From a9170c4330628f66fa13dce29dff59eded1c82e3 Mon Sep 17 00:00:00 2001
From: Chad Brokaw <cbrokaw@gmail.com>
Date: Mon, 7 Nov 2022 23:26:51 -0500
Subject: [PATCH] prevent bad encoding with only move-tos

---
 piet-gpu/src/samples.rs         | 11 +++++++++++
 piet-scene/src/scene/builder.rs | 20 +++++++-------------
 2 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/piet-gpu/src/samples.rs b/piet-gpu/src/samples.rs
index a52ba74..0cbfd57 100644
--- a/piet-gpu/src/samples.rs
+++ b/piet-gpu/src/samples.rs
@@ -13,6 +13,10 @@ pub fn render_funky_paths(sb: &mut SceneBuilder) {
         LineTo((0.0, 400.0).into()),
         LineTo((100.0, 400.0).into()),
     ];
+    let only_movetos = [
+        PathElement::MoveTo((0.0, 0.0).into()),
+        PathElement::MoveTo((100.0, 100.0).into()),
+    ];
     let empty: [PathElement; 0] = [];
     sb.fill(
         Fill::NonZero,
@@ -28,6 +32,13 @@ pub fn render_funky_paths(sb: &mut SceneBuilder) {
         None,
         empty,
     );
+    sb.fill(
+        Fill::NonZero,
+        Affine::IDENTITY,
+        &Color::rgb8(0, 0, 255).into(),
+        None,
+        only_movetos,
+    );
     sb.stroke(
         &simple_stroke(8.0),
         Affine::translate(100.0, 100.0),
diff --git a/piet-scene/src/scene/builder.rs b/piet-scene/src/scene/builder.rs
index 8007753..7100433 100644
--- a/piet-scene/src/scene/builder.rs
+++ b/piet-scene/src/scene/builder.rs
@@ -160,8 +160,8 @@ impl<'a> SceneBuilder<'a> {
                 PathElement::Close => b.close_path(),
             }
         }
+        b.finish();
         if b.n_pathseg != 0 {
-            b.path();
             self.scene.n_path += 1;
             self.scene.n_pathseg += b.n_pathseg;
             true
@@ -434,7 +434,7 @@ impl<'a> PathBuilder<'a> {
         self.state = PathState::Start;
     }
 
-    fn finish(&mut self) {
+    pub fn finish(&mut self) {
         if self.is_fill {
             self.close_path();
         }
@@ -442,17 +442,11 @@ impl<'a> PathBuilder<'a> {
             let new_len = self.pathseg_stream.len() - 8;
             self.pathseg_stream.truncate(new_len);
         }
-        if let Some(tag) = self.tag_stream.last_mut() {
-            *tag |= 4;
+        if self.n_pathseg != 0 {
+            if let Some(tag) = self.tag_stream.last_mut() {
+                *tag |= 4;
+            }
+            self.tag_stream.push(0x10);
         }
     }
-
-    /// Finish encoding a path.
-    ///
-    /// Encode this after encoding path segments.
-    pub fn path(&mut self) {
-        self.finish();
-        // maybe don't encode if path is empty? might throw off sync though
-        self.tag_stream.push(0x10);
-    }
 }