using System.Globalization;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;
using WxAgent.Core;

namespace WxAgent.Windows;

/// <summary>
/// Collects authorized messages directly from verified read-only database keys.
/// It never opens a WeChat window, changes the active account, or writes to the
/// source database; unavailable shards are returned as an explicit partial result.
/// </summary>
public sealed class DatabaseMessageSyncCollector : IRemoteDataCollector
{
    private readonly string? keyFile;
    private readonly int perChatLimit;
    private readonly int overlapRows;

    public DatabaseMessageSyncCollector(string? keyFile = null, int perChatLimit = 100, int overlapRows = 1)
    {
        if (perChatLimit is < 1 or > 500) throw new ArgumentOutOfRangeException(nameof(perChatLimit));
        if (overlapRows is < 0 or > 100) throw new ArgumentOutOfRangeException(nameof(overlapRows));
        this.keyFile = keyFile;
        this.perChatLimit = perChatLimit;
        this.overlapRows = overlapRows;
    }

    public async Task<RemoteDataCollection> CollectAsync(
        string accountId,
        IReadOnlyList<RemoteReportingScope> scopes,
        RemoteDataSyncCheckpoint checkpoint,
        CancellationToken cancellationToken)
    {
        if (string.IsNullOrWhiteSpace(accountId))
            throw new WxAgentException(WxAgentErrorCode.InvalidArgument, "A database sync account id is required.");
        var accounts = await DatabaseKeyStore.LoadAsync(keyFile, cancellationToken).ConfigureAwait(false);
        var account = accounts.SingleOrDefault(item => string.Equals(item.AccountRootFingerprint, accountId, StringComparison.OrdinalIgnoreCase));
        if (account is null)
            throw new WxAgentException(WxAgentErrorCode.DatabaseKeyNotFound, "The verified database account does not match the requested sync account.");
        if (account.Databases.Any(database => database.Confidence != KeyBindingConfidence.PageHmacVerified))
            throw new WxAgentException(WxAgentErrorCode.DatabaseOpenFailed, "The account contains a database key without page-1 HMAC verification.");

        // The verified account fingerprint is the stable source generation for v1;
        // a future key-cache version can deliberately rotate it for a rebind.
        var sourceGeneration = account.AccountRootFingerprint;
        var cursors = string.Equals(checkpoint.SourceGeneration, sourceGeneration, StringComparison.Ordinal)
            ? checkpoint.ConfirmedCursors
            : new Dictionary<string, long>(StringComparer.Ordinal);
        var chatScopes = scopes.DistinctBy(scope => $"{scope.ChatType}:{scope.ChatId}").ToArray();
        var chatIds = chatScopes.Select(scope => scope.ChatId).Distinct(StringComparer.Ordinal).ToArray();
        var page = await WechatMessageDbReader.ReadIncrementalAsync(
            account.AccountRootPath, account.Databases, chatIds, cursors, perChatLimit, cancellationToken, overlapRows).ConfigureAwait(false);
        var scopeByChat = chatScopes
            .GroupBy(scope => scope.ChatId, StringComparer.Ordinal)
            .ToDictionary(group => group.Key, group => group.First(), StringComparer.Ordinal);
        var observedAt = DateTimeOffset.UtcNow;
        var conversations = chatScopes.Select(scope => new RemoteSyncConversation(
            scope.ChatId,
            scope.ChatType,
            scope.ChatId,
            null,
            "authorized-db-scope",
            observedAt,
            "observed")).ToArray();
        var messages = page.Messages.Select(message =>
        {
            var scope = scopeByChat.GetValueOrDefault(message.ChatId)
                ?? new RemoteReportingScope(message.ChatId, ReportingChatType.Private);
            var sourceMessageId = message.LocalId.ToString(CultureInfo.InvariantCulture);
            var messageId = message.ServerId > 0
                ? $"{message.ChatId}:server:{message.ServerId.ToString(CultureInfo.InvariantCulture)}"
                : $"{message.ChatId}:local:{sourceMessageId}";
            var text = message.Content ?? string.Empty;
            return new RemoteSyncMessage(
                messageId,
                message.ChatId,
                scope.ChatType,
                sourceMessageId,
                message.IsSelf == true ? "outgoing" : "incoming",
                message.Type.ToString(CultureInfo.InvariantCulture),
                text,
                message.Timestamp,
                observedAt,
                account.WechatVersion ?? "unknown",
                HashMessage(messageId, text, message.Timestamp));
        }).ToArray();
        return new RemoteDataCollection(sourceGeneration, page.NextCursors, conversations, messages, page.IsComplete, page.NewMessageCount > 0, page.UnavailableDatabases);
    }

    private static string HashMessage(string messageId, string text, DateTimeOffset sourceTime)
    {
        var canonical = JsonSerializer.Serialize(new { messageId, text, sourceTime = sourceTime.ToUniversalTime().ToString("O") });
        return Convert.ToHexString(SHA256.HashData(Encoding.UTF8.GetBytes(canonical))).ToLowerInvariant();
    }
}