[freetype2] hdmx-advances de70a40: [truetype] Binary search through `hdmx` records.

[Werner Lemberg]

branch: hdmx-advances
commit de70a401aad9bb99a295b2cbfb8f7253a9a8d589
Author: Alexei Podtelezhnikov <apodtele@gmail.com>
Commit: Alexei Podtelezhnikov <apodtele@gmail.com>

    [truetype] Binary search through `hdmx` records.
    
    The `hdmx` table is supposed to be sorted by ppem size, which
    enables binary search.  We also drop the check for the sufficient
    length of the record because it is now enforced when the table
    is loaded.
    
    * src/truetype/ttpload.c (tt_face_load_hdmx): Prudently sort records.
    (tt_face_get_device_metrics): Implement binary search to retrieve
    advances.
---
 src/truetype/ttpload.c | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/src/truetype/ttpload.c b/src/truetype/ttpload.c
index 71db75a..2a2af70 100644
--- a/src/truetype/ttpload.c
+++ b/src/truetype/ttpload.c
@@ -498,6 +498,14 @@
   }
 
 
+  FT_COMPARE_DEF( int )
+  compare_ppem( const void*  a,
+                const void*  b )
+  {
+    return *(FT_Byte*)a - *(FT_Byte*)b;
+  }
+
+
   /**************************************************************************
    *
    * @Function:
@@ -577,8 +585,10 @@
     if ( FT_QNEW_ARRAY( face->hdmx_record_sizes, num_records ) )
       goto Fail;
 
-    /* XXX: We do not check if the records are sorted by ppem */
-    /* and cannot use binary search later.                    */
+    /* The records must be already sorted by ppem but it does not */
+    /* hurt to make sure so that the binary search works later.   */
+    ft_qsort( p, num_records, record_size, compare_ppem );
+
     for ( nn = 0; nn < num_records; nn++ )
     {
       if ( p + record_size > limit )
@@ -619,27 +629,36 @@
   /**************************************************************************
    *
    * Return the advance width table for a given pixel size if it is found
-   * in the font's `hdmx' table (if any).
+   * in the font's `hdmx' table (if any).  The records must be sorted for
+   * the binary search to work properly.
    */
   FT_LOCAL_DEF( FT_Byte* )
   tt_face_get_device_metrics( TT_Face  face,
                               FT_UInt  ppem,
                               FT_UInt  gindex )
   {
-    FT_UInt   nn;
+    FT_UInt   min         = 0;
+    FT_UInt   max         = face->hdmx_record_count;
+    FT_UInt   mid;
     FT_Byte*  result      = NULL;
     FT_ULong  record_size = face->hdmx_record_size;
     FT_Byte*  record      = FT_OFFSET( face->hdmx_table, 8 );
 
 
-    for ( nn = 0; nn < face->hdmx_record_count; nn++ )
-      if ( face->hdmx_record_sizes[nn] == ppem )
+    while ( min < max )
+    {
+      mid = ( min + max ) >> 1;
+
+      if ( face->hdmx_record_sizes[mid] > ppem )
+        max = mid;
+      else if ( face->hdmx_record_sizes[mid] < ppem )
+        min = mid + 1;
+      else
       {
-        gindex += 2;
-        if ( gindex < record_size )
-          result = record + nn * record_size + gindex;
+        result = record + mid * record_size + gindex + 2;
         break;
       }
+    }
 
     return result;
   }