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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: can't understand how to move a node of a linked list to another place on the same
Date: Fri, 19 Feb 2021 09:09:23 +0000
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Mehdi Saada <00120260a@gmail.com> writes:

> X means the node of place X, that I want moved before the node of
> place Y.

I don't think it does that; I think it moves it _after_. Weren't you
just having a conversation about clarity of purpose?

May I say that a simple linked list isn't a good structure if you want
to do this sort of thing to it! as you (and I) have found.

Consider a list [p q r s t] and you want to move r after s. I'm going to
use w -> x to mean that the node whose value is 'w' has Next pointing to
the node whose value is 'y'; personally I find it easier to draw the
nodes on paper, it's hard work typing the equivalent. YMMV.

Using your original naming, Before_A -> q, Before_B -> r.

   procedure Insertion_A_On_Place_B (Before_A, Before_B: in T_List) is
      A: constant T_List := Before_A.Next;
      --  r
      B: constant T_List := Before_B.Next;
      --  s
   begin
      Before_A.Next := A.Next;
      --  q -> s
      A.Next := B;
      --  r -> s (no change!)
      Before_B.Next := A;
      --  r -> r
   end Insertion_A_On_Place_B;

and the list is now [p q s t], with r left dangling and pointing to
itself.