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clarify diff implementation approach

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Fischlurch 2014-12-06 23:35:22 +01:00
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@ -7721,7 +7721,7 @@ The most fundamental distinction is the difference between //finding// a diff an
Before we can consider a diffing technique, we need to clarify the primitive operations used as foundation. These primitives form //a language.// This incurs a problem and choice of context. We might base the representation language on the situation where the diff is applied, we may care for the conciseness of the representation, or the effort and space required to apply it. We should consider if the focus is on reading and understanding the diff, if we intend to derive something directly from the diff representation -- say, if and what and when something is changed -- and we should consider how the context where the diff is established relates to the context where it is applied.
</pre>
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<div title="TreeDiffModel" creator="Ichthyostega" modifier="Ichthyostega" created="201410270313" modified="201412010038" tags="Model GuiPattern spec draft" changecount="41">
<div title="TreeDiffModel" creator="Ichthyostega" modifier="Ichthyostega" created="201410270313" modified="201412060222" tags="Model GuiPattern spec draft" changecount="44">
<pre>for the purpose of handling updates in the GUI timeline display efficiently, we need to determine and represent //structural differences//
We build a slightly abstracted representation of tree changes and use this to propagate //change notifications// to the actual widgets. To keep the whole process space efficient, a demand-driven, stateless implementation approach is chosen. This reduces the problem into several layered stages.
* our model is a heterogeneous tree &amp;rArr; use demand-driven recursion
@ -7754,13 +7754,15 @@ Thus, for our specific usage scenario, the foremost relevant question is //how t
|{{{del}}}(a~~2~~) |!| ()|(a~~3~~, a~~4~~, a~~5~~) |
|{{{ins}}}(b~~1~~) |!| (b~~1~~)|(a~~3~~, a~~4~~, a~~5~~) |
|{{{pick}}}(a~~3~~) |!| (b~~1~~, a~~3~~)|(a~~4~~, a~~5~~) |
|{{{push}}}(+1, a~~4~~) |!| (b~~1~~, a~~3~~)|(a~~5~~, a~~4~~) |
|{{{push}}}( a~~5~~) |!| (b~~1~~, a~~3~~)|(a~~5~~, a~~4~~) |
|{{{pick}}}(a~~5~~) |!| (b~~1~~, a~~3~~, a~~5~~)|(a~~4~~) |
|{{{ins}}}(b~~2~~) |!| (b~~1~~, a~~3~~, a~~5~~, b~~2~~)|(a~~4~~) |
|{{{ins}}}(b~~3~~) |!| (b~~1~~, a~~3~~, a~~5~~, b~~2~~, b~~3~~)|(a~~4~~) |
|{{{pick}}}(a~~4~~) |!| (b~~1~~, a~~3~~, a~~5~~, b~~2~~, b~~3~~, a~~4~~)|() |
|{{{ins}}}(b~~4~~) |!| (b~~1~~, a~~3~~, a~~5~~, b~~2~~, b~~3~~, a~~4~~, b~~4~~)|() |
__Implementation note__:The representation chosen here uses terms of constant size for the individual diff steps; in most cases, the argument is redundant and can be used for verification when applying the diff -- exceptions being the {{{push}}} and {{{ins}}} terms, where it actually encodes additional information. Especially the {{{puah}}}-representation is a compromise, since we encode as &quot;push the next term back behind the term a~~5~~&quot;. The more obvious rendering -- &quot;push term a~~4~~ back by +1 steps&quot; -- requires an additional integer argument not neccesary for any of the other diff verbs, defeating a fixed value implementation.
!!!extension to tree changes
Basically we could send messages for recursive descent right after each {{{pick}}} token -- yet, while minimal, such a representation would be unreadable, and requires a dedicated stack storage on both sides. Thus we arrange for the //recursive treatment of children// to be sent //postfix,// after the messages for the current node. Recursive descent is indicated by explicit (and slightly redundant) //bracketing tokens://
*{{{open}}}(node-ID) : recurse into the designated node, which must be present already as result of the preceding changes