using ScadaLink.Commons.Types.Enums;

namespace ScadaLink.DeploymentManager;

/// <summary>
/// WP-4: State transition matrix for instance lifecycle.
///
/// State     | Deploy | Disable | Enable | Delete
/// ----------|--------|---------|--------|-------
/// NotDeploy | OK     | NO      | NO     | OK
/// Enabled   | OK     | OK      | NO     | OK
/// Disabled  | OK*    | NO      | OK     | OK
///
/// * Deploy on a Disabled instance also enables it.
/// Delete removes the instance record entirely; it is valid from any state.
/// </summary>
public static class StateTransitionValidator
{
    public static bool CanDeploy(InstanceState currentState) =>
        currentState is InstanceState.NotDeployed or InstanceState.Enabled or InstanceState.Disabled;

    public static bool CanDisable(InstanceState currentState) =>
        currentState == InstanceState.Enabled;

    public static bool CanEnable(InstanceState currentState) =>
        currentState == InstanceState.Disabled;

    public static bool CanDelete(InstanceState currentState) =>
        currentState is InstanceState.NotDeployed or InstanceState.Enabled or InstanceState.Disabled;

    /// <summary>
    /// Returns a human-readable error message if the transition is invalid, or null if valid.
    /// </summary>
    public static string? ValidateTransition(InstanceState currentState, string operation)
    {
        var allowed = operation.ToLowerInvariant() switch
        {
            "deploy" => CanDeploy(currentState),
            "disable" => CanDisable(currentState),
            "enable" => CanEnable(currentState),
            "delete" => CanDelete(currentState),
            _ => false
        };

        if (allowed) return null;

        return $"Operation '{operation}' is not allowed when instance is in state '{currentState}'.";
    }
}
