using System;
using System.Collections.Generic;
using System.IO;
using Microsoft.Win32;
using Outlook = Microsoft.Office.Interop.Outlook;

namespace BulkArchiveMailer.AddIn
{
    /// <summary>
    /// Reads the user's default "new message" Outlook signature HTML from
    /// disk. Outlook Interop has no API that exposes "the currently
    /// selected default signature" - the only reliable way to get at it
    /// is: look up its *name* in the registry
    /// (HKCU\Software\Microsoft\Office\&lt;version&gt;\Common\MailSettings,
    /// value "NewSignature"), then load the matching .htm file from
    /// %APPDATA%\Microsoft\Signatures\&lt;name&gt;.htm ourselves.
    ///
    /// This is the only file in the solution that touches the Windows
    /// registry - deliberately kept out of BulkArchiveMailer.Core.
    /// </summary>
    public static class SignatureReader
    {
        /// <summary>
        /// Best-effort auto-detection. Never throws - returns null if the
        /// registry value, the version-specific key, or the .htm file is
        /// missing for any reason, so the caller can fall back to letting
        /// the operator pick a signature file manually instead of crashing
        /// the add-in.
        /// </summary>
        public static string TryAutoDetectSignatureHtml(Outlook.Application application)
        {
            try
            {
                var signatureName = TryReadSignatureName(application);
                if (string.IsNullOrEmpty(signatureName))
                {
                    return null;
                }

                var path = ResolveSignaturePath(signatureName);
                return File.Exists(path) ? File.ReadAllText(path) : null;
            }
            catch
            {
                return null;
            }
        }

        /// <summary>Same lookup, but returns the resolved .htm path instead of its contents - useful for SettingsForm to show/prefill the path.</summary>
        public static string TryAutoDetectSignaturePath(Outlook.Application application)
        {
            try
            {
                var signatureName = TryReadSignatureName(application);
                if (string.IsNullOrEmpty(signatureName))
                {
                    return null;
                }

                var path = ResolveSignaturePath(signatureName);
                return File.Exists(path) ? path : null;
            }
            catch
            {
                return null;
            }
        }

        public static string ResolveSignaturePath(string signatureName)
        {
            var signaturesFolder = Path.Combine(
                Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
                "Microsoft",
                "Signatures");
            return Path.Combine(signaturesFolder, signatureName + ".htm");
        }

        private static string TryReadSignatureName(Outlook.Application application)
        {
            var candidateVersions = new List<string>();

            try
            {
                // application.Version looks like "16.0" and lines up
                // exactly with the Office registry hive layout, so prefer
                // it over the fixed fallback list below.
                var version = application?.Version;
                if (!string.IsNullOrEmpty(version))
                {
                    candidateVersions.Add(version);
                }
            }
            catch
            {
                // Interop call failed for some reason - fall through to
                // the fixed list.
            }

            foreach (var fallback in new[] { "16.0", "15.0", "14.0" })
            {
                if (!candidateVersions.Contains(fallback))
                {
                    candidateVersions.Add(fallback);
                }
            }

            foreach (var version in candidateVersions)
            {
                using (var key = Registry.CurrentUser.OpenSubKey(
                    $@"Software\Microsoft\Office\{version}\Common\MailSettings"))
                {
                    var value = key?.GetValue("NewSignature") as string;
                    if (!string.IsNullOrEmpty(value))
                    {
                        return value;
                    }
                }
            }

            return null;
        }
    }
}