vm: add cached ScriptHash() to Context

Avoid recalculating it over and over again in interop.

pkg/core/interop_system.go

@@ -254,7 +254,7 @@ func (ic *interopContext) engineGetScriptContainer(v *vm.VM) error {
 func getContextScriptHash(v *vm.VM, n int) util.Uint160 {
 	ctxIface := v.Istack().Peek(n).Value()
 	ctx := ctxIface.(*vm.Context)
-	return hash.Hash160(ctx.Program())
+	return ctx.ScriptHash()
 }
 
 // pushContextScriptHash pushes to evaluation stack the script hash of the

pkg/vm/context.go

@@ -3,7 +3,9 @@ package vm
 import (
 	"errors"
 
+	"github.com/CityOfZion/neo-go/pkg/crypto/hash"
 	"github.com/CityOfZion/neo-go/pkg/io"
+	"github.com/CityOfZion/neo-go/pkg/util"
 	"github.com/CityOfZion/neo-go/pkg/vm/opcode"
 )
 
@@ -29,6 +31,9 @@ type Context struct {
 
 	// Alt stack pointer.
 	astack *Stack
+
+	// Script hash of the prog.
+	scriptHash util.Uint160
 }
 
 // NewContext returns a new Context object.
@@ -125,6 +130,14 @@ func (c *Context) Program() []byte {
 	return c.prog
 }
 
+// ScriptHash returns a hash of the script in the current context.
+func (c *Context) ScriptHash() util.Uint160 {
+	if c.scriptHash.Equals(util.Uint160{}) {
+		c.scriptHash = hash.Hash160(c.prog)
+	}
+	return c.scriptHash
+}
+
 // Value implements StackItem interface.
 func (c *Context) Value() interface{} {
 	return c