From: ootiib@hot.ee   
      
   On Sunday, 3 April 2022 at 14:16:14 UTC+3, Malcolm McLean wrote:   
   > To implement a simple undo redo, you need two stacks, and a copyable   
   representation of the program state. You then push to the undo stack, with   
   straight forwards extensions for redo.    
   >    
   > However this is memory greedy. So it is better to use the following pattern,   
   on stack push, take the difference between stack top and the object being   
   pushed. Then delete stack top, and replace with the difference between the   
   pushed object and stack    
   top, then push. To pop the stack, apply the difference to stack top, and set   
   it as new stack top.    
   >    
   > This could be made generic, but it involves a user-supplied delta codec   
   which could be hard to write. Another way of doing it is to feed everything   
   through a tostring / fromstring representation.    
   >    
   > The problem is now reduced to writing the delta codec for two strings. The   
   proper way to do this is to use a string alignment algorithm, but that is   
   likely to be overkill. The majority of changes will be either one character or   
   one indel. If we've got    
   more than few changes, we can simply resort to storing the entire string.   
      
   Storing whole program state is pointlessly memory consuming and diffing whole   
   program state is pointlessly expensive.    
   More usual is to make all actions to be required to record their reverse   
   actions.   
       
   So when action is done then reverse action is stored to undo stack and redo   
   stack is cleaned.   
   Undo then means that reverse action is done and removed from undo stack and   
   action is stored to redo stack.   
   Redo then means that action is done and removed from redo stack and reverse   
   action is stored to undo stack.   
      
   That takes designing (and testing) your commands/actions/operations more   
   carefully.   
   But that is anyway not bad at all in the long run.     
      
   --- SoupGate-Win32 v1.05   
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