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perl/Examples/Chap4/permute-flop

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+
+
+###
+### permute-flop
+###
+
+## Chapter 4 section 3.1
+
+sub permute {
+  my @items = @_;
+  my $n = 0;
+  return Iterator {
+    $n++, return @items if $n==0;
+    my $i;
+    my $p = $n;
+    for ($i=1; $i<=@items && $p%$i==0; $i++) {
+      $p /= $i;
+    }
+    my $d = $p % $i;
+    my $j = @items - $i;
+    return if $j < 0;
+
+    @items[$j+1..$#items] = reverse @items[$j+1..$#items];
+    @items[$j,$j+$d] = @items[$j+$d,$j];
+
+    $n++;
+    return @items;
+  };
+}