prog: adjust number of image mutations

Also adjust number of mutations per image.
We do one per 128K of interesting data. But we have no single seed
that has so much interesting data. Here is the amount of interesting data in seeds:
https://gist.github.com/dvyukov/566b20364610d80e5d0534524546c3f0
Reduce this cuhnk size to 4K.

1 files changed, 11 insertions, 2 deletions

diff --git a/prog/heatmap.go b/prog/heatmap.go
index d1c5ea3b9..2bfbceacc 100644
--- a/prog/heatmap.go
+++ b/prog/heatmap.go
@@ -37,8 +37,17 @@ func MakeGenericHeatmap(data []byte, r *rand.Rand) Heatmap {
 }
 
 func (hm *GenericHeatmap) NumMutations() int {
-	// At least two mutations, up to about one mutation every 128 KB of heatmap size.
-	return hm.r.Intn(hm.length/(1<<17)+1) + 2
+	// At least one mutation.
+	n := 1
+	// + up to about one mutation every 4 KB of heatmap size.
+	n += hm.r.Intn(hm.length/(4<<10) + 1)
+	// + up to 4 mutations at random so that even small images can get more than one.
+	n += hm.r.Intn(5)
+	// But don't do too many as it will most likely corrupt the image.
+	if max := 10; n > max {
+		n = max
+	}
+	return n
 }
 
 func (hm *GenericHeatmap) ChooseLocation() int {