apl: quicksort recursive partition — Phase 6 classics complete (+9 tests, 315/315)

lib/apl/tests/programs.sx

@@ -257,3 +257,50 @@
 (apl-test "permutations of 3 has 6" (len (apl-permutations 3)) 6)
 
 (apl-test "permutations of 4 has 24" (len (apl-permutations 4)) 24)
+
+(apl-test
+  "quicksort empty"
+  (mkrv (apl-quicksort (make-array (list 0) (list))))
+  (list))
+
+(apl-test
+  "quicksort single"
+  (mkrv (apl-quicksort (make-array (list 1) (list 42))))
+  (list 42))
+
+(apl-test
+  "quicksort already sorted"
+  (mkrv (apl-quicksort (make-array (list 5) (list 1 2 3 4 5))))
+  (list 1 2 3 4 5))
+
+(apl-test
+  "quicksort reverse sorted"
+  (mkrv (apl-quicksort (make-array (list 5) (list 5 4 3 2 1))))
+  (list 1 2 3 4 5))
+
+(apl-test
+  "quicksort with duplicates"
+  (mkrv (apl-quicksort (make-array (list 7) (list 3 1 4 1 5 9 2))))
+  (list 1 1 2 3 4 5 9))
+
+(apl-test
+  "quicksort all equal"
+  (mkrv (apl-quicksort (make-array (list 5) (list 7 7 7 7 7))))
+  (list 7 7 7 7 7))
+
+(apl-test
+  "quicksort negatives"
+  (mkrv (apl-quicksort (make-array (list 5) (list -3 1 -1 2 0))))
+  (list -3 -1 0 1 2))
+
+(apl-test
+  "quicksort 11-element pi"
+  (mkrv
+    (apl-quicksort (make-array (list 11) (list 3 1 4 1 5 9 2 6 5 3 5))))
+  (list 1 1 2 3 3 4 5 5 5 6 9))
+
+(apl-test
+  "quicksort preserves length"
+  (first
+    (mksh (apl-quicksort (make-array (list 7) (list 3 1 4 1 5 9 2)))))
+  7)

lib/apl/tests/programs/quicksort.apl

@@ -0,0 +1,25 @@
+⍝ Quicksort — the classic Roger Hui one-liner
+⍝
+⍝ Q ← {1≥≢⍵:⍵ ⋄ (∇⍵⌿⍨⍵<p),(p=⍵)/⍵,∇⍵⌿⍨⍵>p←⍵⌷⍨?≢⍵}
+⍝
+⍝ Read right-to-left:
+⍝   ?≢⍵          : pick a random index in 1..length
+⍝   ⍵⌷⍨…         : take that element as pivot p
+⍝   ⍵>p          : boolean — elements greater than pivot
+⍝   ∇⍵⌿⍨…        : recursively sort the > partition
+⍝   (p=⍵)/⍵      : keep elements equal to pivot
+⍝   ⍵<p          : boolean — elements less than pivot
+⍝   ∇⍵⌿⍨…        : recursively sort the < partition
+⍝   ,            : catenate ⟨less⟩ ⟨equal⟩ ⟨greater⟩
+⍝   1≥≢⍵:⍵       : guard — base case for length ≤ 1
+⍝
+⍝ Stability: not stable on duplicates (but eq-class is preserved as a block).
+⍝ Worst case O(N²) on already-sorted input with deterministic pivot;
+⍝ randomized pivot selection gives expected O(N log N).
+⍝
+⍝ Examples:
+⍝   Q 3 1 4 1 5 9 2 6 5 3 5    →  1 1 2 3 3 4 5 5 5 6 9
+⍝   Q ⍳0                        →  ⍬   (empty)
+⍝   Q ,42                       →  42
+
+quicksort ← {1≥≢⍵:⍵ ⋄ p←⍵⌷⍨?≢⍵ ⋄ (∇⍵⌿⍨⍵<p),(p=⍵)/⍵,∇⍵⌿⍨⍵>p}