Sepearate ComputeGasOutputs into sepearate function, add tests

Signed-off-by: Jakub Sztandera <kubuxu@protocol.ai>

chain/vm/burn.go

@@ -0,0 +1,18 @@
+package vm
+
+const (
+	gasOveruseNum   = 3
+	gasOveruseDenom = 10
+)
+
+// ComputeGasOutputs computes amount of gas to be refunded and amount of gas to be burned
+// Result is (refund, burn)
+func ComputeGasOutputs(gasUsed, gasLimit int64) (int64, int64) {
+	allowedGasOverUsed := gasUsed + (gasUsed*gasOveruseNum)/gasOveruseDenom
+	gasToBurn := gasLimit - allowedGasOverUsed
+	if gasToBurn < 0 {
+		gasToBurn = 0
+	}
+
+	return gasLimit - gasUsed - gasToBurn, gasToBurn
+}

chain/vm/burn_test.go

@@ -0,0 +1,34 @@
+package vm
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGasBurn(t *testing.T) {
+	tests := []struct {
+		used   int64
+		limit  int64
+		refund int64
+		burn   int64
+	}{
+		{100, 200, 30, 70},
+		{1000, 1300, 300, 0},
+		{500, 700, 150, 50},
+		{200, 200, 0, 0},
+		{20000, 21000, 1000, 0},
+		{0, 2000, 0, 2000},
+		{500, 651, 150, 1},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(fmt.Sprintf("%v", test), func(t *testing.T) {
+			refund, toBurn := ComputeGasOutputs(test.used, test.limit)
+			assert.Equal(t, test.burn, toBurn)
+			assert.Equal(t, test.refund, refund)
+		})
+	}
+}

chain/vm/vm.go

@@ -37,11 +37,6 @@ var log = logging.Logger("vm")
 var actorLog = logging.Logger("actors")
 var gasOnActorExec = newGasCharge("OnActorExec", 0, 0)
 
-const (
-	gasOveruseNum   = 3
-	gasOveruseDenom = 10
-)
-
 // ResolveToKeyAddr returns the public key type of address (`BLS`/`SECP256K1`) of an account actor identified by `addr`.
 func ResolveToKeyAddr(state types.StateTree, cst cbor.IpldStore, addr address.Address) (address.Address, aerrors.ActorError) {
 	if addr.Protocol() == address.BLS || addr.Protocol() == address.SECP256K1 {
@@ -459,15 +454,10 @@ func (vm *VM) ApplyMessage(ctx context.Context, cmsg types.ChainMsg) (*ApplyRet,
 	if gasUsed < 0 {
 		gasUsed = 0
 	}
-
+	gasRefund, gasToBurn := ComputeGasOutputs(gasUsed, msg.GasLimit)
-	allowedGasOverUsed := gasUsed + (gasUsed*gasOveruseNum)/gasOveruseDenom
-	gasToBurn := msg.GasLimit - allowedGasOverUsed
-	if gasToBurn < 0 {
-		gasToBurn = 0
-	}
 
 	// refund unused gas
-	refund := types.BigMul(types.NewInt(uint64(msg.GasLimit-gasUsed-gasToBurn)), msg.GasPrice)
+	refund := types.BigMul(types.NewInt(uint64(gasRefund)), msg.GasPrice)
 	if err := vm.transferFromGasHolder(msg.From, gasHolder, refund); err != nil {
 		return nil, xerrors.Errorf("failed to refund gas")
 	}