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General List Sorting Utilities                      MLG - 22 December 1981
------------------------------

The module Gsort (use LOAD GSORT) contains a number of general sorting
functions and associated key comparison functions.  The Key comparison
functions are given 2 objects to compare, return NIL if they are not
in correct order:

BeforeFn(a:any,b:any):Extra-Boolean;  %  return NIL if not in order

  The package defines:

  NumberSortFn(N1:number,N2:Number)
  StringSortFn(S1:String,N2:string)  [Sc1 and Sc2 are faster versions]
  IdSortFn(D1:id,D2:id)              [IdC1 and IDc2 are faster]
  AtomSortFn(X1:atom,X2:Atom)

The general sorting functions expect a SortFn (which MUST be an ID)

GsortP(Lst:x-list,BeforeFn:id):Boolean   % T if x-list is sorted 
Gsort(Lst:x-list,BeforeFn:id):x-list     % Tree-sort of x-list
GMergeSort(Lst:x-list,BeforeFn:id):x-list % Merge-sort of x-list
 
Currently, Gsort is often fastest,  but GMergeSort is more stable.

Example: To sort a list of Ids call Gsort(Dlist,'Idsortfn)
         or Gsort(Dlist,'IDc2) for faster sort.
       
         To sort list of records (e.g. pairs), user must define comparison:
         E.g. to sort LP, a List of dotted pairs (Number . Info), define
     
 procedure NPSortFn(P1,P2); NumberSortFn(Car p1, Car P2);

         then execute Gsort(LP,'NPSortfn);

See PU:Gsort.Red for the code.