Self-contained export (#321)

DiscordChatExporter.Domain/Internal/Extensions/ColorExtensions.cs

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+using System.Drawing;
+
+namespace DiscordChatExporter.Domain.Internal.Extensions
+{
+    internal static class ColorExtensions
+    {
+        public static Color WithAlpha(this Color color, int alpha) => Color.FromArgb(alpha, color);
+
+        public static Color ResetAlpha(this Color color) => color.WithAlpha(255);
+
+        public static int ToRgb(this Color color) => color.ToArgb() & 0xffffff;
+    }
+}

DiscordChatExporter.Domain/Internal/Extensions/DateExtensions.cs

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+using System;
+using System.Globalization;
+
+namespace DiscordChatExporter.Domain.Internal.Extensions
+{
+    internal static class DateExtensions
+    {
+        public static string ToSnowflake(this DateTimeOffset dateTime)
+        {
+            var value = ((ulong) dateTime.ToUnixTimeMilliseconds() - 1420070400000UL) << 22;
+            return value.ToString();
+        }
+
+        public static string ToLocalString(this DateTimeOffset dateTime, string format) =>
+            dateTime.ToLocalTime().ToString(format, CultureInfo.InvariantCulture);
+    }
+}

DiscordChatExporter.Domain/Internal/Extensions/GenericExtensions.cs

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+using System;
+
+namespace DiscordChatExporter.Domain.Internal.Extensions
+{
+    internal static class GenericExtensions
+    {
+        public static TOut Pipe<TIn, TOut>(this TIn input, Func<TIn, TOut> transform) => transform(input);
+
+        public static T? NullIf<T>(this T value, Func<T, bool> predicate) where T : struct =>
+            !predicate(value)
+                ? value
+                : (T?) null;
+    }
+}

DiscordChatExporter.Domain/Internal/Extensions/HttpClientExtensions.cs

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+using System.IO;
+using System.Net.Http;
+using System.Text.Json;
+using System.Threading.Tasks;
+
+namespace DiscordChatExporter.Domain.Internal.Extensions
+{
+    internal static class HttpClientExtensions
+    {
+        // HACK: ConfigureAwait() is crucial here to enable sync-over-async in HtmlMessageWriter
+        public static async Task DownloadAsync(this HttpClient httpClient, string uri, string outputFilePath)
+        {
+            await using var input = await httpClient.GetStreamAsync(uri).ConfigureAwait(false);
+            var output = File.Create(outputFilePath);
+
+            await input.CopyToAsync(output).ConfigureAwait(false);
+            await output.DisposeAsync().ConfigureAwait(false);
+        }
+
+        public static async Task<JsonElement> ReadAsJsonAsync(this HttpContent content)
+        {
+            await using var stream = await content.ReadAsStreamAsync();
+            using var doc = await JsonDocument.ParseAsync(stream);
+
+            return doc.RootElement.Clone();
+        }
+    }
+}

DiscordChatExporter.Domain/Internal/Extensions/JsonElementExtensions.cs

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+using System.Text.Json;
+
+namespace DiscordChatExporter.Domain.Internal.Extensions
+{
+    internal static class JsonElementExtensions
+    {
+        public static JsonElement? GetPropertyOrNull(this JsonElement element, string propertyName) =>
+            element.TryGetProperty(propertyName, out var result) && result.ValueKind != JsonValueKind.Null
+                ? result
+                : (JsonElement?) null;
+    }
+}

DiscordChatExporter.Domain/Internal/Extensions/StringExtensions.cs

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+using System.Text;
+
+namespace DiscordChatExporter.Domain.Internal.Extensions
+{
+    internal static class StringExtensions
+    {
+        public static StringBuilder AppendIfNotEmpty(this StringBuilder builder, char value) =>
+            builder.Length > 0
+                ? builder.Append(value)
+                : builder;
+    }
+}

DiscordChatExporter.Domain/Internal/Extensions/Utf8JsonWriterExtensions.cs

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+using System;
+using System.Text.Json;
+
+namespace DiscordChatExporter.Domain.Internal.Extensions
+{
+    internal static class Utf8JsonWriterExtensions
+    {
+        public static void WriteString(this Utf8JsonWriter writer, string propertyName, DateTimeOffset? value)
+        {
+            writer.WritePropertyName(propertyName);
+
+            if (value != null)
+                writer.WriteStringValue(value.Value);
+            else
+                writer.WriteNullValue();
+        }
+
+        public static void WriteNumber(this Utf8JsonWriter writer, string propertyName, int? value)
+        {
+            writer.WritePropertyName(propertyName);
+
+            if (value != null)
+                writer.WriteNumberValue(value.Value);
+            else
+                writer.WriteNullValue();
+        }
+    }
+}