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

using System;
using System.Collections.Generic;

namespace System.Diagnostics
{
    /// <summary>
    /// A text annotation associated with a collection of tags.
    /// </summary>
    [DebuggerDisplay("Name = {Name}, Timestamp = {Timestamp}")]
    public readonly struct ActivityEvent
    {
        private static readonly IEnumerable<KeyValuePair<string, object?>> s_emptyTags = Array.Empty<KeyValuePair<string, object?>>();
        private readonly Activity.TagsLinkedList? _tags;

        /// <summary>
        /// Initializes a new instance of the <see cref="ActivityEvent"/> class.
        /// </summary>
        /// <param name="name">Event name.</param>
        public ActivityEvent(string name) : this(name, DateTimeOffset.UtcNow, tags: null)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="ActivityEvent"/> class.
        /// </summary>
        /// <param name="name">Event name.</param>
        /// <param name="timestamp">Event timestamp. Timestamp MUST only be used for the events that happened in the past, not at the moment of this call.</param>
        /// <param name="tags">Event Tags.</param>
        public ActivityEvent(string name, DateTimeOffset timestamp = default, ActivityTagsCollection? tags = null) : this(name, timestamp, tags, tags is null ? 0 : tags.Count) { }

        internal ActivityEvent(string name, DateTimeOffset timestamp, ref TagList tags) : this(name, timestamp, tags, tags.Count) { }

        private ActivityEvent(string name, DateTimeOffset timestamp, IEnumerable<KeyValuePair<string, object?>>? tags, int tagsCount)
        {
            Name = name ?? string.Empty;
            Timestamp = timestamp != default ? timestamp : DateTimeOffset.UtcNow;

            _tags = tagsCount > 0 ? new Activity.TagsLinkedList(tags!) : null;
        }

        /// <summary>
        /// Gets the <see cref="ActivityEvent"/> name.
        /// </summary>
        public string Name { get; }

        /// <summary>
        /// Gets the <see cref="ActivityEvent"/> timestamp.
        /// </summary>
        public DateTimeOffset Timestamp { get; }

        /// <summary>
        /// Gets the collection of tags associated with the event.
        /// </summary>
        public IEnumerable<KeyValuePair<string, object?>> Tags => _tags ?? s_emptyTags;

        /// <summary>
        /// Enumerate the tags attached to this <see cref="ActivityEvent"/> object.
        /// </summary>
        /// <returns><see cref="Activity.Enumerator{T}"/>.</returns>
        public Activity.Enumerator<KeyValuePair<string, object?>> EnumerateTagObjects() => new Activity.Enumerator<KeyValuePair<string, object?>>(_tags?.First);
    }
}