Optimize DotvvmMetrics.ExponentialBuckets

The stupid prometheus-net library call this function for
every single datapoint...

I didn't want to cache the result arrays, as we'd have to
undo that optimization when we add some configuration
arguments to the TryGetRecommendedBuckets method.

src/Framework/Framework/Hosting/DotvvmMetrics.cs

@@ -112,24 +112,24 @@ public static class DotvvmMetrics
 
             var secStart = 1.0 / 128.0; // about 10ms, so that 1second is a boundary
             if (instrument == ResourceServeDuration)
-                return ExponentialBuckets(secStart, 2, 0.5);
+                return ExponentialBuckets(secStart, 0.5);
 
             if (instrument == ResourceServeDuration)
-                return ExponentialBuckets(secStart, 2, 0.5);
+                return ExponentialBuckets(secStart, 0.5);
 
             if (instrument == ResourceServeDuration)
-                return ExponentialBuckets(secStart, 2, 1);
+                return ExponentialBuckets(secStart, 1);
 
             if (instrument == RequestDuration || instrument == CommandInvocationDuration || instrument == StaticCommandInvocationDuration)
-                return ExponentialBuckets(secStart, 2, 65);
+                return ExponentialBuckets(secStart, 65);
 
             if (instrument.Unit == "seconds")
-                return ExponentialBuckets(secStart, 2, 2.0);
+                return ExponentialBuckets(secStart, 2.0);
 
             if (instrument.Unit == "bytes")
-                return ExponentialBuckets(1024, 2, 130 * 1024 * 1024); // 1KB ... 128MB
+                return ExponentialBuckets(1024, 130 * 1024 * 1024); // 1KB ... 128MB
 
-            return ExponentialBuckets(secStart, 2, 10);
+            return ExponentialBuckets(secStart, 10);
         }
 
         // The Counter from metrics doesn't count anything when there isn't a listener.
@@ -143,12 +143,26 @@ internal class BareCounters
             public static long DotvvmPropertyInitialized = 0;
         }
 
-        internal static double[] ExponentialBuckets(double start, double factor, double end)
+        internal static int IntegerLog2(double value)
         {
-            return Enumerable.Range(0, 1000)
-                .Select(i => start * Math.Pow(factor, i))
-                .TakeWhile(b => b <= end)
-                .ToArray();
+            // float64 is stored as 1 sign bit, 11 exponent bits, 52 mantissa bits
+            // where the exponent is essentially the integer logarithm we want
+            var bits = BitConverter.DoubleToInt64Bits(value);
+            var exponent = (int)((bits >> 52) & 0x7FF);
+            // the exponent is a signed integer, stored as unsigned with 1023 bias
+            return exponent - 1023;
+        }
+
+        internal static double[] ExponentialBuckets(double start, double end)
+        {
+            var buckets = new double[IntegerLog2(end / start) + 1];
+            var bucket = start;
+            for (int i = 0; i < buckets.Length; i++)
+            {
+                buckets[i] = bucket;
+                bucket *= 2;
+            }
+            return buckets;
         }
 
         internal static KeyValuePair<string, object?> RouteLabel(this IDotvvmRequestContext context) =>

src/Tests/Runtime/MetricsTests.cs

@@ -0,0 +1,29 @@
+using System;
+using DotVVM.Framework.Hosting;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace DotVVM.Framework.Tests.Runtime
+{
+    [TestClass]
+    public class MetricsTests
+    {
+        [TestMethod]
+        public void IntLog()
+        {
+            for (int i = 1; i < 100; i++)
+            {
+                var log = DotvvmMetrics.IntegerLog2(i);
+                var expectedLog = (int)Math.Log(i, 2);
+                Assert.AreEqual(expectedLog, log);
+            }
+        }
+
+        [TestMethod]
+        public void ExponentialBuckets()
+        {
+            XAssert.Equal(new double[] { 1, 2, 4, 8, 16, 32, 64 }, DotvvmMetrics.ExponentialBuckets(1, 64));
+            XAssert.Equal(new double[] { 0.0078125, 0.015625, 0.03125, 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8, 16, 32, 64 }, DotvvmMetrics.TryGetRecommendedBuckets(DotvvmMetrics.RequestDuration));
+
+        }
+    }
+}