using System.Globalization;
using System.Text;
using System.Text.Json;
using System.Text.Json.Nodes;

namespace PrivaPub.Federation.Signing
{
	// RFC 8785, the JSON Canonicalization Scheme: members sorted by their names' UTF-16 code units, no whitespace, strings
	// escaped only where JSON must be, numbers as ECMAScript writes them. What an eddsa-jcs-2022 proof signs (FEP-8b32).
	public static class Jcs
	{
		public static string Serialize(JsonNode node)
		{
			var builder = new StringBuilder();
			Write(builder, node);
			return builder.ToString();
		}

		static void Write(StringBuilder builder, JsonNode node)
		{
			switch (node)
			{
				case null:
					builder.Append("null");
					break;
				case JsonObject obj:
					builder.Append('{');
					var first = true;
					foreach (var (name, value) in obj.OrderBy(p => p.Key, StringComparer.Ordinal))
					{
						if (!first)
							builder.Append(',');
						first = false;
						String(builder, name);
						builder.Append(':');
						Write(builder, value);
					}
					builder.Append('}');
					break;
				case JsonArray array:
					builder.Append('[');
					for (var i = 0; i < array.Count; i++)
					{
						if (i > 0)
							builder.Append(',');
						Write(builder, array[i]);
					}
					builder.Append(']');
					break;
				case JsonValue value:
					switch (value.GetValueKind())
					{
						case JsonValueKind.String:
							String(builder, value.GetValue<string>());
							break;
						case JsonValueKind.Number:
							builder.Append(Number(double.Parse(value.ToJsonString(), NumberStyles.Float, CultureInfo.InvariantCulture)));
							break;
						case JsonValueKind.True:
							builder.Append("true");
							break;
						case JsonValueKind.False:
							builder.Append("false");
							break;
						default:
							builder.Append("null");
							break;
					}
					break;
			}
		}

		static void String(StringBuilder builder, string text)
		{
			builder.Append('"');
			foreach (var c in text)
			{
				switch (c)
				{
					case '"':
						builder.Append("\\\"");
						break;
					case '\\':
						builder.Append("\\\\");
						break;
					case '\b':
						builder.Append("\\b");
						break;
					case '\f':
						builder.Append("\\f");
						break;
					case '\n':
						builder.Append("\\n");
						break;
					case '\r':
						builder.Append("\\r");
						break;
					case '\t':
						builder.Append("\\t");
						break;
					default:
						if (c < 0x20)
							builder.Append("\\u").Append(((int)c).ToString("x4", CultureInfo.InvariantCulture));
						else
							builder.Append(c);
						break;
				}
			}
			builder.Append('"');
		}

		// ECMAScript's Number.prototype.toString: integers without a fraction, the shortest digits that read back the same,
		// an exponent from 1e21 up and below 1e-6
		public static string Number(double value)
		{
			if (double.IsNaN(value) || double.IsInfinity(value))
				throw new ArgumentException("JSON has no NaN or Infinity", nameof(value));
			if (value == 0)
				return "0";
			var abs = Math.Abs(value);
			if (abs >= 1e21 || abs < 1e-6)
			{
				var exponential = value.ToString("R", CultureInfo.InvariantCulture).Replace("E", "e");
				var at = exponential.IndexOf('e');
				if (at < 0)
					return exponential;
				var mantissa = exponential[..at];
				var exponent = int.Parse(exponential[(at + 1)..], CultureInfo.InvariantCulture);
				return $"{mantissa}e{(exponent < 0 ? "-" : "+")}{Math.Abs(exponent)}";
			}
			var text = value.ToString("R", CultureInfo.InvariantCulture);
			if (!text.Contains('E'))
				return text;
			// "R" chose an exponent ECMAScript would not: written out in full
			return decimal.Parse(text, NumberStyles.Float, CultureInfo.InvariantCulture).ToString(CultureInfo.InvariantCulture);
		}
	}
}