Added flow checker

vms/avm/flow_checker.go

@@ -0,0 +1,51 @@
+// (c) 2019-2020, Ava Labs, Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package avm
+
+import (
+	"github.com/ava-labs/gecko/ids"
+	"github.com/ava-labs/gecko/utils/math"
+	"github.com/ava-labs/gecko/utils/wrappers"
+)
+
+// FlowChecker ...
+type FlowChecker struct {
+	consumed, produced map[[32]byte]uint64
+	errs               wrappers.Errs
+}
+
+// NewFlowChecker ...
+func NewFlowChecker() *FlowChecker {
+	return &FlowChecker{
+		consumed: make(map[[32]byte]uint64),
+		produced: make(map[[32]byte]uint64),
+	}
+}
+
+// Consume ...
+func (fc *FlowChecker) Consume(assetID ids.ID, amount uint64) { fc.add(fc.consumed, assetID, amount) }
+
+// Produce ...
+func (fc *FlowChecker) Produce(assetID ids.ID, amount uint64) { fc.add(fc.produced, assetID, amount) }
+
+func (fc *FlowChecker) add(value map[[32]byte]uint64, assetID ids.ID, amount uint64) {
+	var err error
+	assetIDKey := assetID.Key()
+	value[assetIDKey], err = math.Add64(value[assetIDKey], amount)
+	fc.errs.Add(err)
+}
+
+// Verify ...
+func (fc *FlowChecker) Verify() error {
+	if !fc.errs.Errored() {
+		for assetID, producedAssetAmount := range fc.produced {
+			consumedAssetAmount := fc.consumed[assetID]
+			if producedAssetAmount > consumedAssetAmount {
+				fc.errs.Add(errInsufficientFunds)
+				break
+			}
+		}
+	}
+	return fc.errs.Err
+}