Fix log_clusters calculation in HarfBuzz-NG code path

The old code wasn't good enough to catch all the glyph (de)composition cases,
thus leading to an assertion in QTextLayout's addNextCluster() helper.

The new code catches all the corner cases and introduces somewhat
better performance to the HB-NG shaper backend.

Change-Id: I5b6c673395a4a039dc55b200abbf74b0ba5d0829
Reviewed-by: Ahmed Saidi <justroftest@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

1 files changed, 16 insertions, 37 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index c2e352eff4..607fe95ab1 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -1105,18 +1105,6 @@ int QTextEngine::shapeTextWithHarfbuzzNG(const QScriptItem &si, const ushort *st
             buffer_flags |= HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES;
         hb_buffer_set_flags(buffer, hb_buffer_flags_t(buffer_flags));
 
-        const uint num_codes = hb_buffer_get_length(buffer);
-        {
-            // adjust clusters
-            hb_glyph_info_t *infos = hb_buffer_get_glyph_infos(buffer, 0);
-            const ushort *uc = string + item_pos;
-            for (uint i = 0, code_pos = 0; i < item_length; ++i, ++code_pos) {
-                if (QChar::isHighSurrogate(uc[i]) && i + 1 < item_length && QChar::isLowSurrogate(uc[i + 1]))
-                    ++i;
-                infos[code_pos].cluster = code_pos + item_glyph_pos;
-            }
-        }
-
 
         // shape
         bool shapedOk = false;
@@ -1139,8 +1127,7 @@ int QTextEngine::shapeTextWithHarfbuzzNG(const QScriptItem &si, const ushort *st
         if (si.analysis.bidiLevel % 2)
             hb_buffer_reverse(buffer);
 
-
-        remaining_glyphs -= num_codes;
+        remaining_glyphs -= item_glyph_pos;
 
         // ensure we have enough space for shaped glyphs and metrics
         const uint num_glyphs = hb_buffer_get_length(buffer);
@@ -1151,44 +1138,36 @@ int QTextEngine::shapeTextWithHarfbuzzNG(const QScriptItem &si, const ushort *st
 
         // fetch the shaped glyphs and metrics
         QGlyphLayout g = availableGlyphs(&si).mid(glyphs_shaped, num_glyphs);
-        if (num_glyphs > num_codes)
-            moveGlyphData(g.mid(num_glyphs), g.mid(num_codes), remaining_glyphs);
+        if (num_glyphs != item_glyph_pos)
+            moveGlyphData(g.mid(num_glyphs), g.mid(item_glyph_pos), remaining_glyphs);
         ushort *log_clusters = logClusters(&si) + item_pos;
 
         hb_glyph_info_t *infos = hb_buffer_get_glyph_infos(buffer, 0);
         hb_glyph_position_t *positions = hb_buffer_get_glyph_positions(buffer, 0);
-        uint last_cluster = -1;
+        uint str_pos = 0;
+        uint last_cluster = ~0u;
+        uint last_glyph_pos = glyphs_shaped;
         for (uint i = 0; i < num_glyphs; ++i) {
             g.glyphs[i] = infos[i].codepoint;
-            log_clusters[i] = infos[i].cluster;
 
             g.advances_x[i] = QFixed::fromFixed(positions[i].x_advance);
             g.advances_y[i] = QFixed::fromFixed(positions[i].y_advance);
             g.offsets[i].x = QFixed::fromFixed(positions[i].x_offset);
             g.offsets[i].y = QFixed::fromFixed(positions[i].y_offset);
 
-            if (infos[i].cluster != last_cluster) {
-                last_cluster = infos[i].cluster;
+            uint cluster = infos[i].cluster;
+            if (last_cluster != cluster) {
+                // fix up clusters so that the cluster indices will be monotonic
+                // and thus we never return out-of-order indices
+                while (last_cluster++ < cluster && str_pos < item_length)
+                    log_clusters[str_pos++] = last_glyph_pos;
+                last_glyph_pos = i + glyphs_shaped;
+                last_cluster = cluster;
                 g.attributes[i].clusterStart = true;
             }
         }
-
-        {
-            // adjust clusters
-            uint glyph_pos = 0;
-            for (uint i = 0; i < item_length; ++i) {
-                if (i + item_pos != infos[glyph_pos].cluster) {
-                    for (uint j = glyph_pos + 1; j < num_glyphs; ++j) {
-                        if (i + item_pos <= infos[j].cluster) {
-                            if (i + item_pos == infos[j].cluster)
-                                glyph_pos = j;
-                            break;
-                        }
-                    }
-                }
-                log_clusters[i] = glyph_pos + item_glyph_pos;
-            }
-        }
+        while (str_pos < item_length)
+            log_clusters[str_pos++] = last_glyph_pos;
 
         if (engineIdx != 0) {
             for (quint32 i = 0; i < num_glyphs; ++i)