Initial data commit

Task/Sorting-algorithms-Quicksort/Icon/sorting-algorithms-quicksort.icon

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+procedure main()                     #: demonstrate various ways to sort a list and string
+   demosort(quicksort,[3, 14, 1, 5, 9, 2, 6, 3],"qwerty")
+end
+
+procedure quicksort(X,op,lower,upper)                      #: return sorted list
+local pivot,x
+
+   if /lower := 1 then {                                   # top level call setup
+      upper := *X
+      op := sortop(op,X)                                   # select how and what we sort
+      }
+
+   if upper - lower > 0 then {
+      every x := quickpartition(X,op,lower,upper) do       # find a pivot and sort ...
+          /pivot | X := x                                  # ... how to return 2 values w/o a structure
+      X := quicksort(X,op,lower,pivot-1)                   # ... left
+      X := quicksort(X,op,pivot,upper)                     # ... right
+      }
+
+   return X
+end
+
+procedure quickpartition(X,op,lower,upper)                 #: quicksort partitioner helper
+local   pivot
+static  pivotL
+initial pivotL := list(3)
+
+   pivotL[1] := X[lower]                                   # endpoints
+   pivotL[2] := X[upper]                                   # ... and
+   pivotL[3] := X[lower+?(upper-lower)]                    # ... random midpoint
+   if op(pivotL[2],pivotL[1]) then pivotL[2] :=: pivotL[1] # mini-
+   if op(pivotL[3],pivotL[2]) then pivotL[3] :=: pivotL[2] # ... sort
+   pivot := pivotL[2]                                      # median is pivot
+
+   lower -:= 1
+   upper +:= 1
+   while lower < upper do {                                # find values on wrong side of pivot ...
+      while op(pivot,X[upper -:= 1])                       # ... rightmost
+      while op(X[lower +:=1],pivot)                        # ... leftmost
+      if lower < upper then                                # not crossed yet
+         X[lower] :=: X[upper]                             # ... swap
+      }
+
+   suspend lower                                           # 1st return pivot point
+   suspend X                                               # 2nd return modified X (in case immutable)
+end