March 2014 update

Task/Permutations-by-swapping/Ruby/permutations-by-swapping.rb

@@ -0,0 +1,26 @@
+def perms(n)
+  p = Array.new(n+1){|i| -i}
+  s = 1
+  loop do
+    yield p[1..-1].map(&:abs), s
+    k = 0
+    for i in 2..n
+      k = i if p[i] < 0 and p[i].abs > p[i-1].abs and p[i].abs > p[k].abs
+    end
+    for i in 1...n
+      k = i if p[i] > 0 and p[i].abs > p[i+1].abs and  p[i].abs > p[k].abs
+    end
+    break if k.zero?
+    for i in 1..n
+      p[i] *= -1 if p[i].abs > p[k].abs
+    end
+    i = k + (p[k] <=> 0)
+    p[k], p[i] = p[i], p[k]
+    s = -s
+  end
+end
+
+for i in 3..4
+  perms(i){|perm, sign| puts "Perm: #{perm}  Sign: #{sign}"}
+  puts
+end