From d277addb2f198e7bb710a79d7ae0c97c85613719 Mon Sep 17 00:00:00 2001
From: Behdad Esfahbod <behdad@behdad.org>
Date: Sat, 30 Jul 2022 10:10:21 -0600
Subject: [PATCH] [buffer] Add HB_BUFFER_FLAG_PRODUCE_SAFE_TO_INSERT_KASHIDA

---
 src/hb-buffer.h    |  3 ++-
 src/hb-buffer.hh   |  5 +++++
 src/hb-ot-shape.cc | 21 +++++++++++++++++----
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index 820a90517..aee2d7392 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -389,8 +389,9 @@ typedef enum { /*< flags >*/
   HB_BUFFER_FLAG_DO_NOT_INSERT_DOTTED_CIRCLE	= 0x00000010u,
   HB_BUFFER_FLAG_VERIFY				= 0x00000020u,
   HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT	= 0x00000040u,
+  HB_BUFFER_FLAG_PRODUCE_SAFE_TO_INSERT_KASHIDA	= 0x00000070u,
 
-  HB_BUFFER_FLAG_DEFINED			= 0x0000007Fu
+  HB_BUFFER_FLAG_DEFINED			= 0x000000FFu
 } hb_buffer_flags_t;
 
 HB_EXTERN void
diff --git a/src/hb-buffer.hh b/src/hb-buffer.hh
index 561c07c43..c370a3148 100644
--- a/src/hb-buffer.hh
+++ b/src/hb-buffer.hh
@@ -464,6 +464,11 @@ struct hb_buffer_t
   }
   void safe_to_insert_kashida (unsigned int start = 0, unsigned int end = -1)
   {
+    if ((flags & HB_BUFFER_FLAG_PRODUCE_SAFE_TO_INSERT_KASHIDA) == 0)
+    {
+      unsafe_to_break (start, end);
+      return;
+    }
     _set_glyph_flags (HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA,
 		      start, end,
 		      true);
diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index 842c74000..09f21a69b 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -1141,6 +1141,16 @@ hb_propagate_flags (hb_buffer_t *buffer)
   if (!(buffer->scratch_flags & HB_BUFFER_SCRATCH_FLAG_HAS_GLYPH_FLAGS))
     return;
 
+  /* If we are producing SAFE_TO_INSERT_KASHIDA, then do two things:
+   *
+   * - If the places that the Arabic shaper marked as SAFE_TO_INSERT_KASHIDA,
+   *   are UNSAFE_TO_BREAK, then clear the SAFE_TO_INSERT_KASHIDA,
+   * - Any place that is SAFE_TO_INSERT_KASHIDA, is also now UNSAFE_TO_BREAK.
+   *
+   * We couldn't make this interaction earlier. It has to be done here.
+   */
+  bool flip_kashida = buffer->flags & HB_BUFFER_FLAG_PRODUCE_SAFE_TO_INSERT_KASHIDA;
+
   hb_glyph_info_t *info = buffer->info;
 
   foreach_cluster (buffer, start, end)
@@ -1149,10 +1159,13 @@ hb_propagate_flags (hb_buffer_t *buffer)
     for (unsigned int i = start; i < end; i++)
       mask |= info[i].mask & HB_GLYPH_FLAG_DEFINED;
 
-    if (mask & HB_GLYPH_FLAG_UNSAFE_TO_BREAK)
-      mask &= ~HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA;
-    if (mask & HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA)
-      mask |= HB_GLYPH_FLAG_UNSAFE_TO_BREAK | HB_GLYPH_FLAG_UNSAFE_TO_CONCAT;
+    if (flip_kashida)
+    {
+      if (mask & HB_GLYPH_FLAG_UNSAFE_TO_BREAK)
+	mask &= ~HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA;
+      if (mask & HB_GLYPH_FLAG_SAFE_TO_INSERT_KASHIDA)
+	mask |= HB_GLYPH_FLAG_UNSAFE_TO_BREAK | HB_GLYPH_FLAG_UNSAFE_TO_CONCAT;
+    }
 
     if (mask)
       for (unsigned int i = start; i < end; i++)