using System.Buffers.Binary;
using AVEVA.Historian.Client.Models;

namespace AVEVA.Historian.Client.Wcf;

/// <remarks>
/// Serializer for the M3 historical (non-streamed original / backfill) value write — the
/// <c>HistoryService.AddStreamValues</c> <c>values</c> buffer. Decoded byte-for-byte from live
/// captures of the native 2023 R2 gRPC client driving
/// <c>HistorianAccess.AddNonStreamedValue → SendValues</c> against sandbox tags of each analog type
/// (see <c>docs/plans/revision-write-path.md</c> §"R3.1 CAPTURED"). The native non-streamed write
/// rides <c>HistoryService.AddStreamValues</c> with an "ON" storage-sample buffer, the analog sibling
/// of the AddS2 "OS" event buffer (<see cref="HistorianEventWriteProtocol"/>).
///
/// <code>
/// values buffer (single analog sample):
///   0x00  UInt16  0x4E4F            // "ON" signature
///   0x02  UInt16  sampleCount = 1
///   0x04  UInt32  10 + valueBlob.Length   // total buffer length
///   0x08  UInt16  valueBlob.Length
///   0x0A  valueBlob:
///     +0x00  GUID    tag GUID  (the per-tag GUID = the value ParseTagInfoRecord reads as "typeId")
///     +0x10  Int64   sample FILETIME (UTC — the VTQ value timestamp)
///     +0x18  UInt16  OpcQuality = 192 (good)
///     +0x1A  4 bytes value descriptor — CONSTANT C0 10 01 00 across all analog types (the server
///                                       interprets the value bytes by the tag's declared type)
///     +0x1E  Int64   received/version FILETIME (UTC)
///     +0x26  UInt32  0                 // value high dword (constant zero)
///     +0x2A  value bytes, native width by tag type:
///                Float → Float32(4) · Double → Float64(8) · Int2 → Int16(2)
///                Int4 → Int32(4) · UInt4 → UInt32(4)
///                Int8 → Int64(8) · UInt8 → UInt64(8)
/// </code>
///
/// Live-captured for Float/Double/Int2/Int4/UInt4; Int8/UInt8 mirror the captured
/// Double value layout (8 LE bytes) and are live-proven (2026-06-25). UInt1 is re-gated:
/// the historian accepts EnsureTags(UInt1) but stores a degenerate tag — writes would fail.
/// Other tag types have no captured value encoding and are rejected.
/// </remarks>
internal static class HistorianHistoricalWriteProtocol
{
    public const ushort BufferSignature = 0x4E4F; // "ON"
    public const ushort OpcQualityGood = 192;

    // Captured constant: the 4-byte value descriptor was identical across all analog types
    // (Float/Double/Int2/Int4/UInt4). The value type is conveyed by the tag's registration, not here.
    private static readonly byte[] ValueDescriptor = [0xC0, 0x10, 0x01, 0x00];

    // valueBlob fixed prefix: GUID(16) + sampleFT(8) + quality(2) + descriptor(4) + receivedFT(8) +
    // value high dword(4) = 42, then the native-width value bytes.
    private const int ValueBlobPrefixLength = 16 + 8 + 2 + 4 + 8 + 4; // 42

    /// <summary>
    /// Builds the <c>AddStreamValues</c> <c>values</c> buffer for a single analog historical sample.
    /// <paramref name="tagGuid"/> is the per-tag GUID (from the gRPC tag-info read),
    /// <paramref name="dataType"/> is the tag's declared analog type (selects the value width), and
    /// <paramref name="receivedTimeUtc"/> is the storage/received timestamp the orchestrator stamps.
    /// Throws <see cref="ProtocolEvidenceMissingException"/> for tag types without a captured encoding
    /// (including UInt1, which is re-gated — the historian creates a degenerate UInt1 analog tag).
    /// Int8/UInt8 carry exact magnitude only up to 2^53 — the value API is <see langword="double"/>;
    /// full 64-bit range is a separate follow-on.
    /// </summary>
    public static byte[] SerializeAddStreamValuesBuffer(
        Guid tagGuid,
        HistorianDataType dataType,
        DateTime sampleTimeUtc,
        double value,
        DateTime receivedTimeUtc,
        ushort quality = OpcQualityGood)
    {
        byte[] valueBytes = EncodeNativeValue(dataType, value);

        byte[] valueBlob = new byte[ValueBlobPrefixLength + valueBytes.Length];
        Span<byte> span = valueBlob;
        tagGuid.ToByteArray().CopyTo(span[..16]);
        BinaryPrimitives.WriteInt64LittleEndian(span.Slice(16, 8), ToFileTime(sampleTimeUtc));
        BinaryPrimitives.WriteUInt16LittleEndian(span.Slice(24, 2), quality);
        ValueDescriptor.CopyTo(span.Slice(26, 4));
        BinaryPrimitives.WriteInt64LittleEndian(span.Slice(30, 8), ToFileTime(receivedTimeUtc));
        BinaryPrimitives.WriteUInt32LittleEndian(span.Slice(38, 4), 0); // value high dword (constant 0)
        valueBytes.CopyTo(span.Slice(42, valueBytes.Length));

        byte[] buffer = new byte[10 + valueBlob.Length];
        BinaryPrimitives.WriteUInt16LittleEndian(buffer.AsSpan(0, 2), BufferSignature);
        BinaryPrimitives.WriteUInt16LittleEndian(buffer.AsSpan(2, 2), 1); // sampleCount
        BinaryPrimitives.WriteUInt32LittleEndian(buffer.AsSpan(4, 4), checked((uint)(10 + valueBlob.Length)));
        BinaryPrimitives.WriteUInt16LittleEndian(buffer.AsSpan(8, 2), checked((ushort)valueBlob.Length));
        valueBlob.CopyTo(buffer.AsSpan(10));
        return buffer;
    }

    private static byte[] EncodeNativeValue(HistorianDataType dataType, double value)
    {
        switch (dataType)
        {
            case HistorianDataType.Float:
            {
                byte[] b = new byte[4];
                BinaryPrimitives.WriteSingleLittleEndian(b, (float)value);
                return b;
            }
            case HistorianDataType.Double:
            {
                byte[] b = new byte[8];
                BinaryPrimitives.WriteDoubleLittleEndian(b, value);
                return b;
            }
            case HistorianDataType.Int2:
            {
                byte[] b = new byte[2];
                BinaryPrimitives.WriteInt16LittleEndian(b, checked((short)value));
                return b;
            }
            case HistorianDataType.Int4:
            {
                byte[] b = new byte[4];
                BinaryPrimitives.WriteInt32LittleEndian(b, checked((int)value));
                return b;
            }
            case HistorianDataType.UInt4:
            {
                byte[] b = new byte[4];
                BinaryPrimitives.WriteUInt32LittleEndian(b, checked((uint)value));
                return b;
            }
            case HistorianDataType.Int8:
            {
                byte[] b = new byte[8];
                BinaryPrimitives.WriteInt64LittleEndian(b, checked((long)value));
                return b;
            }
            case HistorianDataType.UInt8:
            {
                byte[] b = new byte[8];
                BinaryPrimitives.WriteUInt64LittleEndian(b, checked((ulong)value));
                return b;
            }
            default:
                throw new ProtocolEvidenceMissingException(
                    $"AddHistoricalValuesAsync has no captured value encoding for tag data type '{dataType}'. " +
                    "Captured types: Float, Double, Int2, Int4, UInt4, Int8, UInt8.");
        }
    }

    private static long ToFileTime(DateTime value)
    {
        DateTime utc = value.Kind == DateTimeKind.Utc ? value : value.ToUniversalTime();
        return utc.ToFileTimeUtc();
    }
}