namespace BizHawk.Analyzers;

using System.Collections.Immutable;

[DiagnosticAnalyzer(LanguageNames.CSharp)]
public sealed class TernaryInferredTypeMismatchAnalyzer : DiagnosticAnalyzer
{
	private static readonly DiagnosticDescriptor DiagTernaryInferredTypeMismatch = new(
		id: "BHI1210",
		title: "Inferred type of branches of ternary expression in interpolation don't match",
		messageFormat: "{0}",
		category: "Usage",
		defaultSeverity: DiagnosticSeverity.Warning,
		isEnabledByDefault: true);

	public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(DiagTernaryInferredTypeMismatch);

	public override void Initialize(AnalysisContext context)
	{
		context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
		context.EnableConcurrentExecution();
		context.RegisterOperationAction(
			oac =>
			{
				var ifelseOrTernaryOp = (IConditionalOperation) oac.Operation;
				if (ifelseOrTernaryOp.WhenFalse is null) return;
				var parent = ifelseOrTernaryOp.Parent!;
				if (parent.Kind is OperationKind.Conversion) parent = parent.Parent!;
				if (parent.Kind is not OperationKind.Interpolation) return;
				var ternaryOp = ifelseOrTernaryOp;
				var typeTernary = ternaryOp.Type!;
#if false // never hit; either both branches are string and there are no conversions, or conversions are necessary
				if (typeTernary.SpecialType is SpecialType.System_String) return;
#endif
				var lhs = ternaryOp.WhenTrue;
				var rhs = ternaryOp.WhenFalse;

				static IOperation TrimImplicitCast(IOperation op)
					=> op is IConversionOperation { Conversion.IsImplicit: true } implCastOp ? implCastOp.Operand : op;
				var typeLHS = TrimImplicitCast(lhs).Type!;
				var typeRHS = TrimImplicitCast(rhs).Type!;
				if (typeLHS.Matches(typeRHS)) return; // unnecessary conversion operators on each branch? seen with `? this : this`

				const string ERR_MSG_OBJECT = "missing ToString means ternary branches are upcast to object";
				var fatal = false;
				IOperation flaggedOp = ternaryOp;
				string message;
				if (typeLHS.SpecialType is SpecialType.System_String)
				{
					flaggedOp = rhs;
					message = ERR_MSG_OBJECT;
				}
				else if (typeRHS.SpecialType is SpecialType.System_String)
				{
					flaggedOp = lhs;
					message = ERR_MSG_OBJECT;
				}
				else if (typeTernary.SpecialType is SpecialType.System_Object)
				{
					fatal = true;
					message = "ternary branches are upcast to object! add ToString calls, or convert one to the other's type";
				}
				else
				{
					// if one's already an e.g. int literal, flag the e.g. char literal
					if (typeTernary.Matches(typeLHS)) flaggedOp = rhs;
					else if (typeTernary.Matches(typeRHS)) flaggedOp = lhs;
					message = $"ternary branches are converted to {typeTernary} before serialisation, possibly unintended";
				}
				DiagTernaryInferredTypeMismatch.ReportAt(flaggedOp, fatal ? DiagnosticSeverity.Error : DiagnosticSeverity.Warning, oac, message);
			},
			OperationKind.Conditional);
	}
}