[indic] Order left-matras inside-out

Fixes https://github.com/harfbuzz/harfbuzz/issues/3863

src/hb-ot-shaper-indic.cc

@@ -742,14 +742,40 @@ initial_reordering_consonant_syllable (const hb_ot_shape_plan_t *plan,
 
     /* Sit tight, rock 'n roll! */
     hb_stable_sort (info + start, end - start, compare_indic_order);
-    /* Find base again */
+
+    /* Find base again; also flip left-matra sequence. */
+    unsigned first_left_matra = end;
+    unsigned last_left_matra = end;
     base = end;
     for (unsigned int i = start; i < end; i++)
+    {
       if (info[i].indic_position() == POS_BASE_C)
       {
 	base = i;
 	break;
       }
+      else if (info[i].indic_position() == POS_PRE_M)
+      {
+        if (first_left_matra == end)
+	  first_left_matra = i;
+	last_left_matra = i;
+      }
+    }
+    /* https://github.com/harfbuzz/harfbuzz/issues/3863 */
+    if (first_left_matra < last_left_matra)
+    {
+      /* No need to merge clusters, handled later. */
+      buffer->reverse_range (first_left_matra, last_left_matra + 1);
+      /* Reverse back nuktas, etc. */
+      unsigned i = first_left_matra;
+      for (unsigned j = i; j <= last_left_matra; j++)
+        if (info[j].indic_category() == I_Cat(M))
+	{
+	  buffer->reverse_range (i, j + 1);
+	  i = j + 1;
+	}
+    }
+
     /* Things are out-of-control for post base positions, they may shuffle
      * around like crazy.  In old-spec mode, we move halants around, so in
      * that case merge all clusters after base.  Otherwise, check the sort

src/hb-ot-shaper-myanmar.cc

@@ -270,6 +270,33 @@ initial_reordering_consonant_syllable (hb_buffer_t *buffer,
 
   /* Sit tight, rock 'n roll! */
   buffer->sort (start, end, compare_myanmar_order);
+
+  /* Flip left-matra sequence. */
+  unsigned first_left_matra = end;
+  unsigned last_left_matra = end;
+  for (unsigned int i = start; i < end; i++)
+  {
+    if (info[i].myanmar_position() == POS_PRE_M)
+    {
+      if (first_left_matra == end)
+	first_left_matra = i;
+      last_left_matra = i;
+    }
+  }
+  /* https://github.com/harfbuzz/harfbuzz/issues/3863 */
+  if (first_left_matra < last_left_matra)
+  {
+    /* No need to merge clusters, done already? */
+    buffer->reverse_range (first_left_matra, last_left_matra + 1);
+    /* Reverse back VS, etc. */
+    unsigned i = first_left_matra;
+    for (unsigned j = i; j <= last_left_matra; j++)
+      if (info[j].myanmar_category() == M_Cat(VPre))
+      {
+	buffer->reverse_range (i, j + 1);
+	i = j + 1;
+      }
+  }
 }
 
 static void

test/shape/data/in-house/tests/indic-special-cases.tests

@@ -2,3 +2,5 @@
 ../fonts/3cae6bfe5b57c07ba81ddbd54c02fe4f3a1e3bf6.ttf;;U+0CB0,U+200D,U+0CCD,U+0C95;[gid2=0+1334|gid6=0+358]
 ../fonts/3cae6bfe5b57c07ba81ddbd54c02fe4f3a1e3bf6.ttf;;U+0CB0,U+0CCD,U+200D,U+0C95;[gid2=0+1334|gid6=0+358]
 ../fonts/e716f6bd00a108d186b7e9f47b4515565f784f36.ttf;;U+0C1A,U+0C3F,U+0C32,U+0C4D,U+0C15,U+0C42,U+0C30,U+0C4D;[civoweltelu=0+766|latelu=2+709|uuvowelsigntelu=2+661|kasubscripttelu=2+483|rahalanttelu=6+593]
+../fonts/9d8c53cb64b8747abdd2b70755cce2ee0eb42ef7.ttf;;U+0915,U+093F,U+094E,U+093C;[uni094E=0+273|uni093C=0+0|ivowelsign03deva=0+259|uni0915=0+762]
+../fonts/9d8c53cb64b8747abdd2b70755cce2ee0eb42ef7.ttf;;U+0915,U+093F,U+093C,U+094E;[uni094E=0+273|ivowelsign00deva=0+259|uni093C=0+0|uni0915=0+762]

test/shape/data/in-house/tests/myanmar-misc.tests

@@ -1 +1,2 @@
 ../fonts/065b01e54f35f0d849fd43bd5b936212739a50cb.ttf;;U+101A,U+1035;[ya_e_above=0+1000]
+../fonts/a232bb734d4c6c898a44506547d19768f0eba6a6.ttf;;U+1000,U+1031,U+1084;[e_shn=0+592|_e=0+618|ka=0+1124]