internal/ops: expose PC and ResetAt

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/ops/reader.go

@@ -12,7 +12,7 @@ import (
 
 // Reader parses an ops list.
 type Reader struct {
-	pc        pc
+	pc        PC
 	stack     []macro
 	ops       *op.Ops
 	deferOps  op.Ops
@@ -38,33 +38,48 @@ type Key struct {
 // Shadow of op.MacroOp.
 type macroOp struct {
 	ops *op.Ops
-	pc  pc
+	pc  PC
 }
 
-type pc struct {
+// PC is an instruction counter for an operation list.
+type PC struct {
 	data int
 	refs int
 }
 
 type macro struct {
 	ops   *op.Ops
-	retPC pc
-	endPC pc
+	retPC PC
+	endPC PC
 }
 
 type opMacroDef struct {
-	endpc pc
+	endpc PC
 }
 
-// Reset start reading from the op list.
+// Reset start reading from the beginning of ops.
 func (r *Reader) Reset(ops *op.Ops) {
+	r.ResetAt(ops, PC{})
+}
+
+// ResetAt is like Reset, except it starts reading from pc.
+func (r *Reader) ResetAt(ops *op.Ops, pc PC) {
 	r.stack = r.stack[:0]
 	r.deferOps.Reset()
 	r.deferDone = false
-	r.pc = pc{}
+	r.pc = pc
 	r.ops = ops
 }
 
+// NewPC returns a PC representing the current instruction counter of
+// ops.
+func NewPC(ops *op.Ops) PC {
+	return PC{
+		data: len(ops.Data()),
+		refs: len(ops.Refs()),
+	}
+}
+
 func (k Key) SetTransform(t f32.Affine2D) Key {
 	sx, hx, _, hy, sy, _ := t.Elems()
 	k.sx = sx
@@ -99,7 +114,7 @@ func (r *Reader) Decode() (EncodedOp, bool) {
 			r.deferDone = true
 			// Execute deferred macros.
 			r.ops = &r.deferOps
-			r.pc = pc{}
+			r.pc = PC{}
 			continue
 		}
 		key := Key{ops: r.ops, pc: r.pc.data, version: r.ops.Version()}
@@ -174,7 +189,7 @@ func (op *opMacroDef) decode(data []byte) {
 	dataIdx := int(int32(bo.Uint32(data[1:])))
 	refsIdx := int(int32(bo.Uint32(data[5:])))
 	*op = opMacroDef{
-		endpc: pc{
+		endpc: PC{
 			data: dataIdx,
 			refs: refsIdx,
 		},
@@ -191,7 +206,7 @@ func (m *macroOp) decode(data []byte, refs []interface{}) {
 	refsIdx := int(int32(bo.Uint32(data[5:])))
 	*m = macroOp{
 		ops: refs[0].(*op.Ops),
-		pc: pc{
+		pc: PC{
 			data: dataIdx,
 			refs: refsIdx,
 		},