1 files changed, 22 insertions, 4 deletions

diff --git a/pkg/lang/vm/mem/mem.go b/pkg/lang/vm/mem/mem.go
index a645ee2..0f15743 100644
--- a/pkg/lang/vm/mem/mem.go
+++ b/pkg/lang/vm/mem/mem.go
@@ -10,6 +10,7 @@ type Mem interface {
 
 	Retain(ptr Ptr) error
 	Release(ptr Ptr) error
+	RefCount(ptr Ptr) int
 }
 
 type memImpl struct {
@@ -30,15 +31,20 @@ func New() Mem {
 }
 
 func (m *memImpl) Allocate(kind CellKind) (Ptr, error) {
+	if kind == CellKindForbidden || kind == CellKindEmpty {
+		return NullPtr, ErrInvalidCellKind{kind}
+	}
+
 	if len(m.free) > 0 {
 		idx := m.free[len(m.free)-1]
 		m.free = m.free[:len(m.free)-1]
 
 		if m.cells[idx].kind != CellKindEmpty {
-			return NullPtr, ErrFatalNonFreeCell
+			// This should never happen.
+			panic("cell marked as free was not empty")
 		}
 
-		m.cells[idx].kind = kind
+		m.cells[idx] = cell{kind: kind, refs: 1}
 		return Ptr(idx), nil
 	} else {
 		if len(m.cells) > 10000 {
@@ -57,6 +63,10 @@ func (m *memImpl) Set(ptr Ptr, v CellData) error {
 		return err
 	}
 
+	if m.cells[ptr].kind != v.MatchingCellKind() {
+		return ErrDifferingCellKind{Ptr: ptr, Expected: m.cells[ptr].kind, Got: v.MatchingCellKind()}
+	}
+
 	m.cells[ptr].data = v
 	return nil
 }
@@ -71,7 +81,7 @@ func (m *memImpl) Get(ptr Ptr) (CellData, error) {
 
 func (m *memImpl) Is(ptr Ptr, kind CellKind) bool {
 	if ptr >= Ptr(len(m.cells)) {
-		return false
+		return kind == CellKindEmpty
 	}
 
 	return m.cells[ptr].kind == kind
@@ -79,7 +89,7 @@ func (m *memImpl) Is(ptr Ptr, kind CellKind) bool {
 
 func (m *memImpl) Kind(ptr Ptr) CellKind {
 	if ptr >= Ptr(len(m.cells)) {
-		return CellKindForbidden
+		return CellKindEmpty
 	}
 
 	return m.cells[ptr].kind
@@ -111,6 +121,14 @@ func (m *memImpl) Release(ptr Ptr) error {
 	return nil
 }
 
+func (m *memImpl) RefCount(ptr Ptr) int {
+	if err := m.validPtr(ptr); err != nil {
+		return 0
+	}
+
+	return m.cells[ptr].refs
+}
+
 func (m *memImpl) validPtr(ptr Ptr) error {
 	if ptr >= Ptr(len(m.cells)) {
 		return ErrInvalidMemAccess{ptr}