improved BDD Unicode table representation in NonBacktracking engine (#61142)

* improved BDD Unicode table representation in NonBacktracking engine

* remove line

Co-authored-by: Dan Moseley <[email protected]>

* improved bounds-check elimination

Co-authored-by: Stephen Toub <[email protected]>

* clearer notation of numbers

Co-authored-by: Dan Moseley <[email protected]>

* fixed typo

Co-authored-by: Dan Moseley <[email protected]>
Co-authored-by: Stephen Toub <[email protected]>

Author: 3 people

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Symbolic/Algebras/BDD.cs

@@ -69,6 +69,16 @@ internal sealed class BDD : IComparable
         /// </summary>
         private static readonly long[] s_trueRepresentation = new long[] { 1 };
 
+        /// <summary>
+        /// Representation of False for compact serialization of BDDs.
+        /// </summary>
+        private static readonly byte[] s_falseRepresentationCompact = new byte[] { 0 };
+
+        /// <summary>
+        /// Representation of True for compact serialization of BDDs.
+        /// </summary>
+        private static readonly byte[] s_trueRepresentationCompact = new byte[] { 1 };
+
         internal BDD(int ordinal, BDD? one, BDD? zero)
         {
             One = one;
@@ -300,6 +310,49 @@ public long[] Serialize()
             return res;
         }
 
+        /// <summary>
+        /// Serialize this BDD into a byte array.
+        /// This method is not valid for MTBDDs where some elements may be negative.
+        /// </summary>
+        public byte[] SerializeToBytes()
+        {
+            if (IsEmpty)
+                return s_falseRepresentationCompact;
+
+            if (IsFull)
+                return s_trueRepresentationCompact;
+
+            // in other cases make use of the general serializer to long[]
+            long[] serialized = Serialize();
+
+            // get the maximal element from the array
+            long m = 0;
+            for (int i = 0; i < serialized.Length; i++)
+            {
+                // make sure this serialization is not applied to MTBDDs
+                Debug.Assert(serialized[i] > 0);
+                m = Math.Max(serialized[i], m);
+            }
+
+            // k is the number of bytes needed to represent the maximal element
+            int k = m <= 0xFFFF ? 2 : (m <= 0xFF_FFFF ? 3 : (m <= 0xFFFF_FFFF ? 4 : (m <= 0xFF_FFFF_FFFF ? 5 : (m <= 0xFFFF_FFFF_FFFF ? 6 : (m <= 0xFF_FFFF_FFFF_FFFF ? 7 : 8)))));
+
+            // the result will contain k as the first element and the number of serialized elements times k
+            byte[] result = new byte[(k * serialized.Length) + 1];
+            result[0] = (byte)k;
+            for (int i=0; i < serialized.Length; i += 1)
+            {
+                long serialized_i = serialized[i];
+                // add the serialized longs as k-byte subsequences
+                for (int j = 1; j <= k; j++)
+                {
+                    result[(i * k) + j] = (byte)serialized_i;
+                    serialized_i = serialized_i >> 8;
+                }
+            }
+            return result;
+        }
+
         /// <summary>
         /// Recreates a BDD from a ulong array that has been created using Serialize.
         /// Is executed using a lock on algebra (if algebra != null) in a single thread mode.
@@ -353,6 +406,70 @@ public static BDD Deserialize(long[] arcs, BDDAlgebra algebra)
             return nodes[k - 1];
         }
 
+        /// <summary>
+        /// Recreates a BDD from a byte array that has been created using SerializeToBytes.
+        /// </summary>
+        public static BDD Deserialize(byte[] bytes, BDDAlgebra algebra)
+        {
+            if (bytes.Length == 1)
+            {
+                return bytes[0] == 0 ? False : True;
+            }
+
+            // here bytes represents an array of longs with k = the number of bytes used per long
+            int k = (int)bytes[0];
+
+            // gets the i'th element from the underlying array of longs represented by bytes
+            long Get(int i)
+            {
+                long l = 0;
+                for (int j = k; j > 0; j--)
+                {
+                    l = (l << 8) | bytes[(k * i) + j];
+                }
+                return l;
+            }
+
+            // n is the total nr of longs that corresponds also to the total number of BDD nodes needed
+            int n = (bytes.Length - 1) / k;
+
+            // make sure the represented nr of longs divides precisely without remainder
+            Debug.Assert((bytes.Length - 1) % k == 0);
+
+            // the number of bits used for ordinals and node identifiers are stored in the first two longs
+            int ordinal_bits = (int)Get(0);
+            int node_bits = (int)Get(1);
+
+            // create bit masks for the sizes of ordinals and node identifiers
+            long ordinal_mask = (1 << ordinal_bits) - 1;
+            long node_mask = (1 << node_bits) - 1;
+            BitLayout(ordinal_bits, node_bits, out int zero_node_shift, out int one_node_shift, out int ordinal_shift);
+
+            // store BDD nodes by their id when they are created
+            BDD[] nodes = new BDD[n];
+            nodes[0] = False;
+            nodes[1] = True;
+
+            for (int i = 2; i < n; i++)
+            {
+                // represents the triple (ordinal, one, zero)
+                long arc = Get(i);
+
+                // reconstruct the ordinal and child identifiers for a non-terminal
+                int ord = (int)((arc >> ordinal_shift) & ordinal_mask);
+                int oneId = (int)((arc >> one_node_shift) & node_mask);
+                int zeroId = (int)((arc >> zero_node_shift) & node_mask);
+
+                // the BDD nodes for the children are guaranteed to exist already
+                // because of the topological order they were serialized by
+                Debug.Assert(oneId < i && zeroId < i);
+                nodes[i] = algebra.GetOrCreateBDD(ord, nodes[oneId], nodes[zeroId]);
+            }
+
+            //the result is the last BDD in the nodes array
+            return nodes[n - 1];
+        }
+
         /// <summary>
         /// Use this bit layout in the serialization
         /// </summary>

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Symbolic/Algebras/CharSetSolver.cs

@@ -12,8 +12,8 @@ namespace System.Text.RegularExpressions.Symbolic
     /// </summary>
     internal sealed class CharSetSolver : BDDAlgebra, ICharAlgebra<BDD>
     {
-        /// <summary>BDDs for all characters for fast lookup.</summary>
-        private readonly BDD[] _charPredTable = new BDD[char.MaxValue + 1];
+        /// <summary>BDDs for all ASCII characters for fast lookup.</summary>
+        private readonly BDD[] _charPredTable = new BDD[128];
         private readonly Unicode.IgnoreCaseTransformer _ignoreCase;
         internal readonly BDD _nonAscii;
 
@@ -40,10 +40,23 @@ public BDD CharConstraint(char c, bool ignoreCase = false, string? culture = nul
             else
             {
                 //individual character BDDs are always fixed
-                return _charPredTable[c] ??= CreateSetFrom(c, 15);
+                BDD[] charPredTable = _charPredTable;
+                return c < charPredTable.Length ?
+                    charPredTable[c] ??= CreateBDDFromChar(c) :
+                    CreateBDDFromChar(c);
             }
         }
 
+        private BDD CreateBDDFromChar(ushort c)
+        {
+            BDD bdd = BDD.True;
+            for (int k = 0; k < 16; k++)
+            {
+                bdd = (c & (1 << k)) == 0 ? GetOrCreateBDD(k, BDD.False, bdd) : GetOrCreateBDD(k, bdd, BDD.False);
+            }
+            return bdd;
+        }
+
         /// <summary>
         /// Make a CharSet from all the characters in the range from m to n.
         /// Returns the empty set if n is less than m

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Symbolic/Unicode/GeneratorHelper.cs

@@ -17,6 +17,16 @@ public static void WriteInt64ArrayInitSyntax(StreamWriter sw, long[] values)
             }
             sw.Write("}");
         }
+
+        public static void WriteByteArrayInitSyntax(StreamWriter sw, byte[] values)
+        {
+            sw.Write("new byte[] {");
+            for (int i = 0; i < values.Length; i++)
+            {
+                sw.Write($" 0x{values[i]:X}, ");
+            }
+            sw.Write("}");
+        }
     }
 #endif
 }

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Symbolic/Unicode/IgnoreCaseRelation.cs

@@ -41,70 +41,54 @@ private static void WriteIgnoreCaseBDD(StreamWriter sw)
             sw.WriteLine("        /// <summary>Serialized BDD for mapping characters to their case-ignoring equivalence classes in the default (en-US) culture.</summary>");
 
             var solver = new CharSetSolver();
-            Dictionary<char, BDD> ignoreCase = ComputeIgnoreCaseDictionary(solver, new CultureInfo(DefaultCultureName));
+            List<EquivalenceClass> ignoreCaseEquivalenceClasses = ComputeIgnoreCaseEquivalenceClasses(solver, new CultureInfo(DefaultCultureName));
             BDD ignorecase = solver.False;
-            foreach (KeyValuePair<char, BDD> kv in ignoreCase)
+            foreach (EquivalenceClass ec in ignoreCaseEquivalenceClasses)
             {
-                BDD a = solver.CreateCharSetFromRange(kv.Key, kv.Key);
-                BDD b = kv.Value;
-                ignorecase = solver.Or(ignorecase, solver.And(solver.ShiftLeft(a, 16), b));
+                // Create the Cartesian product of ec._set with itself
+                BDD crossproduct = solver.And(solver.ShiftLeft(ec._set, 16), ec._set);
+                // Add the product into the overall lookup table
+                ignorecase = solver.Or(ignorecase, crossproduct);
             }
 
-            sw.Write("        public static readonly long[] IgnoreCaseEnUsSerializedBDD = ");
-            GeneratorHelper.WriteInt64ArrayInitSyntax(sw, ignorecase.Serialize());
+            sw.Write("        public static readonly byte[] IgnoreCaseEnUsSerializedBDD = ");
+            GeneratorHelper.WriteByteArrayInitSyntax(sw, ignorecase.SerializeToBytes());
             sw.WriteLine(";");
         }
 
-        private static Dictionary<char, BDD> ComputeIgnoreCaseDictionary(CharSetSolver solver, CultureInfo culture)
+        private static List<EquivalenceClass> ComputeIgnoreCaseEquivalenceClasses(CharSetSolver solver, CultureInfo culture)
         {
-            CultureInfo originalCulture = CultureInfo.CurrentCulture;
-            try
-            {
-                CultureInfo.CurrentCulture = culture;
+            var ignoreCase = new Dictionary<char, EquivalenceClass>();
+            var sets = new List<EquivalenceClass>();
 
-                var ignoreCase = new Dictionary<char, BDD>();
+            for (uint i = 65; i <= 0xFFFF; i++)
+            {
+                char C = (char)i;
+                char c = char.ToLower(C, culture);
 
-                for (uint i = 0; i <= 0xFFFF; i++)
+                if (c == C)
                 {
-                    char c = (char)i;
-                    char cUpper = char.ToUpper(c);
-                    char cLower = char.ToLower(c);
-
-                    if (cUpper == cLower)
-                    {
-                        continue;
-                    }
-
-                    // c may be different from both cUpper as well as cLower.
-                    // Make sure that the regex engine considers c as being equivalent to cUpper and cLower, else ignore c.
-                    // In some cases c != cU but the regex engine does not consider the chacarters equivalent wrt the ignore-case option.
-                    if (Regex.IsMatch($"{cUpper}{cLower}", $"^(?i:\\u{i:X4}\\u{i:X4})$"))
-                    {
-                        BDD equiv = solver.False;
-
-                        if (ignoreCase.ContainsKey(c))
-                            equiv = solver.Or(equiv, ignoreCase[c]);
-
-                        if (ignoreCase.ContainsKey(cUpper))
-                            equiv = solver.Or(equiv, ignoreCase[cUpper]);
-
-                        if (ignoreCase.ContainsKey(cLower))
-                            equiv = solver.Or(equiv, ignoreCase[cLower]);
-
-                        // Make sure all characters are included initially or when some is still missing
-                        equiv = solver.Or(equiv, solver.Or(solver.CreateCharSetFromRange(c, c), solver.Or(solver.CreateCharSetFromRange(cUpper, cUpper), solver.CreateCharSetFromRange(cLower, cLower))));
-
-                        // Update all the members with their case-invariance equivalence classes
-                        foreach (char d in solver.GenerateAllCharacters(equiv))
-                            ignoreCase[d] = equiv;
-                    }
+                    continue;
                 }
 
-                return ignoreCase;
+                EquivalenceClass? ec;
+                if (!ignoreCase.TryGetValue(c, out ec))
+                {
+                    ec = new EquivalenceClass(solver.CharConstraint(c));
+                    ignoreCase[c] = ec;
+                    sets.Add(ec);
+                }
+                ec._set = solver.Or(ec._set, solver.CharConstraint(C));
             }
-            finally
+            return sets;
+        }
+
+        private class EquivalenceClass
+        {
+            public BDD _set;
+            public EquivalenceClass(BDD set)
             {
-                CultureInfo.CurrentCulture = originalCulture;
+                _set = set;
             }
         }
     };

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Symbolic/Unicode/IgnoreCaseRelationGenerator.cs

@@ -166,6 +166,10 @@ private IgnoreCaseRelation EnsureDefault()
             if (_relationDefault is null)
             {
                 BDD instance = BDD.Deserialize(Unicode.IgnoreCaseRelation.IgnoreCaseEnUsSerializedBDD, _solver);
+                byte[] tmp = instance.SerializeToBytes();
+                BDD instance2 = BDD.Deserialize(tmp, _solver);
+                if (instance != instance2)
+                    throw new Exception();
                 BDD instanceDomain = _solver.ShiftRight(instance, 16); // represents the set of all case-sensitive characters in the default culture.
                 _relationDefault = new IgnoreCaseRelation(instance, instanceDomain);
             }