Added string domain

src/Draco.Compiler/Internal/Binding/Binder_BoundPattern.cs

@@ -1,8 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using Draco.Compiler.Internal.BoundTree;
 using Draco.Compiler.Internal.Diagnostics;
 using Draco.Compiler.Internal.Solver;

src/Draco.Compiler/Internal/Binding/Binder_UntypedPattern.cs

@@ -1,8 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using Draco.Compiler.Api.Syntax;
 using Draco.Compiler.Internal.Diagnostics;
 using Draco.Compiler.Internal.Solver;

src/Draco.Compiler/Internal/FlowAnalysis/Domain/IntegralDomain.cs

@@ -4,9 +4,6 @@
 using System.Linq;
 using System.Numerics;
 using System.Runtime.InteropServices;
-using System.Text;
-using System.Threading.Tasks;
-using Draco.Compiler.Internal.Binding;
 using Draco.Compiler.Internal.BoundTree;
 using Draco.Compiler.Internal.Symbols;
 using Draco.Compiler.Internal.Utilities;
@@ -114,7 +111,7 @@ public override void SubtractPattern(BoundPattern pattern)
 
     public override string ToString()
     {
-        if (this.IsEmpty) return $"[{this.minValue}; {this.maxValue}]";
+        if (this.IsEmpty) return "empty";
 
         var parts = new List<string>();
         if (this.subtracted[0].From != this.minValue) parts.Add($"[{this.minValue}; {this.subtracted[0].From})");

src/Draco.Compiler/Internal/FlowAnalysis/Domain/StringDomain.cs

@@ -0,0 +1,79 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using Draco.Compiler.Internal.BoundTree;
+using Draco.Compiler.Internal.Symbols;
+using Draco.Compiler.Internal.Utilities;
+
+namespace Draco.Compiler.Internal.FlowAnalysis.Domain;
+
+internal sealed class StringDomain : ValueDomain
+{
+    public override bool IsEmpty => this.allSubtracted;
+
+    private readonly TypeSymbol backingType;
+    private readonly HashSet<string> subtracted;
+    private bool allSubtracted;
+
+    private StringDomain(TypeSymbol backingType, HashSet<string> subtracted, bool allSubtracted)
+    {
+        this.backingType = backingType;
+        this.subtracted = subtracted;
+        this.allSubtracted = allSubtracted;
+    }
+
+    public StringDomain(TypeSymbol backingType)
+        : this(backingType, new(), false)
+    {
+    }
+
+    public override string ToString() => this.allSubtracted
+        ? "empty"
+        : $"Universe \\ {{{string.Join(", ", this.subtracted.Select(StringUtils.Unescape))}}}";
+
+    public override ValueDomain Clone() =>
+        new StringDomain(this.backingType, this.subtracted.ToHashSet(), this.allSubtracted);
+
+    public override BoundPattern? SamplePattern()
+    {
+        if (this.allSubtracted) return null;
+
+        // Check some trivial ones
+        if (!this.subtracted.Contains(string.Empty)) return this.ToPattern(string.Empty);
+
+        // We use the same trick as proving that you can't list all possible reals
+        // We construct a string that differs in the first char from the first string,
+        // in the second char of the second string, ...
+
+        var result = new StringBuilder();
+        var i = 0;
+        foreach (var str in this.subtracted)
+        {
+            var ithChar = str.Length < i ? str[i] : '\0';
+            result.Append(ithChar == 'a' ? 'b' : 'a');
+        }
+
+        return this.ToPattern(result.ToString());
+    }
+
+    public override void SubtractPattern(BoundPattern pattern)
+    {
+        if (this.allSubtracted) return;
+
+        switch (pattern)
+        {
+        case BoundDiscardPattern:
+            this.subtracted.Clear();
+            this.allSubtracted = true;
+            break;
+        case BoundLiteralPattern litPattern when litPattern.Value is string str:
+            this.subtracted.Add(str);
+            break;
+        default:
+            throw new ArgumentException("illegal pattern for string domain", nameof(pattern));
+        }
+    }
+
+    private BoundPattern ToPattern(string text) => throw new NotImplementedException();
+}

src/Draco.Compiler/Internal/FlowAnalysis/Domain/ValueDomain.cs

@@ -1,8 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using Draco.Compiler.Internal.BoundTree;
 using Draco.Compiler.Internal.Symbols;
 using Draco.Compiler.Internal.Symbols.Synthetized;

src/Draco.Compiler/Internal/Utilities/BinarySearch.cs

@@ -1,8 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 
 namespace Draco.Compiler.Internal.Utilities;