From 5f0f9e1638b66a75e54c87c5a2f4d60360ecc5cb Mon Sep 17 00:00:00 2001
From: Roman Khimov <roman@nspcc.ru>
Date: Thu, 5 Sep 2019 15:18:04 +0300
Subject: [PATCH] vm: implement PICK instruction

---
 pkg/vm/vm.go      |  8 ++++++++
 pkg/vm/vm_test.go | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/pkg/vm/vm.go b/pkg/vm/vm.go
index a8c1df58c..11c7b3647 100644
--- a/pkg/vm/vm.go
+++ b/pkg/vm/vm.go
@@ -337,6 +337,14 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 		v.estack.Push(b)
 		v.estack.Push(a)
 
+	case PICK:
+		n := int(v.estack.Pop().BigInt().Int64())
+		if n < 0 {
+			panic("negative stack item returned")
+		}
+		a := v.estack.Peek(n)
+		v.estack.Push(a)
+
 	case ROLL:
 		n := int(v.estack.Pop().BigInt().Int64())
 		if n < 0 {
diff --git a/pkg/vm/vm_test.go b/pkg/vm/vm_test.go
index b9ec5ee74..e80eabfb0 100644
--- a/pkg/vm/vm_test.go
+++ b/pkg/vm/vm_test.go
@@ -264,6 +264,39 @@ func TestNZfalse(t *testing.T) {
 	assert.Equal(t, false, vm.estack.Pop().Bool())
 }
 
+func TestPICKbadNoitem(t *testing.T) {
+	prog := makeProgram(PICK)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestPICKbadNegative(t *testing.T) {
+	prog := makeProgram(PICK)
+	vm := load(prog)
+	vm.estack.PushVal(-1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestPICKgood(t *testing.T) {
+	prog := makeProgram(PICK)
+	result := 2
+	vm := load(prog)
+	vm.estack.PushVal(0)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(result)
+	vm.estack.PushVal(3)
+	vm.estack.PushVal(4)
+	vm.estack.PushVal(5)
+	vm.estack.PushVal(3)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(result), vm.estack.Pop().BigInt().Int64())
+}
+
+
 func makeProgram(opcodes ...Instruction) []byte {
 	prog := make([]byte, len(opcodes)+1) // RET
 	for i := 0; i < len(opcodes); i++ {