namespace ScadaLink.SiteRuntime.Repositories;

/// <summary>
/// SiteRuntime-007: deterministic synthetic-ID generation for site-local artifacts.
///
/// The site SQLite tables are keyed by name rather than an auto-increment integer, but
/// the shared repository contracts (<c>IExternalSystemRepository</c>,
/// <c>INotificationRepository</c>) expose integer-keyed lookups. A synthetic integer ID
/// is therefore derived from the entity name. It MUST be stable across process restarts
/// — <see cref="string.GetHashCode()"/> is randomized per process on .NET Core and so
/// cannot be used.
/// </summary>
internal static class SyntheticId
{
    // FNV-1a 32-bit constants.
    private const uint FnvOffsetBasis = 2166136261;
    private const uint FnvPrime = 16777619;

    /// <summary>
    /// Computes a deterministic, process-stable positive 31-bit integer ID for the
    /// given name using the FNV-1a hash over its UTF-8 bytes.
    /// </summary>
    public static int From(string name)
    {
        var hash = FnvOffsetBasis;
        foreach (var b in System.Text.Encoding.UTF8.GetBytes(name))
        {
            hash ^= b;
            hash *= FnvPrime;
        }

        // Mask to a positive 31-bit value so the ID is always non-negative.
        return (int)(hash & 0x7FFFFFFF);
    }
}