using System.Text;

namespace LehrerApp.Templating;

public sealed record TemplateRichTextRun(string Text, string? Placeholder, string? Format,
    bool Bold, bool Italic, bool Underline)
{
    public bool IsPlaceholder => Placeholder is not null;
}

public static class TemplateRichText
{
    public static IReadOnlyList<TemplateRichTextRun> Parse(string source)
    {
        var runs = new List<TemplateRichTextRun>();
        var literal = new StringBuilder();
        var styles = new Stack<char>();

        void Flush()
        {
            if (literal.Length == 0) return;
            AddRun(runs, new(literal.ToString(), null, null,
                styles.Contains('b'), styles.Contains('i'), styles.Contains('u')));
            literal.Clear();
        }

        for (var index = 0; index < source.Length;)
        {
            if (source[index] == '\\' && index + 1 < source.Length
                && source[index + 1] is '\\' or '[' or '$')
            { literal.Append(source[index + 1]); index += 2; continue; }

            if (TryTag(source, index, out var tag, out var closing, out var tagLength))
            {
                Flush();
                if (closing)
                {
                    if (styles.Count == 0 || styles.Peek() != tag)
                        throw new InvalidDataException($"Rich-Text-Tag [/{tag}] ist nicht passend geöffnet.");
                    styles.Pop();
                }
                else styles.Push(tag);
                index += tagLength; continue;
            }

            if (source[index] == '$' && TryPlaceholder(source, index, out var name, out var format, out var length))
            {
                Flush();
                AddRun(runs, new("", name, format,
                    styles.Contains('b'), styles.Contains('i'), styles.Contains('u')));
                index += length; continue;
            }

            literal.Append(source[index++]);
        }

        Flush();
        if (styles.Count > 0) throw new InvalidDataException($"Rich-Text-Tag [{styles.Peek()}] wurde nicht geschlossen.");
        return runs;
    }

    public static IReadOnlyList<string> UsedPlaceholders(string source) => Parse(source)
        .Where(x => x.Placeholder is not null).Select(x => x.Placeholder!).Distinct(StringComparer.Ordinal).ToList();

    private static bool TryTag(string source, int index, out char tag, out bool closing, out int length)
    {
        tag = default; closing = false; length = 0;
        if (index + 2 < source.Length && source[index] == '[' && source[index + 2] == ']'
            && source[index + 1] is 'b' or 'i' or 'u')
        { tag = source[index + 1]; length = 3; return true; }
        if (index + 3 < source.Length && source[index] == '[' && source[index + 1] == '/'
            && source[index + 3] == ']' && source[index + 2] is 'b' or 'i' or 'u')
        { tag = source[index + 2]; closing = true; length = 4; return true; }
        return false;
    }

    private static bool TryPlaceholder(string source, int index, out string name, out string? format, out int length)
    {
        name = ""; format = null; length = 0;
        if (index + 1 >= source.Length) return false;
        if (source[index + 1] == '{')
        {
            var end = source.IndexOf('}', index + 2);
            if (end < 0) throw new InvalidDataException("Platzhalter mit '${' wurde nicht mit '}' geschlossen.");
            var content = source[(index + 2)..end];
            var parts = content.Split('|', 2);
            name = parts[0].Trim(); format = parts.Length == 2 ? parts[1] : null;
            if (name.Length == 0) throw new InvalidDataException("Ein eingebetteter Platzhaltername darf nicht leer sein.");
            length = end - index + 1; return true;
        }
        if (!IsNameStart(source[index + 1])) return false;
        var cursor = index + 2;
        while (cursor < source.Length && IsNamePart(source[cursor])) cursor++;
        name = source[(index + 1)..cursor]; length = cursor - index; return true;
    }

    private static bool IsNameStart(char value) => char.IsLetter(value) || value == '_';
    private static bool IsNamePart(char value) => char.IsLetterOrDigit(value) || value is '_' or '.' or '-';

    private static void AddRun(List<TemplateRichTextRun> runs, TemplateRichTextRun run)
    {
        if (!run.IsPlaceholder && runs.LastOrDefault() is { IsPlaceholder: false } previous
            && previous.Bold == run.Bold && previous.Italic == run.Italic && previous.Underline == run.Underline)
            runs[^1] = previous with { Text = previous.Text + run.Text };
        else runs.Add(run);
    }
}