using System.Diagnostics.CodeAnalysis;
using System.Text;

namespace Houston.Model.Documents;

public readonly record struct OrgSlug
{
    public const int MaxLength = 128;

    private const string CharactersToReplace = @"/\{}^%`][""><~#|";

    private const char ReplacementCharacter = '-';

    /// <summary>
    /// Characters that must not start or end a folder name, because they are either invisible or
    /// break path semantics on the client side.
    /// </summary>
    private static readonly char[] UntrimmableEdgeCharacters = [' ', '.', ReplacementCharacter];

    public string Value { get; }

    private OrgSlug(string value) => Value = value;

    public static bool TryFromS3Key(string key, [NotNullWhen(true)] out OrgSlug? result)
    {
        result = null;

        if (!key.EndsWith('/')) return false;

        var parts = key.Split('/', StringSplitOptions.RemoveEmptyEntries);

        return parts is [var name] && TryFromFolderName(name, out result);
    }

    public static bool TryFromFolderName(string name, [NotNullWhen(true)] out OrgSlug? result)
    {
        result = null;

        if (String.IsNullOrWhiteSpace(name)) return false;
        if (name.Contains('/')) return false;
        if (name == "..") return false;

        result = new OrgSlug(name);
        return true;
    }

    /// <summary>
    /// Builds the canonical folder name for an organization.
    /// </summary>
    /// <returns>
    /// <c>false</c> if the name contains nothing that survives the normalization, in which case
    /// there is no folder name Houston could safely claim.
    /// </returns>
    public static bool TryFrom(OrgName name, [NotNullWhen(true)] out OrgSlug? result)
    {
        result = null;

        var slug = Normalize(name.Value, MaxLength);
        if (slug.Length == 0)
            return false;

        result = new OrgSlug(slug);
        return true;
    }

    public static bool TryFrom(OrgName name, OrgId id, [NotNullWhen(true)] out OrgSlug? result)
    {
        result = null;

        var normalizedId = Normalize(id.Value, MaxLength);
        if (normalizedId.Length == 0)
            return false;

        // The id is what makes the folder name unique, so the name is what gets truncated.
        var suffix = $" ({normalizedId})";
        if (suffix.Length >= MaxLength)
            return false;

        var slug = Normalize(name.Value, MaxLength - suffix.Length);

        result = new OrgSlug(slug.Length == 0 ? normalizedId : slug + suffix);
        return true;
    }

    private static string Normalize(string value, int maxLength)
    {
        var builder = new StringBuilder(Math.Min(value.Length, maxLength));

        foreach (var character in value.Trim().TakeWhile(c => builder.Length != maxLength))
        {
            builder.Append(Char.IsControl(character) || CharactersToReplace.Contains(character)
                ? ReplacementCharacter
                : character);
        }

        // Truncating can uncover a trailing space or dot, so trimming happens afterwards.
        return builder.ToString().Trim().Trim(UntrimmableEdgeCharacters).Trim();
    }

    /// <summary>
    /// S3 prefix of the folder, including the trailing slash.
    /// </summary>
    public override string ToString()
    {
        // A default(OrgSlug) would resolve to the bucket root and expose every customer's
        // documents, so it must never be used as a prefix.
        if (String.IsNullOrEmpty(Value))
            throw new InvalidOperationException("An uninitialized OrgSlug cannot be used as an S3 prefix.");

        return Value + '/';
    }
}