Improve report to show duration/i (e.g: 12.24 ns/i).

```
Warming up --------------------------------------
          string.new     1.406M i/100ms
       string.concat     1.091M i/100ms
Calculating -------------------------------------
          string.new  45.48 ns/i    21.990M (± 4.0%) i/s -     44.997M in   2.049861s
       string.concat  64.32 ns/i    15.546M (± 5.9%) i/s -     31.644M in   2.045207s
```

lib/benchmark/ips.rb

@@ -93,6 +93,19 @@ def scale(value)
         "%10.3f#{suffix}" % scaled_value
       end
       module_function :scale
+
+      def humanize_duration(duration_ns)
+        if duration_ns < 1000
+          "%4.2f ns" % duration_ns
+        elsif duration_ns < 1_000_000
+          "%4.2f μs" % (duration_ns / 1000)
+        elsif duration_ns < 1_000_000_000
+          "%4.2f ms" % (duration_ns / 1_000_000)
+        else
+          "%4.2f s" % (duration_ns / 1_000_000_000)
+        end
+      end
+      module_function :humanize_duration
     end
   end
 

lib/benchmark/ips/report.rb

@@ -86,23 +86,25 @@ def error_percentage
         # percentage of standard deviation, iterations in runtime.
         # @return [String] Left justified body.
         def body
+          iter_duration = " %s/i" % [Helpers.humanize_duration(1_000_000_000 / @stats.central_tendency)]
+
           case Benchmark::IPS.options[:format]
           when :human
             left = "%s (±%4.1f%%) i/s" % [Helpers.scale(@stats.central_tendency), @stats.error_percentage]
             iters = Helpers.scale(@iterations)
 
             if @show_total_time
-              left.ljust(20) + (" - %s in %10.6fs" % [iters, runtime])
+              iter_duration + left.ljust(20) + (" - %s in %10.6fs" % [iters, runtime])
             else
-              left.ljust(20) + (" - %s" % iters)
+              iter_duration + left.ljust(20) + (" - %s" % iters)
             end
           else
             left = "%10.1f (±%.1f%%) i/s" % [@stats.central_tendency, @stats.error_percentage]
 
             if @show_total_time
-              left.ljust(20) + (" - %10d in %10.6fs" % [@iterations, runtime])
+              iter_duration + left.ljust(20) + (" - %10d in %10.6fs" % [@iterations, runtime])
             else
-              left.ljust(20) + (" - %10d" % @iterations)
+              iter_duration + left.ljust(20) + (" - %10d" % @iterations)
             end
           end
         end

test/test_benchmark_ips.rb

@@ -268,4 +268,12 @@ def test_small_warmup_and_time
     end
     assert_equal 1, report.entries[0].iterations
   end
+
+  def test_humanize_duration
+    assert_equal Benchmark::IPS::Helpers.humanize_duration(0.000000001), "0.00 ns"
+    assert_equal Benchmark::IPS::Helpers.humanize_duration(123.456789), "123.46 ns"
+    assert_equal Benchmark::IPS::Helpers.humanize_duration(12345.67890123), "12.35 μs"
+    assert_equal Benchmark::IPS::Helpers.humanize_duration(123456789.0123456789), "123.46 ms"
+    assert_equal Benchmark::IPS::Helpers.humanize_duration(123456789012.3456789012), "123.46 s"
+  end
 end