[glyf] Adopt a state machine like style for the path builder

Toward making glyf path reader alloc free at least on gvar absence.

src/hb-ot-glyf-table.hh

@@ -1031,6 +1031,41 @@ struct glyf
         add_gid_and_children (item.glyphIndex, gids_to_retain, depth);
     }
 
+    struct path_builder_t
+    {
+      hb_font_t *font;
+      const hb_draw_funcs_t *funcs;
+      void *user_data;
+      void (*quad_to) (hb_font_t *, const hb_draw_funcs_t *,
+		       float, float, float, float, float, float,
+		       void *);
+      float last_x, last_y;
+
+      struct optional_point_t
+      {
+	optional_point_t () { is_null = true; }
+	optional_point_t (float x_, float y_) { x = x_; y = y_; is_null = false; }
+
+	bool is_null;
+	float x;
+	float y;
+
+	optional_point_t lerp (optional_point_t p, float t)
+	{ return optional_point_t (x + t * (p.x - x), y + t * (p.y - y)); }
+      } first_oncurve, first_offcurve, last_offcurve;
+
+      path_builder_t (hb_font_t *font_, const hb_draw_funcs_t *funcs_, void *user_data_)
+      {
+	font = font_;
+	funcs = funcs_;
+	user_data = user_data_;
+	quad_to = funcs->quadratic_to ? _normal_quadratic_to_call : _translate_quadratic_to_cubic;
+	last_x = last_y = 0;
+	end_of_contour ();
+      }
+
+      void end_of_contour () { first_oncurve = first_offcurve = last_offcurve = optional_point_t (); }
+
       static void
       _normal_quadratic_to_call (hb_font_t *font, const hb_draw_funcs_t *funcs,
 				 float from_x HB_UNUSED, float from_y HB_UNUSED,
@@ -1059,53 +1094,15 @@ struct glyf
 			 user_data);
       }
 
-    struct optional_point_t
+      /* based on https://github.com/RazrFalcon/ttf-parser/blob/master/src/glyf.rs#L292 */
-    {
+      void consume_point (contour_point_t &point)
-      optional_point_t () { is_null = true; }
-      optional_point_t (float x_, float y_) { x = x_; y = y_; is_null = false; }
-
-      bool is_null;
-      float x;
-      float y;
-
-      optional_point_t lerp (optional_point_t p, float t)
-      { return optional_point_t (x + t * (p.x - x), y + t * (p.y - y)); }
-    };
-
-    bool
-    get_path (hb_font_t *font, hb_codepoint_t gid,
-	      const hb_draw_funcs_t *funcs, void *user_data) const
-    {
-      /* Making this completely alloc free is not that easy
-	 https://github.com/harfbuzz/harfbuzz/issues/2095
-	 mostly because of gvar handling in VF fonts,
-	 perhaps a separate path can be considered */
-      contour_point_vector_t all_points;
-      if (unlikely (!get_points (font, gid, all_points))) return false;
-      hb_array_t<contour_point_t> points = all_points.sub_array (0, all_points.length - 4);
-
-      void (*quad_to) (hb_font_t *, const hb_draw_funcs_t *,
-		       float, float, float, float, float, float,
-		       void *) = funcs->quadratic_to ? _normal_quadratic_to_call : _translate_quadratic_to_cubic;
-
-      /* based on https://github.com/RazrFalcon/ttf-parser/blob/master/src/glyf.rs#L292 which is base on font-rs */
-      unsigned point_index = 0;
-      while (point_index < points.length)
       {
         /* Skip empty contours */
-	if (points[point_index].is_end_point)
+        if (unlikely (point.is_end_point && first_oncurve.is_null && first_offcurve.is_null))
-	{
+	  return;
-	  ++point_index;
+
-	  continue;
+	bool is_on_curve = point.flag & Glyph::FLAG_ON_CURVE;
-	}
+	optional_point_t p (point.x, point.y);
-	optional_point_t first_oncurve = optional_point_t ();
-	optional_point_t first_offcurve = optional_point_t ();
-	optional_point_t last_offcurve = optional_point_t ();
-	float last_x = 0, last_y = 0;
-	do
-	{
-	  bool is_on_curve = points[point_index].flag & Glyph::FLAG_ON_CURVE;
-	  optional_point_t p (points[point_index].x, points[point_index].y);
 	if (first_oncurve.is_null)
 	{
 	  if (is_on_curve)
@@ -1165,8 +1162,9 @@ struct glyf
 	    }
 	  }
 	}
-	} while (!points[point_index++].is_end_point);
 
+	if (point.is_end_point)
+	{
 	  while (true)
 	  {
 	    if (!first_offcurve.is_null && !last_offcurve.is_null)
@@ -1197,8 +1195,21 @@ struct glyf
 	      break;
 	    }
 	  }
+	  end_of_contour ();
 	  funcs->close_path (user_data);
 	}
+      }
+    };
+
+    bool
+    get_path (hb_font_t *font, hb_codepoint_t gid,
+	      const hb_draw_funcs_t *funcs, void *user_data) const
+    {
+      path_builder_t builder (font, funcs, user_data);
+      contour_point_vector_t all_points;
+      if (unlikely (!get_points (font, gid, all_points))) return false;
+      for (unsigned point_index = 0; point_index < all_points.length - 4; ++point_index)
+        builder.consume_point (all_points[point_index]);
       return true;
     }