From 44c519e9b7606cfbe8599700e727c4124dfb0e09 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Mon, 17 Jun 2024 14:04:42 -0400
Subject: [PATCH] Extend SearchValues<string> use in Regex to NonBacktracking
 (#103496)

We previously only enabled this for the compiler. Support it with NonBacktracking, too.
---
 .../RegexFindOptimizations.cs                 | 49 ++++++++++++++++---
 1 file changed, 42 insertions(+), 7 deletions(-)

diff --git a/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexFindOptimizations.cs b/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexFindOptimizations.cs
index fa90486e7407a..1318769136235 100644
--- a/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexFindOptimizations.cs
+++ b/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexFindOptimizations.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Buffers;
 using System.Collections.Generic;
 using System.Diagnostics;
 
@@ -83,6 +84,7 @@ public RegexFindOptimizations(RegexNode root, RegexOptions options)
             bool dfa = (options & RegexOptions.NonBacktracking) != 0;
             bool compiled = (options & RegexOptions.Compiled) != 0 && !dfa; // for now, we never generate code for NonBacktracking, so treat it as non-compiled
             bool interpreter = !compiled && !dfa;
+            bool usesRfoTryFind = !compiled;
 
             // For interpreter, we want to employ optimizations, but we don't want to make construction significantly
             // more expensive; someone who wants to pay to do more work can specify Compiled.  So for the interpreter
@@ -140,12 +142,18 @@ public RegexFindOptimizations(RegexNode root, RegexOptions options)
             // We're now left-to-right only and looking for multiple prefixes and/or sets.
 
             // If there are multiple leading strings, we can search for any of them.
-            if (compiled)
+            if (!interpreter) // this works in the interpreter, but we avoid it due to additional cost during construction
             {
                 if (RegexPrefixAnalyzer.FindPrefixes(root, ignoreCase: true) is { Length: > 1 } caseInsensitivePrefixes)
                 {
                     LeadingPrefixes = caseInsensitivePrefixes;
                     FindMode = FindNextStartingPositionMode.LeadingStrings_OrdinalIgnoreCase_LeftToRight;
+#if SYSTEM_TEXT_REGULAREXPRESSIONS
+                    if (usesRfoTryFind)
+                    {
+                        LeadingStrings = SearchValues.Create(LeadingPrefixes, StringComparison.OrdinalIgnoreCase);
+                    }
+#endif
                     return;
                 }
 
@@ -156,6 +164,12 @@ public RegexFindOptimizations(RegexNode root, RegexOptions options)
                 //{
                 //    LeadingPrefixes = caseSensitivePrefixes;
                 //    FindMode = FindNextStartingPositionMode.LeadingStrings_LeftToRight;
+#if SYSTEM_TEXT_REGULAREXPRESSIONS
+                //    if (usesRfoTryFind)
+                //    {
+                //        LeadingStrings = SearchValues.Create(LeadingPrefixes, StringComparison.Ordinal);
+                //    }
+#endif
                 //    return;
                 //}
             }
@@ -275,6 +289,11 @@ public RegexFindOptimizations(RegexNode root, RegexOptions options)
         /// <remarks>The case-insensitivity of the 0th entry will always match the mode selected, but subsequent entries may not.</remarks>
         public List<FixedDistanceSet>? FixedDistanceSets { get; }
 
+#if SYSTEM_TEXT_REGULAREXPRESSIONS
+        /// <summary>When in leading strings mode, gets the search values to use for searching the input.</summary>
+        public SearchValues<string>? LeadingStrings { get; }
+#endif
+
         /// <summary>Data about a character class at a fixed offset from the start of any match to a pattern.</summary>
         public struct FixedDistanceSet(char[]? chars, string set, int distance)
         {
@@ -676,6 +695,28 @@ public bool TryFindNextStartingPositionLeftToRight(ReadOnlySpan<char> textSpan,
                         return false;
                     }
 
+                // There are multiple possible strings at the beginning. Search for one.
+                case FindNextStartingPositionMode.LeadingStrings_LeftToRight:
+                case FindNextStartingPositionMode.LeadingStrings_OrdinalIgnoreCase_LeftToRight:
+                {
+                    if (LeadingStrings is not SearchValues<string> searchValues)
+                    {
+                        // This should be exceedingly rare and only happen if a Compiled regex selected this
+                        // option but then failed to compile (e.g. due to too deep stacks) and fell back to the interpreter.
+                        return true;
+                    }
+
+                    int i = textSpan.Slice(pos).IndexOfAny(searchValues);
+                    if (i >= 0)
+                    {
+                        pos += i;
+                        return true;
+                    }
+
+                    pos = textSpan.Length;
+                    return false;
+                }
+
                 // There are one or more sets at fixed offsets from the start of the pattern.
 
                 case FindNextStartingPositionMode.FixedDistanceSets_LeftToRight:
@@ -800,12 +841,6 @@ public bool TryFindNextStartingPositionLeftToRight(ReadOnlySpan<char> textSpan,
                         return false;
                     }
 
-                // Not supported in the interpreter, but we could end up here for patterns so complex the compiler gave up on them.
-
-                case FindNextStartingPositionMode.LeadingStrings_LeftToRight:
-                case FindNextStartingPositionMode.LeadingStrings_OrdinalIgnoreCase_LeftToRight:
-                    return true;
-
                 // Nothing special to look for.  Just return true indicating this is a valid position to try to match.
 
                 default: