From 1ff0caf40e69239be962263d25888975fe5a9144 Mon Sep 17 00:00:00 2001
From: BlockChainDev <kevtheappdev@gmail.com>
Date: Fri, 15 Mar 2019 22:32:08 +0000
Subject: [PATCH] Add Context stack Item

---
 pkg/vm/stack/context.go      | 137 +++++++++++++++++++++++++++++++++++
 pkg/vm/stack/context_test.go |  20 +++++
 pkg/vm/stack/stackitem.go    |   7 ++
 3 files changed, 164 insertions(+)
 create mode 100644 pkg/vm/stack/context.go
 create mode 100644 pkg/vm/stack/context_test.go

diff --git a/pkg/vm/stack/context.go b/pkg/vm/stack/context.go
new file mode 100644
index 000000000..37e342739
--- /dev/null
+++ b/pkg/vm/stack/context.go
@@ -0,0 +1,137 @@
+package stack
+
+import (
+	"encoding/binary"
+)
+
+// Context represent the current execution context of the VM.
+// context will be treated as stack item
+// and placed onto the invocation stack
+type Context struct {
+	*abstractItem
+
+	// Instruction pointer.
+	ip int
+
+	// The raw program script.
+	prog []byte
+
+	// Breakpoints
+	breakPoints []int
+
+	// Evaluation Stack
+	Estack RandomAccess
+}
+
+// NewContext return a new Context object.
+func NewContext(b []byte) *Context {
+	return &Context{
+		abstractItem: &abstractItem{},
+		ip:           -1,
+		prog:         b,
+		breakPoints:  []int{},
+	}
+}
+
+// Context overrides the default implementation
+// to return a context item
+func (c *Context) Context() (*Context, error) {
+	return c, nil
+}
+
+// Next return the next instruction to execute.
+func (c *Context) Next() Instruction {
+	c.ip++
+	if c.ip >= len(c.prog) {
+		return RET
+	}
+	return Instruction(c.prog[c.ip])
+}
+
+// IP returns the absolute instruction without taking 0 into account.
+// If that program starts the ip = 0 but IP() will return 1, cause its
+// the first instruction.
+func (c *Context) IP() int {
+	return c.ip + 1
+}
+
+// LenInstr returns the number of instructions loaded.
+func (c *Context) LenInstr() int {
+	return len(c.prog)
+}
+
+// CurrInstr returns the current instruction and opcode.
+func (c *Context) CurrInstr() (int, Instruction) {
+	if c.ip < 0 {
+		return c.ip, Instruction(0x00)
+	}
+	return c.ip, Instruction(c.prog[c.ip])
+}
+
+// Copy returns an new exact copy of c.
+func (c *Context) Copy() *Context {
+	return &Context{
+		ip:          c.ip,
+		prog:        c.prog,
+		breakPoints: c.breakPoints,
+	}
+}
+
+// Program returns the loaded program.
+func (c *Context) Program() []byte {
+	return c.prog
+}
+
+func (c *Context) atBreakPoint() bool {
+	for _, n := range c.breakPoints {
+		if n == c.ip {
+			return true
+		}
+	}
+	return false
+}
+
+func (c *Context) String() string {
+	return "execution context"
+}
+
+func (c *Context) readUint32() uint32 {
+	start, end := c.IP(), c.IP()+4
+	if end > len(c.prog) {
+		return 0
+	}
+	val := binary.LittleEndian.Uint32(c.prog[start:end])
+	c.ip += 4
+	return val
+}
+
+func (c *Context) readUint16() uint16 {
+	start, end := c.IP(), c.IP()+2
+	if end > len(c.prog) {
+		return 0
+	}
+	val := binary.LittleEndian.Uint16(c.prog[start:end])
+	c.ip += 2
+	return val
+}
+
+func (c *Context) readByte() byte {
+	return c.readBytes(1)[0]
+}
+
+func (c *Context) readBytes(n int) []byte {
+	start, end := c.IP(), c.IP()+n
+	if end > len(c.prog) {
+		return nil
+	}
+
+	out := make([]byte, n)
+	copy(out, c.prog[start:end])
+	c.ip += n
+	return out
+}
+
+func (c *Context) readVarBytes() []byte {
+	n := c.readByte()
+	return c.readBytes(int(n))
+}
diff --git a/pkg/vm/stack/context_test.go b/pkg/vm/stack/context_test.go
new file mode 100644
index 000000000..6dbe36bb6
--- /dev/null
+++ b/pkg/vm/stack/context_test.go
@@ -0,0 +1,20 @@
+package stack
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNextInstruction(t *testing.T) {
+	// PUSHBYTES1 2
+	builder := NewBuilder()
+	builder.EmitBytes([]byte{0x02}) //[]byte{0x01, 0x02}
+
+	ctx := NewContext(builder.Bytes())
+	op := ctx.Next()
+	byt := ctx.readByte()
+
+	assert.Equal(t, PUSHBYTES1, op)
+	assert.Equal(t, byte(2), byt)
+}
diff --git a/pkg/vm/stack/stackitem.go b/pkg/vm/stack/stackitem.go
index 9f29f9d3b..beed13363 100644
--- a/pkg/vm/stack/stackitem.go
+++ b/pkg/vm/stack/stackitem.go
@@ -10,6 +10,7 @@ type Item interface {
 	Boolean() (*Boolean, error)
 	ByteArray() (*ByteArray, error)
 	Array() (*Array, error)
+	Context() (*Context, error)
 }
 
 // Represents an `abstract` stack item
@@ -40,3 +41,9 @@ func (a *abstractItem) ByteArray() (*ByteArray, error) {
 func (a *abstractItem) Array() (*Array, error) {
 	return nil, errors.New("This stack item is not an array")
 }
+
+// Context is the default implementation for a stackItem
+// Implements Item interface
+func (a *abstractItem) Context() (*Context, error) {
+	return nil, errors.New("This stack item is not of type context")
+}