using System.Buffers.Binary;
using System.Buffers.Text;

namespace WebDrop.Core;

/// <summary>16 字节标识(频道 id、条目 id),可作字典键。</summary>
public readonly struct Id16 : IEquatable<Id16>
{
    public const int Size = 16;
    private const int Base64Length = 22;

    private readonly ulong _hi;
    private readonly ulong _lo;

    private Id16(ulong hi, ulong lo)
    {
        _hi = hi;
        _lo = lo;
    }

    public static Id16 FromBytes(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length != Size) throw new ArgumentException("Id16 需要 16 字节", nameof(bytes));
        return new Id16(BinaryPrimitives.ReadUInt64BigEndian(bytes), BinaryPrimitives.ReadUInt64BigEndian(bytes[8..]));
    }

    /// <summary>只接受规范的 22 字符 base64url,拒绝同一值的非规范编码。</summary>
    /// <summary>32 个十六进制字符(文件服务和 blob 键里的写法)</summary>
    public static bool TryParseHex(string? hex, out Id16 id)
    {
        id = default;
        if (hex is not { Length: 32 }) return false;
        try { id = FromBytes(Convert.FromHexString(hex)); return true; }
        catch (FormatException) { return false; }
    }

    public static bool TryParse(ReadOnlySpan<char> text, out Id16 id)
    {
        id = default;
        if (text.Length != Base64Length) return false;
        Span<byte> buf = stackalloc byte[Size];
        if (!Codec.TryDecodeExact(text, buf)) return false;
        id = FromBytes(buf);
        return true;
    }

    public void WriteTo(Span<byte> destination)
    {
        BinaryPrimitives.WriteUInt64BigEndian(destination, _hi);
        BinaryPrimitives.WriteUInt64BigEndian(destination[8..], _lo);
    }

    public byte[] ToArray()
    {
        var bytes = new byte[Size];
        WriteTo(bytes);
        return bytes;
    }

    public string ToBase64Url() => Codec.Encode(ToArray());

    /// <summary>小写十六进制。用于文件路径,避免大小写不敏感的文件系统造成不同 id 冲突。</summary>
    public string ToHex() => Convert.ToHexStringLower(ToArray());

    public bool Equals(Id16 other) => _hi == other._hi && _lo == other._lo;
    public override bool Equals(object? obj) => obj is Id16 other && Equals(other);
    public override int GetHashCode() => HashCode.Combine(_hi, _lo);
    public static bool operator ==(Id16 a, Id16 b) => a.Equals(b);
    public static bool operator !=(Id16 a, Id16 b) => !a.Equals(b);
    public override string ToString() => ToBase64Url();
}

/// <summary>base64url(无填充)编解码,解码要求规范编码。</summary>
public static class Codec
{
    public static string Encode(ReadOnlySpan<byte> bytes) => Base64Url.EncodeToString(bytes);

    /// <summary>解码为恰好 destination.Length 字节,且重新编码后必须与输入逐字符一致。</summary>
    public static bool TryDecodeExact(ReadOnlySpan<char> text, Span<byte> destination)
    {
        // 注意:Base64Url.TryDecodeFromChars 遇到非法输入会抛 FormatException,这里用返回状态码的版本
        var status = Base64Url.DecodeFromChars(text, destination, out int consumed, out int written, isFinalBlock: true);
        if (status != System.Buffers.OperationStatus.Done || consumed != text.Length || written != destination.Length)
            return false;
        Span<char> check = stackalloc char[Base64Url.GetEncodedLength(destination.Length)];
        return Base64Url.TryEncodeToChars(destination, check, out int n)
               && n == text.Length
               && check[..n].SequenceEqual(text);
    }

    public static bool TryDecodeExact(string? text, int length, out byte[] bytes)
    {
        bytes = [];
        if (string.IsNullOrEmpty(text) || text.Length != Base64Url.GetEncodedLength(length)) return false;
        var buf = new byte[length];
        if (!TryDecodeExact(text.AsSpan(), buf)) return false;
        bytes = buf;
        return true;
    }
}