namespace SuiteLink.Client.Protocol;

public static class SuiteLinkSubscriptionCodec
{
    public const ushort AdviseMessageType = 0x8010;
    public const ushort AdviseAckMessageType = 0x0003;
    public const ushort UnadviseMessageType = 0x8004;

    public static byte[] EncodeAdvise(string itemName)
    {
        // Convenience overload for initial bring-up scenarios.
        return EncodeAdvise(0, itemName);
    }

    public static byte[] EncodeAdvise(uint tagId, string itemName)
    {
        if (string.IsNullOrWhiteSpace(itemName))
        {
            throw new ArgumentException("Value cannot be null or whitespace.", nameof(itemName));
        }

        var itemNameBytes = SuiteLinkEncoding.EncodeLengthPrefixedUtf16(itemName);
        var payload = new byte[sizeof(uint) + itemNameBytes.Length];
        SuiteLinkEncoding.WriteUInt32LittleEndian(payload.AsSpan(0, sizeof(uint)), tagId);
        itemNameBytes.CopyTo(payload, sizeof(uint));

        return SuiteLinkFrameWriter.WriteFrame(AdviseMessageType, payload);
    }

    public static byte[] EncodeUnadvise(uint tagId)
    {
        Span<byte> payload = stackalloc byte[sizeof(uint)];
        SuiteLinkEncoding.WriteUInt32LittleEndian(payload, tagId);
        return SuiteLinkFrameWriter.WriteFrame(UnadviseMessageType, payload);
    }

    public static IReadOnlyList<AdviseAck> DecodeAdviseAckMany(ReadOnlySpan<byte> bytes)
    {
        var frame = SuiteLinkFrameReader.ParseFrame(bytes);
        if (frame.MessageType != AdviseAckMessageType)
        {
            throw new FormatException(
                $"Unexpected advise ack message type 0x{frame.MessageType:x4}; expected 0x{AdviseAckMessageType:x4}.");
        }

        var payload = frame.Payload.Span;
        if (payload.Length == 0)
        {
            return [];
        }

        if (payload.Length % 5 != 0)
        {
            throw new FormatException(
                $"Unexpected advise ack payload length {payload.Length}; payload must be a multiple of 5 bytes.");
        }

        var itemCount = payload.Length / 5;
        var items = new List<AdviseAck>(itemCount);
        var offset = 0;
        while (offset < payload.Length)
        {
            var tagId = SuiteLinkEncoding.ReadUInt32LittleEndian(payload.Slice(offset, 4));
            items.Add(new AdviseAck(tagId));
            offset += 5;
        }

        return items;
    }

    public static AdviseAck DecodeAdviseAck(ReadOnlySpan<byte> bytes)
    {
        var items = DecodeAdviseAckMany(bytes);
        if (items.Count != 1)
        {
            throw new FormatException(
                $"Expected a single advise ack item but decoded {items.Count} items.");
        }

        return items[0];
    }
}

public readonly record struct AdviseAck(uint TagId);