Improve report to show duration/i, e.g: 12.34 ns/i. (#132)

```
Warming up --------------------------------------
          string.new     1.485M i/100ms
       string.concat     1.086M i/100ms
        string.split   649.289k i/100ms
Calculating -------------------------------------
          string.new     15.947M (± 1.2%) i/s   (62.71 ns/i) -     32.665M in   2.048640s
       string.concat     10.846M (± 0.7%) i/s   (92.20 ns/i) -     21.718M in   2.002441s
        string.split      6.506M (± 0.3%) i/s  (153.70 ns/i) -     13.635M in   2.095740s
```

README.md

@@ -64,24 +64,24 @@ end
 This will generate the following report:
 
 ```
-Calculating -------------------------------------
-            addition    71.254k i/100ms
-           addition2    68.658k i/100ms
-           addition3    83.079k i/100ms
+Warming up --------------------------------------
+            addition     3.572M i/100ms
+           addition2     3.672M i/100ms
+           addition3     3.677M i/100ms
 addition-test-long-label
-                        70.129k i/100ms
--------------------------------------------------
-            addition     4.955M (± 8.7%) i/s -     24.155M
-           addition2    24.011M (± 9.5%) i/s -    114.246M
-           addition3    23.958M (±10.1%) i/s -    115.064M
+                         3.511M i/100ms
+Calculating -------------------------------------
+            addition     36.209M (± 2.8%) i/s   (27.62 ns/i) -    182.253M in   5.037433s
+           addition2     36.552M (± 7.8%) i/s   (27.36 ns/i) -    183.541M in   5.069987s
+           addition3     36.639M (± 4.8%) i/s   (27.29 ns/i) -    182.994M in   5.009234s
 addition-test-long-label
-                         5.014M (± 9.1%) i/s -     24.545M
+                         36.164M (± 5.8%) i/s   (27.65 ns/i) -    181.312M in   5.038364s
 
 Comparison:
-           addition2: 24011974.8 i/s
-           addition3: 23958619.8 i/s - 1.00x slower
-addition-test-long-label:  5014756.0 i/s - 4.79x slower
-            addition:  4955278.9 i/s - 4.85x slower
+           addition2: 36558904.5 i/s
+           addition3: 36359284.0 i/s - same-ish: difference falls within error
+addition-test-long-label: 36135428.8 i/s - same-ish: difference falls within error
+            addition: 34666931.3 i/s - same-ish: difference falls within error
 ```
 
 Benchmark/ips will report the number of iterations per second for a given block

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
+          "%.2f ns" % duration_ns
+        elsif duration_ns < 1_000_000
+          "%.2f μs" % (duration_ns / 1000)
+        elsif duration_ns < 1_000_000_000
+          "%.2f ms" % (duration_ns / 1_000_000)
+        else
+          "%.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
+          per_iter = (" (%s/i)" % Helpers.humanize_duration(1_000_000_000 / @stats.central_tendency)).rjust(15)
+
           case Benchmark::IPS.options[:format]
           when :human
-            left = "%s (±%4.1f%%) i/s" % [Helpers.scale(@stats.central_tendency), @stats.error_percentage]
+            left = ("%s (±%4.1f%%) i/s" % [Helpers.scale(@stats.central_tendency), @stats.error_percentage]).ljust(20)
             iters = Helpers.scale(@iterations)
 
             if @show_total_time
-              left.ljust(20) + (" - %s in %10.6fs" % [iters, runtime])
+              left + per_iter + (" - %s in %10.6fs" % [iters, runtime])
             else
-              left.ljust(20) + (" - %s" % iters)
+              left + per_iter + (" - %s" % iters)
             end
           else
-            left = "%10.1f (±%.1f%%) i/s" % [@stats.central_tendency, @stats.error_percentage]
+            left = ("%10.1f (±%.1f%%) i/s" % [@stats.central_tendency, @stats.error_percentage]).ljust(20)
 
             if @show_total_time
-              left.ljust(20) + (" - %10d in %10.6fs" % [@iterations, runtime])
+              left + per_iter + (" - %10d in %10.6fs" % [@iterations, runtime])
             else
-              left.ljust(20) + (" - %10d" % @iterations)
+              left + per_iter + (" - %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