namespace LehrerApp.Templating;

internal static class SystemVariables
{
    internal const string CurrentPage = "curPage";
    internal const string MaximumPageNumber = "maxPageNum";
    internal const string Today = "today";

    private static readonly string[] Names = [CurrentPage, MaximumPageNumber, Today];

    internal static bool IsStandalone(string value) =>
        TryRead(value, 0, out var length) && length == value.Length;

    internal static bool Contains(string value) => Names.Any(name =>
        value.Contains("$$" + name, StringComparison.Ordinal));

    internal static bool TryRead(string source, int index, out int length)
    {
        length = 0;
        if (index < 0 || index + 2 > source.Length || source[index] != '$' || source[index + 1] != '$')
            return false;
        foreach (var name in Names)
        {
            var token = "$$" + name;
            if (!source.AsSpan(index).StartsWith(token, StringComparison.Ordinal)) continue;
            var end = index + token.Length;
            if (end < source.Length && (char.IsLetterOrDigit(source[end]) || source[end] is '_' or '-' or '.'))
                continue;
            length = token.Length;
            return true;
        }
        return false;
    }

    internal static IEnumerable<(string Text, string? Name)> Split(string source)
    {
        var literalStart = 0;
        for (var index = 0; index < source.Length;)
        {
            if (!TryRead(source, index, out var length)) { index++; continue; }
            if (index > literalStart) yield return (source[literalStart..index], null);
            yield return (source.Substring(index, length), source.Substring(index + 2, length - 2));
            index += length;
            literalStart = index;
        }
        if (literalStart < source.Length) yield return (source[literalStart..], null);
    }
}