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Task/Sorting-algorithms-Merge-sort/ERRE/sorting-algorithms-merge-sort.erre

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+PROGRAM MERGESORT_DEMO
+
+! Example of merge sort usage.
+
+CONST SIZE%=100,S1%=50
+
+DIM DTA%[SIZE%],FH%[S1%],STACK%[20,2]
+
+
+PROCEDURE MERGE(START%,MIDDLE%,ENDS%)
+
+LOCAL FHSIZE%
+
+  FHSIZE%=MIDDLE%-START%+1
+
+  FOR I%=0 TO FHSIZE%-1 DO
+     FH%[I%]=DTA%[START%+I%]
+  END FOR
+
+  I%=0
+  J%=MIDDLE%+1
+  K%=START%
+
+  REPEAT
+    IF FH%[I%]<=DTA%[J%] THEN
+        DTA%[K%]=FH%[I%]
+        I%=I%+1
+        K%=K%+1
+      ELSE
+        DTA%[K%]=DTA%[J%]
+        J%=J%+1
+        K%=K%+1
+    END IF
+  UNTIL I%=FHSIZE% OR J%>ENDS%
+
+  WHILE I%<FHSIZE% DO
+     DTA%[K%]=FH%[I%]
+     I%=I%+1
+     K%=K%+1
+  END WHILE
+
+END PROCEDURE
+
+PROCEDURE MERGE_SORT(LEV->LEV)
+
+! *****************************************************************
+! This procedure Merge Sorts the chunk of DTA% bounded by
+! Start% & Ends%.
+! *****************************************************************
+
+   LOCAL MIDDLE%
+
+   IF ENDS%=START% THEN LEV=LEV-1 EXIT PROCEDURE END IF
+
+   IF ENDS%-START%=1 THEN
+      IF DTA%[ENDS%]<DTA%[START%] THEN
+         SWAP(DTA%[START%],DTA%[ENDS%])
+      END IF
+      LEV=LEV-1
+      EXIT PROCEDURE
+   END IF
+
+   MIDDLE%=START%+(ENDS%-START%)/2
+
+   STACK%[LEV,0]=START%  STACK%[LEV,1]=ENDS%  STACK%[LEV,2]=MIDDLE%
+   START%=START%  ENDS%=MIDDLE%
+   MERGE_SORT(LEV+1->LEV)
+   START%=STACK%[LEV,0]  ENDS%=STACK%[LEV,1]  MIDDLE%=STACK%[LEV,2]
+
+   STACK%[LEV,0]=START%  STACK%[LEV,1]=ENDS%  STACK%[LEV,2]=MIDDLE%
+   START%=MIDDLE%+1  ENDS%=ENDS%
+   MERGE_SORT(LEV+1->LEV)
+   START%=STACK%[LEV,0]  ENDS%=STACK%[LEV,1]  MIDDLE%=STACK%[LEV,2]
+
+   MERGE(START%,MIDDLE%,ENDS%)
+
+   LEV=LEV-1
+END PROCEDURE
+
+BEGIN
+  FOR I%=1 TO SIZE% DO
+     DTA%[I%]=RND(1)*10000
+  END FOR
+
+  START%=1  ENDS%=SIZE%
+  MERGE_SORT(0->LEV)
+
+  FOR I%=1 TO SIZE% DO
+     WRITE("#####";DTA%[I%];)
+  END FOR
+  PRINT
+END PROGRAM