// dnlib: See LICENSE.txt for more info

using System.Collections.Generic;

namespace dnlib.DotNet {
	readonly struct GenericArgumentsStack {
		readonly List<IList<TypeSig>> argsStack;
		readonly bool isTypeVar;

		/// <summary>
		/// Constructor
		/// </summary>
		/// <param name="isTypeVar"><c>true</c> if it's for generic types, <c>false</c> if generic methods</param>
		public GenericArgumentsStack(bool isTypeVar) {
			argsStack = new List<IList<TypeSig>>();
			this.isTypeVar = isTypeVar;
		}

		/// <summary>
		/// Pushes generic arguments
		/// </summary>
		/// <param name="args">The generic arguments</param>
		public void Push(IList<TypeSig> args) => argsStack.Add(args);

		/// <summary>
		/// Pops generic arguments
		/// </summary>
		/// <returns>The popped generic arguments</returns>
		public IList<TypeSig> Pop() {
			int index = argsStack.Count - 1;
			var result = argsStack[index];
			argsStack.RemoveAt(index);
			return result;
		}

		/// <summary>
		/// Resolves a generic argument
		/// </summary>
		/// <param name="number">Generic variable number</param>
		/// <returns>A <see cref="TypeSig"/> or <c>null</c> if none was found</returns>
		public TypeSig Resolve(uint number) {
			TypeSig result = null;
			for (int i = argsStack.Count - 1; i >= 0; i--) {
				var args = argsStack[i];
				if (number >= args.Count)
					return null;
				var typeSig = args[(int)number];
				var gvar = typeSig as GenericSig;
				if (gvar is null || gvar.IsTypeVar != isTypeVar)
					return typeSig;
				result = gvar;
				number = gvar.Number;
			}
			return result;
		}
	}

	/// <summary>
	/// Replaces generic type/method var with its generic argument
	/// </summary>
	sealed class GenericArguments {
		GenericArgumentsStack typeArgsStack = new GenericArgumentsStack(true);
		GenericArgumentsStack methodArgsStack = new GenericArgumentsStack(false);

		/// <summary>
		/// Pushes generic arguments
		/// </summary>
		/// <param name="typeArgs">The generic arguments</param>
		public void PushTypeArgs(IList<TypeSig> typeArgs) => typeArgsStack.Push(typeArgs);

		/// <summary>
		/// Pops generic arguments
		/// </summary>
		/// <returns>The popped generic arguments</returns>
		public IList<TypeSig> PopTypeArgs() => typeArgsStack.Pop();

		/// <summary>
		/// Pushes generic arguments
		/// </summary>
		/// <param name="methodArgs">The generic arguments</param>
		public void PushMethodArgs(IList<TypeSig> methodArgs) => methodArgsStack.Push(methodArgs);

		/// <summary>
		/// Pops generic arguments
		/// </summary>
		/// <returns>The popped generic arguments</returns>
		public IList<TypeSig> PopMethodArgs() => methodArgsStack.Pop();

		/// <summary>
		/// Replaces a generic type/method var with its generic argument (if any). If
		/// <paramref name="typeSig"/> isn't a generic type/method var or if it can't
		/// be resolved, it itself is returned. Else the resolved type is returned.
		/// </summary>
		/// <param name="typeSig">Type signature</param>
		/// <returns>New <see cref="TypeSig"/> which is never <c>null</c> unless
		/// <paramref name="typeSig"/> is <c>null</c></returns>
		public TypeSig Resolve(TypeSig typeSig) {
			if (typeSig is null)
				return null;

			var sig = typeSig;

			if (sig is GenericMVar genericMVar) {
				var newSig = methodArgsStack.Resolve(genericMVar.Number);
				if (newSig is null || newSig == sig)
					return sig;
				return newSig;
			}

			if (sig is GenericVar genericVar) {
				var newSig = typeArgsStack.Resolve(genericVar.Number);
				if (newSig is null || newSig == sig)
					return sig;
				return newSig;
			}

			return sig;
		}
	}
}