VM: first pass at Random Access Stack object

pkg/vm/stack/stack.go

@@ -0,0 +1,123 @@
+package stack
+
+import (
+	"errors"
+	"fmt"
+)
+
+const (
+	// StackAverageSize is used to set the capacity of the stack
+	// setting this number too low, will cause extra allocations
+	StackAverageSize = 20
+)
+
+// RandomAccess represents a Random Access Stack
+type RandomAccess struct {
+	vals []Item
+}
+
+// New will return a new random access stack
+func New() *RandomAccess {
+	return &RandomAccess{
+		vals: make([]Item, 0, StackAverageSize),
+	}
+}
+
+// Items will return all items in the stack
+func (ras *RandomAccess) items() []Item {
+	return ras.vals
+}
+
+//Len will return the length of the stack
+func (ras *RandomAccess) Len() int {
+	if ras.vals == nil {
+		return -1
+	}
+	return len(ras.vals)
+}
+
+// Clear will remove all items in the stack
+func (ras *RandomAccess) Clear() {
+	ras.vals = make([]Item, 0, StackAverageSize)
+}
+
+// Pop will remove the last stack item that was added
+func (ras *RandomAccess) Pop() (Item, error) {
+	if len(ras.vals) == 0 {
+		return nil, errors.New("There are no items on the stack to pop")
+	}
+	if ras.vals == nil {
+		return nil, errors.New("Cannot pop from a nil stack")
+	}
+
+	l := len(ras.vals)
+	item := ras.vals[l-1]
+	ras.vals = ras.vals[:l-1]
+
+	return item, nil
+}
+
+// Push will put a stack item onto the top of the stack
+func (ras *RandomAccess) Push(item Item) *RandomAccess {
+	if ras.vals == nil {
+		ras.vals = make([]Item, 0, StackAverageSize)
+	}
+
+	ras.vals = append(ras.vals, item)
+
+	return ras
+}
+
+// Insert will push a stackItem onto the stack at position `n`
+// Note; index 0 is the top of the stack, which is the end of slice
+// REDO:
+func (ras *RandomAccess) Insert(n uint16, item Item) (*RandomAccess, error) {
+
+	if n == 0 {
+		return ras.Push(item), nil
+	}
+
+	if ras.vals == nil {
+		ras.vals = make([]Item, 0, StackAverageSize)
+	}
+
+	// Check that we are not inserting out of the bounds
+	stackSize := uint16(len(ras.vals))
+	if n > stackSize-1 {
+		return nil, fmt.Errorf("Tried to insert at index %d when length of stack is %d", n, len(ras.vals))
+	}
+
+	index := stackSize - n
+
+	ras.vals = append(ras.vals, item)
+	copy(ras.vals[index:], ras.vals[index-1:])
+	ras.vals[index] = item
+
+	return ras, nil
+}
+
+// Peek will check an element at a given index
+// Note: 0 is the top of the stack, which is the end of the slice
+func (ras *RandomAccess) Peek(n uint16) (Item, error) {
+
+	stackSize := uint16(len(ras.vals))
+
+	if n == 0 {
+		index := stackSize - 1
+		return ras.vals[index], nil
+	}
+
+	if ras.vals == nil {
+		return nil, errors.New("Cannot peak at a nil stack")
+	}
+
+	// Check that we are not peeking out of the bounds
+
+	if n > stackSize-1 {
+		return nil, fmt.Errorf("Tried to peek at index %d when length of stack is %d", n, len(ras.vals))
+	}
+
+	index := stackSize - n - 1
+
+	return ras.vals[index], nil
+}