using System.Security.Cryptography;
using backend.Data;
using backend.Devices;
using Microsoft.EntityFrameworkCore;

namespace backend.Pairing;

public record PairResult(Guid DeviceId, string DeviceToken);

public class PairingService
{
    private const string Alphabet = "ABCDEFGHJKMNPQRSTUVWXYZ23456789"; // no O, 0, I, 1, L
    private const int CodeLength = 8;
    public static readonly TimeSpan CodeTtl = TimeSpan.FromMinutes(10);

    private readonly AppDbContext? _db;
    private readonly DeviceTokenService? _tokens;

    public PairingService() { }

    public PairingService(AppDbContext db, DeviceTokenService tokens)
    {
        _db = db;
        _tokens = tokens;
    }

    public string GenerateCode()
    {
        Span<byte> bytes = stackalloc byte[CodeLength];
        RandomNumberGenerator.Fill(bytes);
        Span<char> chars = stackalloc char[CodeLength];
        for (var i = 0; i < CodeLength; i++)
            chars[i] = Alphabet[bytes[i] % Alphabet.Length];
        return new string(chars);
    }

    public async Task<PairingCode> IssueAsync(Guid ownerUserId, string deviceName, CancellationToken ct)
    {
        if (_db is null) throw new InvalidOperationException("PairingService not constructed with DB context.");
        var entity = new PairingCode
        {
            Code = GenerateCode(),
            OwnerUserId = ownerUserId,
            DeviceName = deviceName,
            ExpiresAt = DateTimeOffset.UtcNow.Add(CodeTtl),
        };
        _db.PairingCodes.Add(entity);
        await _db.SaveChangesAsync(ct);
        return entity;
    }

    public async Task<PairResult?> ConsumeAsync(string code, CancellationToken ct)
    {
        if (_db is null || _tokens is null)
            throw new InvalidOperationException("PairingService not constructed with DB context + token service.");
        var now = DateTimeOffset.UtcNow;
        var pc = await _db.PairingCodes.FirstOrDefaultAsync(p => p.Code == code, ct);
        if (pc is null || pc.ConsumedAt is not null || pc.ExpiresAt < now)
            return null;

        var (plaintext, hash) = _tokens.Generate();
        var defaults = await _db.SystemSettings.FirstAsync(ct);

        var device = new Device
        {
            OwnerUserId = pc.OwnerUserId,
            Name = pc.DeviceName,
            PairedAt = now,
            TokenHash = hash,
        };
        device.Config = new DeviceConfig
        {
            DeviceId = device.Id,
            SystemPrompt = defaults.DefaultSystemPrompt,
            Voice = defaults.DefaultVoice,
            Model = defaults.DefaultModel,
            IdleTimeoutSeconds = defaults.DefaultIdleTimeoutSeconds,
        };
        _db.Devices.Add(device);
        pc.ConsumedAt = now;
        await _db.SaveChangesAsync(ct);
        return new PairResult(device.Id, plaintext);
    }
}