CaseExpression.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

namespace Microsoft.EntityFrameworkCore.Cosmos.Query.Internal;

/// <summary>
///     <para>
///         An expression that represents a CASE statement in a SQL tree.
///     </para>
///     <para>
///         This type is typically used by database providers (and other extensions). It is generally
///         not used in application code.
///     </para>
/// </summary>
public class CaseExpression : SqlExpression
{
    private readonly List<CaseWhenClause> _whenClauses = [];

    /// <summary>
    ///     Creates a new instance of the <see cref="CaseExpression" /> class which represents a simple CASE expression.
    /// </summary>
    /// <param name="operand">An expression to compare with <see cref="CaseWhenClause.Test" /> in <see cref="WhenClauses" />.</param>
    /// <param name="whenClauses">A list of <see cref="CaseWhenClause" /> to compare or evaluate and get result from.</param>
    /// <param name="elseResult">A value to return if no <see cref="WhenClauses" /> matches, if any.</param>
    public CaseExpression(
        SqlExpression? operand,
        IReadOnlyList<CaseWhenClause> whenClauses,
        SqlExpression? elseResult = null)
        : base(whenClauses[0].Result.Type, whenClauses[0].Result.TypeMapping)
    {
        Operand = operand;
        _whenClauses.AddRange(whenClauses);
        ElseResult = elseResult;
    }

    /// <summary>
    ///     Creates a new instance of the <see cref="CaseExpression" /> class which represents a searched CASE expression.
    /// </summary>
    /// <param name="whenClauses">A list of <see cref="CaseWhenClause" /> to evaluate condition and get result from.</param>
    /// <param name="elseResult">A value to return if no <see cref="WhenClauses" /> matches, if any.</param>
    public CaseExpression(
        IReadOnlyList<CaseWhenClause> whenClauses,
        SqlExpression? elseResult = null)
        : this(null, whenClauses, elseResult)
    {
    }

    /// <summary>
    ///     The value to compare in <see cref="WhenClauses" />.
    /// </summary>
    public virtual SqlExpression? Operand { get; }

    /// <summary>
    ///     The list of <see cref="CaseWhenClause" /> to match <see cref="Operand" /> or evaluate condition to get result.
    /// </summary>
    public virtual IReadOnlyList<CaseWhenClause> WhenClauses
        => _whenClauses;

    /// <summary>
    ///     The value to return if none of the <see cref="WhenClauses" /> matches.
    /// </summary>
    public virtual SqlExpression? ElseResult { get; }

    /// <inheritdoc />
    protected override Expression VisitChildren(ExpressionVisitor visitor)
    {
        var operand = (SqlExpression?)visitor.Visit(Operand);
        var changed = operand != Operand;
        var whenClauses = new List<CaseWhenClause>();
        foreach (var whenClause in WhenClauses)
        {
            var test = (SqlExpression)visitor.Visit(whenClause.Test);
            var result = (SqlExpression)visitor.Visit(whenClause.Result);

            if (test != whenClause.Test
                || result != whenClause.Result)
            {
                changed = true;
                whenClauses.Add(new CaseWhenClause(test, result));
            }
            else
            {
                whenClauses.Add(whenClause);
            }
        }

        var elseResult = (SqlExpression?)visitor.Visit(ElseResult);
        changed |= elseResult != ElseResult;

        return changed
            ? new CaseExpression(operand, whenClauses, elseResult)
            : this;
    }

    /// <summary>
    ///     Creates a new expression that is like this one, but using the supplied children. If all of the children are the same, it will
    ///     return this expression.
    /// </summary>
    /// <param name="operand">The <see cref="Operand" /> property of the result.</param>
    /// <param name="whenClauses">The <see cref="WhenClauses" /> property of the result.</param>
    /// <param name="elseResult">The <see cref="ElseResult" /> property of the result.</param>
    /// <returns>This expression if no children changed, or an expression with the updated children.</returns>
    public virtual CaseExpression Update(
        SqlExpression? operand,
        IReadOnlyList<CaseWhenClause> whenClauses,
        SqlExpression? elseResult)
        => operand != Operand || !whenClauses.SequenceEqual(WhenClauses) || elseResult != ElseResult
            ? new CaseExpression(operand, whenClauses, elseResult)
            : this;

    /// <inheritdoc />
    protected override void Print(ExpressionPrinter expressionPrinter)
    {
        expressionPrinter.Append("CASE");
        if (Operand != null)
        {
            expressionPrinter.Append(" ");
            expressionPrinter.Visit(Operand);
        }

        using (expressionPrinter.Indent())
        {
            foreach (var whenClause in WhenClauses)
            {
                expressionPrinter.AppendLine().Append("WHEN ");
                expressionPrinter.Visit(whenClause.Test);
                expressionPrinter.Append(" THEN ");
                expressionPrinter.Visit(whenClause.Result);
            }

            if (ElseResult != null)
            {
                expressionPrinter.AppendLine().Append("ELSE ");
                expressionPrinter.Visit(ElseResult);
            }
        }

        expressionPrinter.AppendLine().Append("END");
    }

    /// <inheritdoc />
    public override bool Equals(object? obj)
        => obj != null
            && (ReferenceEquals(this, obj)
                || obj is CaseExpression caseExpression
                && Equals(caseExpression));

    private bool Equals(CaseExpression caseExpression)
        => base.Equals(caseExpression)
            && (Operand?.Equals(caseExpression.Operand) ?? caseExpression.Operand == null)
            && WhenClauses.SequenceEqual(caseExpression.WhenClauses)
            && (ElseResult?.Equals(caseExpression.ElseResult) ?? caseExpression.ElseResult == null);

    /// <inheritdoc />
    public override int GetHashCode()
    {
        var hash = new HashCode();
        hash.Add(base.GetHashCode());
        hash.Add(Operand);
        for (var i = 0; i < WhenClauses.Count; i++)
        {
            hash.Add(WhenClauses[i]);
        }

        hash.Add(ElseResult);
        return hash.ToHashCode();
    }
}