using System.Text.RegularExpressions;

namespace Core.Printing;

/// <summary>
/// Replaces <c>{{Token}}</c> placeholders from a case-insensitive map.
/// Unknown tokens are left unchanged. Known empty values become blank.
/// <c>{{PageBreak}}</c> becomes a print page break after HTML conversion.
/// <c>{{AnswerSpace}}</c> becomes ruled write-in space after HTML conversion.
/// </summary>
public static class NoteTemplateMerger
{
    public const string PageBreakToken = "PageBreak";
    public const string PageBreakSentinel = "<!--tsa-page-break-->";
    public const string PageBreakHtml = "<div class=\"pagebreak\"></div>";

    public const string AnswerSpaceToken = "AnswerSpace";
    public const string AnswerSpaceSentinel = "<!--tsa-answer-space-->";
    public const string AnswerSpaceHtml = "<div class=\"print-answer-space\"></div>";

    private readonly record struct LayoutToken(string Name, string Sentinel, string Html);

    private static readonly LayoutToken[] LayoutTokens =
    [
        new(PageBreakToken, PageBreakSentinel, PageBreakHtml),
        new(AnswerSpaceToken, AnswerSpaceSentinel, AnswerSpaceHtml)
    ];

    private static readonly Regex TokenRegex = new(@"\{\{([^}]+)\}\}", RegexOptions.Compiled);

    public static string Merge(string? template, IReadOnlyDictionary<string, string> tokens)
    {
        if (string.IsNullOrEmpty(template))
            return string.Empty;

        ArgumentNullException.ThrowIfNull(tokens);

        return TokenRegex.Replace(template, match =>
        {
            var key = match.Groups[1].Value.Trim();
            if (key.Length == 0)
                return match.Value;

            foreach (var layout in LayoutTokens)
            {
                if (key.Equals(layout.Name, StringComparison.OrdinalIgnoreCase))
                    return layout.Sentinel;
            }

            return tokens.TryGetValue(key, out var value)
                ? value ?? string.Empty
                : match.Value;
        });
    }

    /// <summary>
    /// Turns layout sentinels into HTML after markdown has been rendered.
    /// </summary>
    public static string ApplyLayout(string? html)
    {
        if (string.IsNullOrEmpty(html))
            return string.Empty;

        foreach (var layout in LayoutTokens)
        {
            html = html
                .Replace($"<p>{layout.Sentinel}</p>", layout.Html, StringComparison.Ordinal)
                .Replace(layout.Sentinel, layout.Html, StringComparison.Ordinal);
        }

        return html;
    }
}