apl: primes sieve (2=+⌿0=A∘.|A)/A←⍳N + apl-compress (+11 tests, 280/280)

lib/apl/runtime.sx

@@ -796,6 +796,34 @@
         ((result (filter (fn (x) (not (index-of b-ravel x))) a-ravel)))
         (make-array (list (len result)) result)))))
 
+(define
+  apl-compress
+  (fn
+    (mask arr)
+    (let
+      ((mask-ravel (get mask :ravel)) (arr-ravel (get arr :ravel)))
+      (let
+        ((kept (filter (fn (i) (not (= 0 (nth mask-ravel i)))) (range 0 (len arr-ravel)))))
+        (let
+          ((picked (map (fn (i) (nth arr-ravel i)) kept)))
+          (make-array (list (len picked)) picked))))))
+
+(define
+  apl-primes
+  (fn
+    (n)
+    (let
+      ((a (apl-iota (apl-scalar n))))
+      (let
+        ((mod-table (apl-outer apl-mod a a)))
+        (let
+          ((zero-mask (apl-eq (apl-scalar 0) mod-table)))
+          (let
+            ((divisor-counts (apl-reduce-first apl-add zero-mask)))
+            (let
+              ((prime-mask (apl-eq (apl-scalar 2) divisor-counts)))
+              (apl-compress prime-mask a))))))))
+
 (define
   apl-reduce
   (fn