From 58f2a73fb95af42e264a91cdef7bb5a89e965601 Mon Sep 17 00:00:00 2001
From: Behdad Esfahbod <behdad@behdad.org>
Date: Tue, 25 Aug 2015 18:55:34 +0100
Subject: [PATCH] [GPOS] Rewrite cursive attachment slightly differently

In anticipation for upcoming fix for bug reported by
Khaled in thread "Issue with cursive attachment".
---
 src/hb-ot-layout-gpos-table.hh | 36 ++++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/src/hb-ot-layout-gpos-table.hh b/src/hb-ot-layout-gpos-table.hh
index da9506c7..6dcada6d 100644
--- a/src/hb-ot-layout-gpos-table.hh
+++ b/src/hb-ot-layout-gpos-table.hh
@@ -960,20 +960,32 @@ struct CursivePosFormat1
     }
 
     /* Cross-direction adjustment */
-    if  (c->lookup_props & LookupFlag::RightToLeft) {
-      pos[i].cursive_chain() = j - i;
-      if (likely (HB_DIRECTION_IS_HORIZONTAL (c->direction)))
-	pos[i].y_offset = entry_y - exit_y;
-      else
-	pos[i].x_offset = entry_x - exit_x;
-    } else {
-      pos[j].cursive_chain() = i - j;
-      if (likely (HB_DIRECTION_IS_HORIZONTAL (c->direction)))
-	pos[j].y_offset = exit_y - entry_y;
-      else
-	pos[j].x_offset = exit_x - entry_x;
+
+    /* We attach child to parent (think graph theory and rooted trees whereas
+     * the root stays on baseline and each node aligns itself against its
+     * parent.
+     *
+     * Optimize things for the case of RightToLeft, as that's most common in
+     * Arabinc. */
+    unsigned int child  = i;
+    unsigned int parent = j;
+    hb_position_t x_offset = entry_x - exit_x;
+    hb_position_t y_offset = entry_y - exit_y;
+    if  (!(c->lookup_props & LookupFlag::RightToLeft))
+    {
+      unsigned int k = child;
+      child = parent;
+      parent = k;
+      x_offset = -x_offset;
+      y_offset = -y_offset;
     }
 
+    pos[child].cursive_chain() = parent - child;
+    if (likely (HB_DIRECTION_IS_HORIZONTAL (c->direction)))
+      pos[child].y_offset = y_offset;
+    else
+      pos[child].x_offset = x_offset;
+
     buffer->idx = j;
     return TRACE_RETURN (true);
   }
-- 
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