prog: move all flag mutation logic into flags() (#1362)

This makes it a bit easier to understand.

Also fix an issue with using flag value as bit number.

2 files changed, 22 insertions, 24 deletions

diff --git a/prog/mutation.go b/prog/mutation.go
index 8a33c3e2e..6c356c4b4 100644
--- a/prog/mutation.go
+++ b/prog/mutation.go
@@ -236,29 +236,9 @@ func (t *IntType) mutate(r *randGen, s *state, arg Arg, ctx ArgCtx) (calls []*Ca
 
 func (t *FlagsType) mutate(r *randGen, s *state, arg Arg, ctx ArgCtx) (calls []*Call, retry, preserve bool) {
 	a := arg.(*ConstArg)
-
 	for oldVal := a.Val; oldVal == a.Val; {
-		// Generate a new value.
-		if r.nOutOf(1, 5) {
-			a.Val = r.flags(t.Vals, t.BitMask, 0)
-			continue
-		}
-
-		if !t.BitMask || (t.BitMask && r.nOutOf(1, 4)) {
-			a.Val = r.flags(t.Vals, t.BitMask, a.Val)
-			continue
-		}
-
-		for stop := false; !stop; stop = r.oneOf(3) {
-			idx := r.rand(len(t.Vals))
-			if r.bin() {
-				a.Val |= 1 << t.Vals[idx]
-			} else {
-				a.Val &= ^(1 << t.Vals[idx])
-			}
-		}
+		a.Val = r.flags(t.Vals, t.BitMask, a.Val)
 	}
-
 	return
 }
 
diff --git a/prog/rand.go b/prog/rand.go
index 819b1104f..cbfc960a8 100644
--- a/prog/rand.go
+++ b/prog/rand.go
@@ -140,13 +140,31 @@ func (r *randGen) randPageCount() (n uint64) {
 // Change a flag value or generate a new one.
 func (r *randGen) flags(vv []uint64, bitmask bool, oldVal uint64) (v uint64) {
 	v = oldVal
+	if r.oneOf(5) {
+		// Ignore the old value sometimes.
+		v = 0
+	}
 	switch {
 	case (bitmask && r.nOutOf(7, 10)) || (!bitmask && r.nOutOf(1, 5)):
-		for stop := false; !stop; stop = r.bin() {
-			v |= vv[r.rand(len(vv))] // prioritized when bitmask = true
+		// Try flipping randomly chosen flags.
+		// Prioritized when bitmask == true.
+		for stop := false; !stop; stop = r.oneOf(3) {
+			flag := vv[r.rand(len(vv))]
+			if r.oneOf(5) {
+				// Try choosing adjacent bit values in case we forgot
+				// to add all relevant flags to the descriptions.
+				if r.bin() {
+					flag >>= 1
+				} else {
+					flag <<= 1
+				}
+			}
+			v ^= flag
 		}
 	case (bitmask && r.nOutOf(2, 3)) || (!bitmask && r.nOutOf(7, 8)):
-		v = vv[r.rand(len(vv))] // prioritized when bitmask = false
+		// Chose a random flag.
+		// Prioritized when bitmask == false.
+		v = vv[r.rand(len(vv))]
 	case r.bin():
 		v = 0
 	default: