using System.Buffers.Binary;

namespace ZB.MOM.WW.OtOpcUa.Driver.S7;

/// <summary>
///     Byte-level codecs for the six Siemens S7 date/time-shaped types: DTL, DATE_AND_TIME
///     (DT), S5TIME, TIME, TIME_OF_DAY (TOD), DATE. Pulled out of <see cref="S7Driver"/> so
///     the encoding rules are unit-testable against golden byte vectors without standing
///     up a Plc instance — same pattern as <see cref="S7StringCodec"/>.
/// </summary>
/// <remarks>
///     Wire formats (all big-endian, matching S7's native byte order):
///     <list type="bullet">
///         <item>
///             <b>DTL</b> (12 bytes): year UInt16 BE / month / day / day-of-week / hour /
///             minute / second (1 byte each) / nanoseconds UInt32 BE. Year range 1970-2554.
///         </item>
///         <item>
///             <b>DATE_AND_TIME (DT)</b> (8 bytes BCD): year-since-1990 / month / day / hour /
///             minute / second (1 BCD byte each) + ms (3 BCD digits packed in 1.5 bytes) +
///             day-of-week (1 BCD digit, 1=Sunday..7=Saturday). Years 90-99 → 1990-1999;
///             years 00-89 → 2000-2089.
///         </item>
///         <item>
///             <b>S5TIME</b> (16 bits): bits 15..14 reserved (0), bits 13..12 timebase
///             (00=10ms, 01=100ms, 10=1s, 11=10s), bits 11..0 = 3-digit BCD count (0-999).
///             Total range 0..9990s.
///         </item>
///         <item>
///             <b>TIME</b> (Int32 ms BE): signed milliseconds. Negative durations allowed.
///         </item>
///         <item>
///             <b>TOD</b> (UInt32 ms BE): milliseconds since midnight, 0..86399999.
///         </item>
///         <item>
///             <b>DATE</b> (UInt16 BE): days since 1990-01-01. Range 0..65535 (1990-2168).
///         </item>
///     </list>
///     <para>
///         <b>Uninitialized PLC bytes</b>: an all-zero DTL or DT buffer (year 0 / month 0)
///         is rejected as <see cref="InvalidDataException"/> rather than decoded as
///         year-0001 garbage — operators see "BadOutOfRange" instead of a misleading
///         valid-but-wrong timestamp.
///     </para>
/// </remarks>
public static class S7DateTimeCodec
{
    // ---- DTL (12 bytes) ----

    /// <summary>Wire size of an S7 DTL value.</summary>
    public const int DtlSize = 12;

    /// <summary>
    ///     Decode a 12-byte DTL buffer into a DateTime. Throws
    ///     <see cref="InvalidDataException"/> when the buffer is uninitialized
    ///     (all-zero year+month) or when components are out of range.
    /// </summary>
    public static DateTime DecodeDtl(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length != DtlSize)
            throw new InvalidDataException($"S7 DTL expected {DtlSize} bytes, got {bytes.Length}");

        int year = BinaryPrimitives.ReadUInt16BigEndian(bytes.Slice(0, 2));
        int month = bytes[2];
        int day = bytes[3];
        // bytes[4] = day-of-week (1=Sunday..7=Saturday); ignored on read — the .NET
        // DateTime carries its own and the PLC value can be inconsistent on uninit data.
        int hour = bytes[5];
        int minute = bytes[6];
        int second = bytes[7];
        uint nanos = BinaryPrimitives.ReadUInt32BigEndian(bytes.Slice(8, 4));

        if (year == 0 && month == 0 && day == 0)
            throw new InvalidDataException("S7 DTL is uninitialized (all-zero year/month/day)");
        if (year is < 1970 or > 2554)
            throw new InvalidDataException($"S7 DTL year {year} out of range 1970..2554");
        if (month is < 1 or > 12)
            throw new InvalidDataException($"S7 DTL month {month} out of range 1..12");
        if (day is < 1 or > 31)
            throw new InvalidDataException($"S7 DTL day {day} out of range 1..31");
        if (hour > 23) throw new InvalidDataException($"S7 DTL hour {hour} out of range 0..23");
        if (minute > 59) throw new InvalidDataException($"S7 DTL minute {minute} out of range 0..59");
        if (second > 59) throw new InvalidDataException($"S7 DTL second {second} out of range 0..59");
        if (nanos > 999_999_999)
            throw new InvalidDataException($"S7 DTL nanoseconds {nanos} out of range 0..999999999");

        // .NET DateTime resolution is 100 ns ticks (1 tick = 100 ns).
        var dt = new DateTime(year, month, day, hour, minute, second, DateTimeKind.Unspecified);
        return dt.AddTicks(nanos / 100);
    }

    /// <summary>Encode a DateTime as a 12-byte DTL buffer.</summary>
    public static byte[] EncodeDtl(DateTime value)
    {
        if (value.Year is < 1970 or > 2554)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 DTL year must be 1970..2554");

        var buf = new byte[DtlSize];
        BinaryPrimitives.WriteUInt16BigEndian(buf.AsSpan(0, 2), (ushort)value.Year);
        buf[2] = (byte)value.Month;
        buf[3] = (byte)value.Day;
        // S7 day-of-week: 1=Sunday..7=Saturday. .NET DayOfWeek: Sunday=0..Saturday=6.
        buf[4] = (byte)((int)value.DayOfWeek + 1);
        buf[5] = (byte)value.Hour;
        buf[6] = (byte)value.Minute;
        buf[7] = (byte)value.Second;

        // Sub-second portion → nanoseconds. 1 tick = 100 ns, so ticks % 10_000_000 gives
        // the fractional second in ticks; multiply by 100 for nanoseconds.
        long fracTicks = value.Ticks % TimeSpan.TicksPerSecond;
        uint nanos = (uint)(fracTicks * 100);
        BinaryPrimitives.WriteUInt32BigEndian(buf.AsSpan(8, 4), nanos);
        return buf;
    }

    // ---- DATE_AND_TIME / DT (8 bytes BCD) ----

    /// <summary>Wire size of an S7 DATE_AND_TIME value.</summary>
    public const int DtSize = 8;

    /// <summary>
    ///     Decode an 8-byte DATE_AND_TIME (BCD) buffer into a DateTime. Year encoding:
    ///     90..99 → 1990..1999, 00..89 → 2000..2089 (per Siemens spec).
    /// </summary>
    public static DateTime DecodeDt(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length != DtSize)
            throw new InvalidDataException($"S7 DATE_AND_TIME expected {DtSize} bytes, got {bytes.Length}");

        int yy = FromBcd(bytes[0]);
        int month = FromBcd(bytes[1]);
        int day = FromBcd(bytes[2]);
        int hour = FromBcd(bytes[3]);
        int minute = FromBcd(bytes[4]);
        int second = FromBcd(bytes[5]);

        // bytes[6] and high nibble of bytes[7] = milliseconds (3 BCD digits).
        // Low nibble of bytes[7] = day-of-week (1=Sunday..7=Saturday); ignored on read.
        int msHigh = (bytes[6] >> 4) & 0xF;
        int msMid = bytes[6] & 0xF;
        int msLow = (bytes[7] >> 4) & 0xF;
        if (msHigh > 9 || msMid > 9 || msLow > 9)
            throw new InvalidDataException($"S7 DT ms BCD digits invalid: {msHigh:X}{msMid:X}{msLow:X}");
        int ms = msHigh * 100 + msMid * 10 + msLow;

        if (yy == 0 && month == 0 && day == 0)
            throw new InvalidDataException("S7 DT is uninitialized (all-zero year/month/day)");

        int year = yy >= 90 ? 1900 + yy : 2000 + yy;
        if (month is < 1 or > 12) throw new InvalidDataException($"S7 DT month {month} out of range 1..12");
        if (day is < 1 or > 31) throw new InvalidDataException($"S7 DT day {day} out of range 1..31");
        if (hour > 23) throw new InvalidDataException($"S7 DT hour {hour} out of range 0..23");
        if (minute > 59) throw new InvalidDataException($"S7 DT minute {minute} out of range 0..59");
        if (second > 59) throw new InvalidDataException($"S7 DT second {second} out of range 0..59");

        return new DateTime(year, month, day, hour, minute, second, ms, DateTimeKind.Unspecified);
    }

    /// <summary>Encode a DateTime as an 8-byte DATE_AND_TIME (BCD) buffer.</summary>
    public static byte[] EncodeDt(DateTime value)
    {
        if (value.Year is < 1990 or > 2089)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 DATE_AND_TIME year must be 1990..2089");

        int yy = value.Year >= 2000 ? value.Year - 2000 : value.Year - 1900;
        int ms = value.Millisecond;
        // S7 day-of-week: 1=Sunday..7=Saturday.
        int dow = (int)value.DayOfWeek + 1;

        var buf = new byte[DtSize];
        buf[0] = ToBcd(yy);
        buf[1] = ToBcd(value.Month);
        buf[2] = ToBcd(value.Day);
        buf[3] = ToBcd(value.Hour);
        buf[4] = ToBcd(value.Minute);
        buf[5] = ToBcd(value.Second);
        // ms = 3 digits packed across bytes [6] (high+mid nibbles) and [7] high nibble.
        buf[6] = (byte)(((ms / 100) << 4) | ((ms / 10) % 10));
        buf[7] = (byte)((((ms % 10) & 0xF) << 4) | (dow & 0xF));
        return buf;
    }

    // ---- S5TIME (16 bits BCD) ----

    /// <summary>Wire size of an S7 S5TIME value.</summary>
    public const int S5TimeSize = 2;

    /// <summary>
    ///     Decode a 2-byte S5TIME buffer into a TimeSpan. Layout:
    ///     <c>0000 TTBB BBBB BBBB</c> where TT is the timebase (00=10ms, 01=100ms,
    ///     10=1s, 11=10s) and BBB is the 3-digit BCD count (0..999).
    /// </summary>
    public static TimeSpan DecodeS5Time(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length != S5TimeSize)
            throw new InvalidDataException($"S7 S5TIME expected {S5TimeSize} bytes, got {bytes.Length}");

        int hi = bytes[0];
        int lo = bytes[1];
        int tb = (hi >> 4) & 0x3;
        int d2 = hi & 0xF;
        int d1 = (lo >> 4) & 0xF;
        int d0 = lo & 0xF;
        if (d2 > 9 || d1 > 9 || d0 > 9)
            throw new InvalidDataException($"S7 S5TIME BCD digits invalid: {d2:X}{d1:X}{d0:X}");

        int count = d2 * 100 + d1 * 10 + d0;
        long unitMs = tb switch
        {
            0 => 10L,
            1 => 100L,
            2 => 1000L,
            3 => 10_000L,
            _ => throw new InvalidDataException($"S7 S5TIME timebase {tb} invalid"),
        };
        return TimeSpan.FromMilliseconds(count * unitMs);
    }

    /// <summary>
    ///     Encode a TimeSpan as a 2-byte S5TIME. Picks the smallest timebase that fits
    ///     <paramref name="value"/> in 999 units. Rejects negative or &gt; 9990s durations
    ///     and any value not a multiple of the chosen timebase.
    /// </summary>
    public static byte[] EncodeS5Time(TimeSpan value)
    {
        if (value < TimeSpan.Zero)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 S5TIME must be non-negative");
        long totalMs = (long)value.TotalMilliseconds;
        if (totalMs > 9_990_000)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 S5TIME max is 9990 seconds");

        int tb;
        long unit;
        if (totalMs <= 9_990 && totalMs % 10 == 0) { tb = 0; unit = 10; }
        else if (totalMs <= 99_900 && totalMs % 100 == 0) { tb = 1; unit = 100; }
        else if (totalMs <= 999_000 && totalMs % 1000 == 0) { tb = 2; unit = 1_000; }
        else if (totalMs % 10_000 == 0) { tb = 3; unit = 10_000; }
        else
            throw new ArgumentException(
                $"S7 S5TIME duration {value} cannot be represented in any timebase without truncation",
                nameof(value));

        long count = totalMs / unit;
        if (count > 999)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 S5TIME count exceeds 999 in chosen timebase");

        int d2 = (int)(count / 100);
        int d1 = (int)((count / 10) % 10);
        int d0 = (int)(count % 10);

        var buf = new byte[2];
        buf[0] = (byte)(((tb & 0x3) << 4) | (d2 & 0xF));
        buf[1] = (byte)(((d1 & 0xF) << 4) | (d0 & 0xF));
        return buf;
    }

    // ---- TIME (Int32 ms BE) ----

    /// <summary>Wire size of an S7 TIME value.</summary>
    public const int TimeSize = 4;

    /// <summary>Decode a 4-byte TIME buffer into a TimeSpan (signed milliseconds).</summary>
    public static TimeSpan DecodeTime(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length != TimeSize)
            throw new InvalidDataException($"S7 TIME expected {TimeSize} bytes, got {bytes.Length}");
        int ms = BinaryPrimitives.ReadInt32BigEndian(bytes);
        return TimeSpan.FromMilliseconds(ms);
    }

    /// <summary>Encode a TimeSpan as a 4-byte TIME (signed Int32 milliseconds, big-endian).</summary>
    public static byte[] EncodeTime(TimeSpan value)
    {
        long totalMs = (long)value.TotalMilliseconds;
        if (totalMs is < int.MinValue or > int.MaxValue)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 TIME exceeds Int32 ms range");
        var buf = new byte[TimeSize];
        BinaryPrimitives.WriteInt32BigEndian(buf, (int)totalMs);
        return buf;
    }

    // ---- TOD / TIME_OF_DAY (UInt32 ms BE, 0..86399999) ----

    /// <summary>Wire size of an S7 TIME_OF_DAY value.</summary>
    public const int TodSize = 4;

    /// <summary>Decode a 4-byte TOD buffer into a TimeSpan (ms since midnight).</summary>
    public static TimeSpan DecodeTod(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length != TodSize)
            throw new InvalidDataException($"S7 TOD expected {TodSize} bytes, got {bytes.Length}");
        uint ms = BinaryPrimitives.ReadUInt32BigEndian(bytes);
        if (ms > 86_399_999)
            throw new InvalidDataException($"S7 TOD value {ms} exceeds 86399999 ms (one day)");
        return TimeSpan.FromMilliseconds(ms);
    }

    /// <summary>Encode a TimeSpan as a 4-byte TOD (UInt32 ms since midnight, big-endian).</summary>
    public static byte[] EncodeTod(TimeSpan value)
    {
        if (value < TimeSpan.Zero)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 TOD must be non-negative");
        long totalMs = (long)value.TotalMilliseconds;
        if (totalMs > 86_399_999)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 TOD max is 86399999 ms (23:59:59.999)");
        var buf = new byte[TodSize];
        BinaryPrimitives.WriteUInt32BigEndian(buf, (uint)totalMs);
        return buf;
    }

    // ---- DATE (UInt16 BE, days since 1990-01-01) ----

    /// <summary>Wire size of an S7 DATE value.</summary>
    public const int DateSize = 2;

    /// <summary>S7 DATE epoch — 1990-01-01 (UTC-unspecified per Siemens spec).</summary>
    public static readonly DateTime DateEpoch = new(1990, 1, 1, 0, 0, 0, DateTimeKind.Unspecified);

    /// <summary>Decode a 2-byte DATE buffer into a DateTime.</summary>
    public static DateTime DecodeDate(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length != DateSize)
            throw new InvalidDataException($"S7 DATE expected {DateSize} bytes, got {bytes.Length}");
        ushort days = BinaryPrimitives.ReadUInt16BigEndian(bytes);
        return DateEpoch.AddDays(days);
    }

    /// <summary>Encode a DateTime as a 2-byte DATE (UInt16 days since 1990-01-01, big-endian).</summary>
    public static byte[] EncodeDate(DateTime value)
    {
        var days = (value.Date - DateEpoch).TotalDays;
        if (days is < 0 or > ushort.MaxValue)
            throw new ArgumentOutOfRangeException(nameof(value), value, "S7 DATE must be 1990-01-01..2168-06-06");
        var buf = new byte[DateSize];
        BinaryPrimitives.WriteUInt16BigEndian(buf, (ushort)days);
        return buf;
    }

    // ---- BCD helpers ----

    /// <summary>Decode a single BCD byte (each nibble must be a decimal digit 0-9).</summary>
    private static int FromBcd(byte b)
    {
        int hi = (b >> 4) & 0xF;
        int lo = b & 0xF;
        if (hi > 9 || lo > 9)
            throw new InvalidDataException($"S7 BCD byte 0x{b:X2} has non-decimal nibble");
        return hi * 10 + lo;
    }

    /// <summary>Encode a 0-99 value as a single BCD byte.</summary>
    private static byte ToBcd(int value)
    {
        if (value is < 0 or > 99)
            throw new ArgumentOutOfRangeException(nameof(value), value, "BCD byte source must be 0..99");
        return (byte)(((value / 10) << 4) | (value % 10));
    }
}