Document normalization design

src/hb-ot-shape-normalize.cc

@@ -25,20 +25,55 @@
  */
 
 #include "hb-ot-shape-private.hh"
+#include "hb-ot-shape-complex-private.hh"
 
 HB_BEGIN_DECLS
 
+/*
+ * HIGHLEVEL DESIGN:
+ *
+ * This file exports one main function: _hb_ot_shape_normalize().
+ *
+ * This function closely reflects the Unicode Normalization Algorithm,
+ * yet it's different.  The shaper an either prefer decomposed (NFD) or
+ * composed (NFC).
+ *
+ * In general what happens is that: each grapheme is decomposed in a chain
+ * of 1:2 decompositions, marks reordered, and then recomposed if desires,
+ * so far it's like Unicode Normalization.  However, the decomposition and
+ * recomposition only happens if the font supports the resulting characters.
+ *
+ * The goals are:
+ *
+ *   - Try to render all canonically equivalent strings similarly.  To really
+ *     achieve this we have to always do the full decomposition and then
+ *     selectively recompose from there.  It's kinda too expensive though, so
+ *     we skip some cases.  For example, if composed is desired, we simply
+ *     don't touch 1-character clusters that are supported by the font, even
+ *     though their NFC may be different.
+ *
+ *   - When a font has a precomposed character for a sequence but the 'ccmp'
+ *     feature in the font is not adequate, form use the precomposed character
+ *     which typically has better mark positioning.
+ *
+ *   - When a font does not support a character but supports its decomposition,
+ *     well, use the decomposition.
+ *
+ *   - The Indic shaper requests decomposed output.  This will handle splitting
+ *     matra for the Indic shaper.
+ */
+
 static bool
 get_glyph (hb_ot_shape_context_t *c, unsigned int i)
 {
-  hb_buffer_t *b = c->buffer;
   hb_codepoint_t glyph;
 
-  return hb_font_get_glyph (c->font, b->info[i].codepoint, 0, &glyph);
+  return hb_font_get_glyph (c->font, c->buffer->info[i].codepoint, 0, &glyph);
 }
 
 static bool
 decompose_single_char_cluster (hb_ot_shape_context_t *c,
+			       bool recompose,
 			       unsigned int i)
 {
   return FALSE;
@@ -46,22 +81,24 @@ decompose_single_char_cluster (hb_ot_shape_context_t *c,
 
 static bool
 handle_single_char_cluster (hb_ot_shape_context_t *c,
+			    bool recompose,
 			    unsigned int i)
 {
-  /* If the single char is supported by the font, we're good. */
+  /* If recomposing and the single char is supported by the font, we're good. */
-  if (get_glyph (c, i))
+  if (recompose && get_glyph (c, i))
     return FALSE;
 
   /* Decompose */
-  return decompose_single_char_cluster (c, i);
+  return decompose_single_char_cluster (c, recompose, i);
 }
 
 static bool
 handle_multi_char_cluster (hb_ot_shape_context_t *c,
+			   bool recompose,
 			   unsigned int start,
 			   unsigned int end)
 {
-  /* If there's a variation-selector, give-up, it's just too hard. */
+  /* TODO Currently if there's a variation-selector we give-up, it's just too hard. */
   for (unsigned int i = start; i < end; i++)
     if (unlikely (is_variation_selector (c->buffer->info[i].codepoint)))
       return FALSE;
@@ -70,24 +107,33 @@ handle_multi_char_cluster (hb_ot_shape_context_t *c,
 }
 
 bool
-_hb_normalize (hb_ot_shape_context_t *c)
+_hb_ot_shape_normalize (hb_ot_shape_context_t *c)
 {
-  hb_buffer_t *b = c->buffer;
+  hb_buffer_t *buffer = c->buffer;
   bool changed = FALSE;
+  bool recompose = !hb_ot_shape_complex_prefer_decomposed (c->plan->shaper);
+
+  buffer->clear_output ();
+
+  unsigned int count = buffer->len;
+  for (buffer->i = 0; buffer->i < count;)
+  {
 
-  unsigned int count = b->len;
-  for (unsigned int i = 0; i < count;) {
     unsigned int end;
-    for (end = i + 1; end < count; end++)
+    for (end = buffer->i + 1; end < count; end++)
-      if (b->info[i].cluster != b->info[end].cluster)
+      if (buffer->info[buffer->i].cluster != buffer->info[end].cluster)
         break;
-    if (i + 1 == end)
+
-      changed |= handle_single_char_cluster (c, i);
+    if (buffer->i + 1 == end)
+      changed |= handle_single_char_cluster (c, recompose, buffer->i);
     else
-      changed |= handle_multi_char_cluster (c, i, end);
+      changed |= handle_multi_char_cluster (c, recompose, buffer->i, end);
-    i = end;
+    while (buffer->i < end)
+      c->buffer->next_glyph ();
   }
 
+  buffer->swap ();
+
   return changed;
 }
 

src/hb-ot-shape-private.hh

@@ -100,7 +100,7 @@ is_variation_selector (hb_codepoint_t unicode)
 }
 
 
-HB_INTERNAL bool _hb_normalize (hb_ot_shape_context_t *c);
+HB_INTERNAL bool _hb_ot_shape_normalize (hb_ot_shape_context_t *c);
 
 HB_END_DECLS
 

src/hb-ot-shape.cc

@@ -254,11 +254,13 @@ static void
 hb_map_glyphs (hb_font_t    *font,
 	       hb_buffer_t  *buffer)
 {
+  hb_codepoint_t glyph;
+
   if (unlikely (!buffer->len))
     return;
 
-  hb_codepoint_t glyph;
   buffer->clear_output ();
+
   unsigned int count = buffer->len - 1;
   for (buffer->i = 0; buffer->i < count;) {
     if (unlikely (is_variation_selector (buffer->info[buffer->i + 1].codepoint))) {
@@ -363,7 +365,7 @@ hb_ot_shape_execute_internal (hb_ot_shape_context_t *c)
 
   hb_ensure_native_direction (c->buffer);
 
-  if (_hb_normalize (c))
+  if (_hb_ot_shape_normalize (c))
     /* Buffer contents changed, reset unicode_props */
     hb_set_unicode_props (c->buffer); /* BUFFER: Set general_category and combining_class in var1 */