Now without bug.

Author: lrhn

pkgs/characters/lib/src/characters_impl.dart

@@ -509,16 +509,16 @@ class StringCharacterRange implements CharacterRange {
       var index = _end;
       while (index < _string.length) {
         var char = _string.codeUnitAt(index);
-        var surrogateBits = char ^ 0xD800;
+        var surrogate = char ^ 0xD800;
         var category = categoryControl;
         var nextIndex = index + 1;
-        if (surrogateBits >= 0x3FF) {
+        if (surrogate > 0x3FF) {
           category = low(char);
         } else if (nextIndex < _string.length) {
-          var nextChar = _string.codeUnitAt(nextIndex) ^ 0xDC00;
-          if (nextChar <= 0x3FF) {
+          var nextSurrogate = _string.codeUnitAt(nextIndex) ^ 0xDC00;
+          if (nextSurrogate <= 0x3FF) {
             nextIndex += 1;
-            category = high(char, nextChar);
+            category = high(surrogate, nextSurrogate);
           }
         }
         state = move(state, category);

pkgs/characters/lib/src/grapheme_clusters/breaks.dart

@@ -77,7 +77,7 @@ class Breaks {
     assert(cursor < end);
     var char = base.codeUnitAt(cursor++);
     var surrogate = char ^ 0xD800;
-    if (surrogate >= 0x3FF) {
+    if (surrogate > 0x3FF) {
       state = move(state, low(char));
       return;
     }
@@ -114,31 +114,35 @@ class Breaks {
     var cursorBefore = cursor - 1;
     var prevChar = base.codeUnitAt(cursorBefore);
     var prevSurrogate = prevChar ^ 0xD800;
-    int prevCategory;
     if (prevSurrogate > 0x7FF) {
       // Not surrogate.
-      prevCategory = low(prevChar);
-    } else if (prevSurrogate <= 0x3FF) {
-      // Lead surrogate. Check for a following tail surrogate.
-      int tailSurrogate;
-      if (cursor < end &&
-          (tailSurrogate = base.codeUnitAt(cursor) ^ 0xDC00) <= 0x3FF) {
-        cursor += 1;
-        prevCategory = high(prevSurrogate, tailSurrogate);
-      } else {
-        prevCategory = categoryControl;
-      }
-    } else {
+      var prevCategory = low(prevChar);
+      state = move(stateCAny, prevCategory);
+      return cursorBefore;
+    }
+    int prevCategory;
+    if (prevSurrogate > 0x3FF) {
       // Tail surrogate, check for prior lead surrogate.
       int leadSurrogate;
       var leadIndex = cursorBefore - 1;
+      prevSurrogate &= 0x3FF;
       if (leadIndex >= start &&
           (leadSurrogate = base.codeUnitAt(leadIndex) ^ 0xD800) <= 0x3FF) {
         prevCategory = high(leadSurrogate, prevSurrogate);
         cursorBefore = leadIndex;
       } else {
         prevCategory = categoryControl;
       }
+    } else {
+      // Lead surrogate. Check for a following tail surrogate.
+      int tailSurrogate;
+      if (cursor < end &&
+          (tailSurrogate = base.codeUnitAt(cursor) ^ 0xDC00) <= 0x3FF) {
+        cursor += 1;
+        prevCategory = high(prevSurrogate, tailSurrogate);
+      } else {
+        prevCategory = categoryControl;
+      }
     }
     state = move(stateCAny, prevCategory);
     return cursorBefore;
@@ -354,7 +358,7 @@ int previousBreak(String text, int start, int end, int index) {
       if (indexAfter < end) {
         var secondSurrogate = text.codeUnitAt(indexAfter) ^ 0xDC00;
         if (secondSurrogate <= 0x3FF) {
-          category = high(nextChar, secondSurrogate);
+          category = high(nextSurrogate, secondSurrogate);
         }
       }
     } else {

pkgs/characters/tool/bin/generate_tables.dart

@@ -320,18 +320,17 @@ String _lookupSurrogatesMethod(
   int startOffset,
   int chunkSize,
 ) {
-  if (chunkSize == 1024) {
+  var shift = chunkSize.bitLength - 1;
+  assert(shift <= 10);
+  if (shift == 10) {
     return '''
 $preferInline
 int $name(int lead, int tail) {
-  var chunkStart = $startName.codeUnitAt($startOffset + (0x3ff & lead));
-  var index = chunkStart + (0x3ff & tail);
-  return $dataName.codeUnitAt(index);
+  var chunkStart = $startName.codeUnitAt($startOffset + lead);
+  return $dataName.codeUnitAt(chunkStart + tail);
 }
 ''';
   }
-  var shift = chunkSize.bitLength - 1;
-  assert(shift <= 10);
   if (shift < 10) {
     return '''
 $preferInline
@@ -341,18 +340,11 @@ int $name(int lead, int tail) {
   var chunkStart = $startName.codeUnitAt($startOffset + offset);
   return $dataName.codeUnitAt(chunkStart + tail);
 }
-''';
-  } else {
-    assert(shift == 10);
-    return '''
-$preferInline
-int $name(int lead, int tail) {
-  var chunkStart = $startName.codeUnitAt($startOffset + lead);
-  return $dataName.codeUnitAt(chunkStart + tail);
-}
 ''';
   }
   // Add code if shift > 10 ever becomes optimal for table size.
+  // Example code:
+  throw UnimplementedError('No code for chunk sizes > 10 bits');
 }
 
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