2 files changed, 53 insertions, 0 deletions
diff --git a/challenge-020/fjwhittle/ch-1.p6 b/challenge-020/fjwhittle/ch-1.p6
new file mode 100644
index 0000000000..b18af980d9
--- /dev/null
+++ b/challenge-020/fjwhittle/ch-1.p6
@@ -0,0 +1,7 @@
+#!/usr/bin/env perl6
+
+unit sub MAIN(Str $string);
+
+my Str @split = $string.comb: / (.) $0 * /;
+
+@split».put
diff --git a/challenge-020/fjwhittle/ch-2.p6 b/challenge-020/fjwhittle/ch-2.p6
new file mode 100644
index 0000000000..f7cf8c68fb
--- /dev/null
+++ b/challenge-020/fjwhittle/ch-2.p6
@@ -0,0 +1,46 @@
+#!/usr/bin/env perl6
+
+use experimental :cached;
+
+sub proper-divisor-sum(Int $n --> Int) is cached {
+  [+] (1, |(2..($n.sqrt)).race.map: -> $m {
+	  |($n %% $m ?? ($m, Int($n / $m)) !! ())
+	}).unique
+}
+
+# Euler version; somewhat disappointing density of results, but produces the
+# first pair instantly.
+#| Generate <count> amicable number pairs.
+multi MAIN(
+  $count = 1,             #= Generate this many numbers; defaults to 1.
+  Bool :$euler where *.so #= Use an Euler generation instead of filtering out
+                          #= non-amicable pairs; sparse results (1st, 8th, ...)
+) {
+  my @amicable = lazy gather for hyper 1^..Inf -> $n {
+    for hyper 0^..^$n -> $m {
+      my $prefix := (2 ** ($n - $m) + 1);
+      my $p = $prefix      * 2 **  $m       - 1;
+      my $q = $prefix      * 2 **  $n       - 1;
+      my $r = $prefix ** 2 * 2 ** ($m + $n) - 1;
+
+      take (2 ** $n * $p * $q) => (2 ** $n * $r)
+        if all($p, $q, $r).is-prime
+    }
+  }
+
+  @amicable[^$count]».fmt('%d, %d')».put
+};
+
+# Naive version, that produces many more results.
+multi MAIN($count = 1, :$no-euler) is hidden-from-USAGE {
+  my SetHash $ak;
+  
+  my @amicable = lazy (1..Inf).hyper.map(-> $n { $n => proper-divisor-sum($n) })
+    .grep({ !$ak{.key}
+  	    and .key != .value
+            and proper-divisor-sum(.value) == .key
+  	    and $ak{.key, .value}»++
+          });
+
+  @amicable[^$count]».fmt('%d, %d')».put
+}