DiscordChatExporter/DiscordChatExporter.Core.Models/Emoji.cs

using System.Collections.Generic;
using System.Linq;
using Tyrrrz.Extensions;

namespace DiscordChatExporter.Core.Models
{
    // https://discordapp.com/developers/docs/resources/emoji#emoji-object

    public partial class Emoji
    {
        public string? Id { get; }

        public string Name { get; }

        public bool IsAnimated { get; }

        public string ImageUrl { get; }

        public Emoji(string? id, string name, bool isAnimated)
        {
            Id = id;
            Name = name;
            IsAnimated = isAnimated;

            ImageUrl = GetImageUrl(id, name, isAnimated);
        }
    }

    public partial class Emoji
    {
        private static IEnumerable<int> GetCodePoints(string emoji)
        {
            for (var i = 0; i < emoji.Length; i += char.IsHighSurrogate(emoji[i]) ? 2 : 1)
                yield return char.ConvertToUtf32(emoji, i);
        }

        private static string GetTwemojiName(string emoji) =>
            GetCodePoints(emoji).Select(i => i.ToString("x")).JoinToString("-");

        public static string GetImageUrl(string? id, string name, bool isAnimated)
        {
            // Custom emoji
            if (!string.IsNullOrWhiteSpace(id))
            {
                // Animated
                if (isAnimated)
                    return $"https://cdn.discordapp.com/emojis/{id}.gif";

                // Non-animated
                return $"https://cdn.discordapp.com/emojis/{id}.png";
            }

            // Standard unicode emoji (via twemoji)
            var twemojiName = GetTwemojiName(name);
            return $"https://twemoji.maxcdn.com/2/72x72/{twemojiName}.png";
        }
    }
}