using System;
using System.Collections.Generic;

namespace Capnp
{
    /// <summary>
    /// Entry point for building Cap'n Proto messages.
    /// </summary>
    public class MessageBuilder
    {
        readonly ISegmentAllocator _allocator;
        readonly DynamicSerializerState _rootPtrBuilder;
        List<Rpc.ConsumedCapability>? _capTable;

        MessageBuilder(ISegmentAllocator allocator)
        {
            _allocator = allocator;
            _rootPtrBuilder = new DynamicSerializerState(this);
            _rootPtrBuilder.SetStruct(0, 1);
            _rootPtrBuilder.Allocate();
        }

        /// <summary>
        /// Constructs an instance using a custom segment allocator and reserves space for the root pointer.
        /// </summary>
        /// <typeparam name="T">Segment allocator implementation type</typeparam>
        public static MessageBuilder Create<T>() where T: ISegmentAllocator, new()
        {
            return new MessageBuilder(new T());
        }

        /// <summary>
        /// Constructs an instance using the default segment allocator and reserves space for the root pointer.
        /// </summary>
        /// <param name="defaultSegmentSize">Default segment size, <see cref="SegmentAllocator"/></param>
        public static MessageBuilder Create(int defaultSegmentSize = 128)
        {
            return new MessageBuilder(new SegmentAllocator(defaultSegmentSize));
        }

        /// <summary>
        /// Creates a new object inside the message.
        /// </summary>
        /// <typeparam name="TS">Serializer state specialization</typeparam>
        /// <returns>Serializer state instance representing the new object</returns>
        public TS CreateObject<TS>() where TS: SerializerState, new()
        {
            var ts = new TS();
            ts.Bind(this);
            return ts;
        }

        /// <summary>
        /// Gets or sets the root object. The root object must be set exactly once per message.
        /// Setting it manually is only required (and allowed) when it was created with <see cref="CreateObject{TS}"/>.
        /// </summary>
        /// <exception cref="ArgumentNullException">Attempt to set null reference</exception>
        public SerializerState? Root
        {
            get => _rootPtrBuilder.TryGetPointer(0);
            set => _rootPtrBuilder.Link(0, value ?? throw new ArgumentNullException(nameof(value)));
        }

        /// <summary>
        /// Creates an object and sets it as root object.
        /// </summary>
        /// <typeparam name="TS">Serializer state specialization (must be a struct)</typeparam>
        /// <returns>Serializer state instance representing the new object</returns>
        public TS BuildRoot<TS>() where TS: SerializerState, new()
        {
            if (Root != null)
                throw new InvalidOperationException("Root already set");

            var root = CreateObject<TS>();
            if (root.Kind != ObjectKind.Struct)
                throw new InvalidOperationException("Root object must be a struct");

            Root = root;
            return root;
        }

        /// <summary>
        /// Returns the wire representation of the built message.
        /// </summary>
        public WireFrame Frame => new WireFrame(_allocator.Segments);

        /// <summary>
        /// Initializes the capability table for using the message builder in RPC context.
        /// </summary>
        public void InitCapTable()
        {
            if (_capTable != null)
                throw new InvalidOperationException("Capability table was already initialized");

            _capTable = new List<Rpc.ConsumedCapability>();
        }

        /// <summary>
        /// Returns this message builder's segment allocator.
        /// </summary>
        public ISegmentAllocator Allocator => _allocator;
        internal List<Rpc.ConsumedCapability>? Caps => _capTable;
    }
}