~~~~~~~~~
By using the indices of the old and the new sequence, we are able to _classify_ each element:

- elements only present in the new sequence are treated as *inserts*
- elements only present in the old sequence are treated as *deletes*
- elements present in both sequences form the *permutation*

Processing pattern
~~~~~~~~~~~~~~~~~~
We _consume both the old and the new sequence synchronously, while emitting the diff sequence_.

The diff describes a sequence of operations, which, when applied, consume a sequence congruent
to the old sequence, while emitting a sequence congruent to the new sequence. We use the
following *list diff language* here:

verb `ins(elm)`::
  insert the given argument element `elm` at the _current processing position_
  into the target sequence. This operation allows to inject new data
verb `del(elm)`::
  delete the _next_ element `elm` at _current position._
  For sake of verification, the element to be deleted is also included as argument (redundancy).
verb `pick(elm)`::
  accepts the _next_ element at _current position_ into the resulting altered sequence.
  The element is given redundantly as argument.
verb `push(elm)`::
  effect a re-ordering of the target list contents. This verb requires to take
  the _next_ element, which happens to sit at _current processing position_ and
  _push it back_ further into the list, to be placed at a position _behind_ the
  _anchor element_ `elm` given as argument.

Since _inserts_ and _deletes_ can be detected and emitted right at the processing frontier,
for the rest of this theoretical discussion, we consider the insert / delete part filtered
away conceptually, and concentrate on generating the permutation part.

Handling sequence permutation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This paragraph describes how to consume two permutations of the same sequence simultaneously,
while emitting `push` and `pick` verbs to describe the re-ordering. Consider the sequences
split into an already-processed part, and a part still-to-be-processed.

.Invariant
Matters are arranged such, that, in the to-be-processed part, each element appearing at the
front of the ``new'' sequence _can be picked right away_.

Now, to arrive at that invariant, we have especially to deal with the case that a different
(not matching) element appears at the front of the ``old'' list. We have to emit additional
`push` verbs to get rid of non-matching elements in the ``old'' order, until we get into a state
where the invariant is re-established (and we're able to `pick` to consume the same element
from the existing sequence and the target sequence). Obviously, the tricky part is how to
determine the *anchor element* for this `push` directrive...

- we need to be sure the anchor _is indeed present_ in the current shape of the sequence in processing.
- the anchor must be in the right place, so to conform to the target sequence at the point of picking it.
- it is desirable to emit at most one `push` directive for any given element; we want it to settle at the
  right place with a single shot