From 6eb2810baf40a51763623363a4502dcf75224a55 Mon Sep 17 00:00:00 2001
From: LPeter1997 <lenkefi.peti@gmail.com>
Date: Wed, 4 Oct 2023 19:12:30 +0200
Subject: [PATCH] Added outline for decision tree

---
 .../Internal/FlowAnalysis/DecisionTree.cs     | 137 ++++++++++++++++++
 .../FlowAnalysis/Domain/ValueDomain.cs        |  10 --
 2 files changed, 137 insertions(+), 10 deletions(-)
 create mode 100644 src/Draco.Compiler/Internal/FlowAnalysis/DecisionTree.cs

diff --git a/src/Draco.Compiler/Internal/FlowAnalysis/DecisionTree.cs b/src/Draco.Compiler/Internal/FlowAnalysis/DecisionTree.cs
new file mode 100644
index 000000000..0a8c5ece5
--- /dev/null
+++ b/src/Draco.Compiler/Internal/FlowAnalysis/DecisionTree.cs
@@ -0,0 +1,137 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Draco.Compiler.Internal.BoundTree;
+
+namespace Draco.Compiler.Internal.FlowAnalysis;
+
+/// <summary>
+/// Represents a decision tree computed for a match expression.
+/// It helps guiding the rewriter to generate code as well as reporting things like redundant branches or
+/// missing cases.
+/// </summary>
+/// <typeparam name="TAction">The action type being performed, when a branch is matched.</typeparam>
+internal sealed class DecisionTree<TAction>
+{
+    /// <summary>
+    /// A single arm in the match construct.
+    /// </summary>
+    /// <param name="Pattern">The matched pattern.</param>
+    /// <param name="Action">The taken action if <paramref name="Pattern"/> matches.</param>
+    public readonly record struct Arm(BoundPattern Pattern, TAction Action);
+
+    /// <summary>
+    /// Represents a redundant arm.
+    /// </summary>
+    /// <param name="CoveredBy">The arm that covers the <paramref name="Redundant"/> one already.</param>
+    /// <param name="Redundant">The arm that is redundant, because <paramref name="CoveredBy"/> already
+    /// matches.</param>
+    public readonly record struct Redundance(Arm CoveredBy, Arm Redundant);
+
+    /// <summary>
+    /// A single node in the decision tree.
+    /// </summary>
+    public abstract class Node
+    {
+        /// <summary>
+        /// The parent node of this one.
+        /// </summary>
+        public virtual Node? Parent => null;
+
+        /// <summary>
+        /// The index of the row this node originated from in the parent.
+        /// </summary>
+        public virtual int? ParentRowIndex => null;
+
+        /// <summary>
+        /// True, if this is an action node, meaning that there is a match.
+        /// </summary>
+        public virtual bool IsAction => false;
+
+        /// <summary>
+        /// The action that's associated with the node.
+        /// </summary>
+        [MemberNotNullWhen(true, nameof(IsAction))]
+        public virtual TAction? Action => default;
+
+        /// <summary>
+        /// True, if this is a failure node.
+        /// </summary>
+        public virtual bool IsFail => false;
+
+        /// <summary>
+        /// The counterexample of this node, if it's a failure node and a counterexample could be produced.
+        /// The counterexample in this case means a pattern that was not covered.
+        /// </summary>
+        public virtual BoundPattern? Counterexample => null;
+
+        /// <summary>
+        /// The expression being matched on, if any.
+        /// </summary>
+        public virtual BoundExpression? MatchedOn => null;
+
+        /// <summary>
+        /// The child nodes of this one, associated to the pattern to match to take the route to the child.
+        /// </summary>
+        public virtual ImmutableArray<KeyValuePair<BoundPattern, Node>> Children =>
+            ImmutableArray<KeyValuePair<BoundPattern, Node>>.Empty;
+    }
+
+    /// <summary>
+    /// Builds a decision tree from the given data.
+    /// </summary>
+    /// <param name="matchedValue">The matched value.</param>
+    /// <param name="arms">The arms of the match.</param>
+    /// <returns>The constructed decision tree.</returns>
+    public static DecisionTree<TAction> Build(BoundExpression matchedValue, ImmutableArray<Arm> arms) =>
+        throw new NotImplementedException();
+
+    /// <summary>
+    /// Stringifies the given <paramref name="pattern"/> to a user-readable format.
+    /// </summary>
+    /// <param name="pattern">The pattern to stringify.</param>
+    /// <returns>The user-readable form of <paramref name="pattern"/>.</returns>
+    public static string ToDisplayString(BoundPattern pattern) => pattern switch
+    {
+        _ => throw new ArgumentOutOfRangeException(nameof(pattern)),
+    };
+
+    /// <summary>
+    /// The matched value.
+    /// </summary>
+    public BoundExpression MatchedValue { get; }
+
+    /// <summary>
+    /// The arms of the root construct.
+    /// </summary>
+    public ImmutableArray<Arm> Arms { get; }
+
+    /// <summary>
+    /// The root node of this tree.
+    /// </summary>
+    public Node Root { get; }
+
+    /// <summary>
+    /// All redundancies in the tree.
+    /// </summary>
+    public ImmutableArray<Redundance> Redundancies { get; }
+
+    /// <summary>
+    /// True, if this tree is exhaustive.
+    /// </summary>
+    public bool IsExhaustive { get; }
+
+    /// <summary>
+    /// An example of an uncovered pattern, if any.
+    /// </summary>
+    public BoundPattern? UncoveredExample { get; }
+
+    private DecisionTree()
+    {
+        // TODO
+    }
+}
diff --git a/src/Draco.Compiler/Internal/FlowAnalysis/Domain/ValueDomain.cs b/src/Draco.Compiler/Internal/FlowAnalysis/Domain/ValueDomain.cs
index fd02dc797..32fcb909e 100644
--- a/src/Draco.Compiler/Internal/FlowAnalysis/Domain/ValueDomain.cs
+++ b/src/Draco.Compiler/Internal/FlowAnalysis/Domain/ValueDomain.cs
@@ -26,16 +26,6 @@ public static ValueDomain CreateDomain(IntrinsicSymbols intrinsics, TypeSymbol t
         throw new ArgumentOutOfRangeException(nameof(type));
     }
 
-    /// <summary>
-    /// Stringifies the given <paramref name="pattern"/> to a user-readable format.
-    /// </summary>
-    /// <param name="pattern">The pattern to stringify.</param>
-    /// <returns>The user-readable form of <paramref name="pattern"/>.</returns>
-    public static string ToDisplayString(BoundPattern pattern) => pattern switch
-    {
-        _ => throw new ArgumentOutOfRangeException(nameof(pattern)),
-    };
-
     /// <summary>
     /// True, if this domain has been emptied.
     /// </summary>