using Editor.Prism.Core;

namespace Editor.Prism.Toolchain;

/// <summary>
/// The validator used when no Slang toolchain is present.
/// <para>
/// This is not a stub, it is the contract. Slang <em>emission</em> is pure text generation from the
/// typed IR and has no dependency on any toolchain, so <c>.slang</c> files are still written on save
/// and every graph still compiles, previews and renders. What is lost is an independent second opinion
/// on our own emitter — which is worth having and worth offering, but never worth blocking on.
/// </para>
/// <para>
/// <see cref="Validate"/> therefore returns an empty list rather than an error: an absent optional tool
/// is not a problem with the user's shader. <see cref="Explain"/> exists for the one place that should
/// mention it — a grey status chip with a one-click install link, no modal and no nag.
/// </para>
/// </summary>
public sealed class NullSlangValidator : ISlangValidator
{
	static readonly Task<IReadOnlyList<Diagnostic>> s_empty =
		Task.FromResult<IReadOnlyList<Diagnostic>>( Array.Empty<Diagnostic>() );

	/// <summary>The shared instance. This type holds no state worth allocating twice.</summary>
	public static readonly NullSlangValidator Instance = new();

	/// <summary>Create a fallback validator, optionally recording why there is no toolchain.</summary>
	public NullSlangValidator( string reason = null )
	{
		Reason = string.IsNullOrWhiteSpace( reason )
			? "No Slang toolchain was found."
			: reason.Trim();
	}

	/// <summary>Always false. Callers show a "not validated" chip rather than an error.</summary>
	public bool Available => false;

	/// <summary>Always null.</summary>
	public string Version => null;

	/// <summary>Why validation is unavailable, phrased for a tooltip.</summary>
	public string Reason { get; }

	/// <summary>Never produces diagnostics and never throws.</summary>
	public Task<IReadOnlyList<Diagnostic>> Validate( SlangValidationRequest request, CancellationToken ct ) => s_empty;

	/// <summary>
	/// A single informational diagnostic for the Slang tab's banner. Deliberately not returned from
	/// <see cref="Validate"/>: the diagnostics list is for problems with the shader, and this is not one.
	/// </summary>
	public IReadOnlyList<Diagnostic> Explain() => new[]
	{
		Diagnostic.Info( DiagnosticCode.SlangUnavailable,
			"Slang output is generated but not validated",
			null,
			$"{Reason} Install it from Preferences to have slangc independently check the module Prism " +
			"emits. Nothing about graph editing, .shader generation, compiling or the preview depends on it." )
	};

	/// <inheritdoc/>
	public override string ToString() => "slang: not validated";
}