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

lib/apl/tests/programs/quicksort.apl

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+⍝ 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}