blockchain: Add CalculateMinEntries

When allocating maps for Utxo caches, you may want to purposely allocate
a size thats lower than the one that'd fit in the memory limit.  Ex:
user gives a utxomaxcachesize of 1000MiB. You can allocate ~800MiB but
you may want to allocate ~400MiB to leave room for UtxoEntry sizes.
Having a function that returns the minimum amount of entries to allocate
~800MiB gives the caller the option to do -1 and allocate half as much
memory for the map.

blockchain/sizehelper.go

@@ -129,6 +129,36 @@ func CalculateEntries(totalBytes int, bucketSize int) int {
 	return int(loadFactorNum * (bucketShift(B) / loadFactorDen))
 }
 
+// CalculateMinEntries returns the minimum number of entries that will make the
+// map allocate the given total bytes.  -1 on the returned entry count will
+// make the map allocate half as much total bytes (for returned entry count that's
+// greater than 0).
+func CalculateMinEntries(totalBytes int, bucketSize int) int {
+	// 48 is the number of bytes needed for the map header in a
+	// 64 bit system. Refer to hmap in runtime/map.go in the go
+	// standard library.
+	totalBytes -= 48
+
+	numBuckets := totalBytes / bucketSize
+	B := uint8(math.Log2(float64(numBuckets)))
+	if B < 4 {
+		switch B {
+		case 0:
+			return 0
+		case 1:
+			return 9
+		case 2:
+			return 14
+		default:
+			return 27
+		}
+	}
+
+	B -= 1
+
+	return (int(loadFactorNum * (bucketShift(B) / loadFactorDen))) + 1
+}
+
 // mulUintptr returns a * b and whether the multiplication overflowed.
 // On supported platforms this is an intrinsic lowered by the compiler.
 func mulUintptr(a, b uintptr) (uintptr, bool) {

blockchain/sizehelper_test.go

@@ -29,5 +29,21 @@ func TestCalculateEntries(t *testing.T) {
 			t.Errorf("For hint %d incremented by 1, expected %v, got %v\n",
 				i, gotRoughMapSizeWrong*2, gotRoughMapSizeWrong)
 		}
+
+		minEntries := CalculateMinEntries(roughMapSize, bucketSize)
+		gotMinRoughMapSize := CalculateRoughMapSize(minEntries, bucketSize)
+		if roughMapSize != gotMinRoughMapSize {
+			t.Errorf("For hint of %d, expected %v, got %v\n",
+				i, roughMapSize, gotMinRoughMapSize)
+		}
+
+		// Can only test if they'll be half the size if the entries aren't 0.
+		if minEntries > 0 {
+			gotMinRoughMapSizeWrong := CalculateRoughMapSize(minEntries-1, bucketSize)
+			if gotMinRoughMapSize == gotMinRoughMapSizeWrong {
+				t.Errorf("For hint %d decremented by 1, expected %v, got %v\n",
+					i, gotRoughMapSizeWrong/2, gotRoughMapSizeWrong)
+			}
+		}
 	}
 }