// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
 
#nullable disable
 
using System.Collections.Immutable;
using Microsoft.CodeAnalysis.Operations;
using Microsoft.CodeAnalysis.PooledObjects;
using Roslyn.Utilities;
 
#if DEBUG
using System.Diagnostics;
#endif
 
namespace Microsoft.CodeAnalysis.FlowAnalysis;
 
/// <summary>
/// Helper class to detect <see cref="IFlowCaptureOperation"/>s that are l-value captures.
/// L-value captures are essentially captures of a symbol's location/address.
/// Corresponding <see cref="IFlowCaptureReferenceOperation"/>s which share the same
/// <see cref="CaptureId"/> as this flow capture, dereferences and writes to this location
/// subsequently in the flow graph.
/// For example, consider the below code:
///     a[i] = x ?? a[j];
/// The control flow graph contains an initial flow capture of "a[i]" to capture the l-value
/// of this array element:
///     FC0 (a[i])
/// Then it evaluates the right hand side, which can have different
/// values on different control flow paths, and the resultant value is then written
/// to the captured location:
///     FCR0 = result
/// </summary>
/// <remarks>
/// NOTE: This type is a workaround for https://github.com/dotnet/roslyn/issues/31007
/// and it can be deleted once that feature is implemented.
/// </remarks>
internal static class LValueFlowCapturesProvider
{
    public static ImmutableDictionary<CaptureId, FlowCaptureKind> CreateLValueFlowCaptures(ControlFlowGraph cfg)
    {
        // This method identifies flow capture reference operations that are target of an assignment
        // and marks them as lvalue flow captures.
        // Control flow graph can also contain flow captures
        // that are r-value captures at some point and l-values captures at other point in
        // the flow graph. Specifically, for an ICoalesceOperation a flow capture acts
        // as both an r-value and l-value flow capture.
 
        ImmutableDictionary<CaptureId, FlowCaptureKind>.Builder lvalueFlowCaptureIdBuilder = null;
        var rvalueFlowCaptureIds = PooledHashSet<CaptureId>.GetInstance();
 
        try
        {
            foreach (var flowCaptureReference in cfg.DescendantOperations<IFlowCaptureReferenceOperation>(OperationKind.FlowCaptureReference))
            {
                if (flowCaptureReference.Parent is IAssignmentOperation assignment &&
                    assignment.Target == flowCaptureReference ||
                    flowCaptureReference.IsInLeftOfDeconstructionAssignment(out _))
                {
                    lvalueFlowCaptureIdBuilder ??= ImmutableDictionary.CreateBuilder<CaptureId, FlowCaptureKind>();
                    var captureKind = flowCaptureReference.Parent.IsAnyCompoundAssignment() || rvalueFlowCaptureIds.Contains(flowCaptureReference.Id)
                        ? FlowCaptureKind.LValueAndRValueCapture
                        : FlowCaptureKind.LValueCapture;
                    lvalueFlowCaptureIdBuilder.Add(flowCaptureReference.Id, captureKind);
                }
                else
                {
                    rvalueFlowCaptureIds.Add(flowCaptureReference.Id);
                }
            }
 
#if DEBUG
            if (lvalueFlowCaptureIdBuilder != null)
            {
                foreach (var (captureId, kind) in lvalueFlowCaptureIdBuilder)
                    Debug.Assert(kind == FlowCaptureKind.LValueAndRValueCapture || !rvalueFlowCaptureIds.Contains(captureId), "Flow capture used as both an r-value and an l-value, but with incorrect flow capture kind");
            }
#endif
 
            return lvalueFlowCaptureIdBuilder != null ? lvalueFlowCaptureIdBuilder.ToImmutable() : ImmutableDictionary<CaptureId, FlowCaptureKind>.Empty;
        }
        finally
        {
            rvalueFlowCaptureIds.Free();
        }
    }
}