chapter-organizer/Core/Parsers/EventOccurrenceGrammar.cs

using Sprache;
using Core.Entities;
using Core.Models;

namespace Core.Parsers;

/// <summary>
/// Grammar definitions for parsing event occurrence DSL using parser combinators.
/// Provides composable parsers for each grammar rule.
/// </summary>
public static class EventOccurrenceGrammar
{
    // Months - all 12 months supported
    private static readonly Parser<string> January = Parse.String("January").Text().Token();
    private static readonly Parser<string> February = Parse.String("February").Text().Token();
    private static readonly Parser<string> March = Parse.String("March").Text().Token();
    private static readonly Parser<string> April = Parse.String("April").Text().Token();
    private static readonly Parser<string> May = Parse.String("May").Text().Token();
    private static readonly Parser<string> June = Parse.String("June").Text().Token();
    private static readonly Parser<string> July = Parse.String("July").Text().Token();
    private static readonly Parser<string> August = Parse.String("August").Text().Token();
    private static readonly Parser<string> September = Parse.String("September").Text().Token();
    private static readonly Parser<string> October = Parse.String("October").Text().Token();
    private static readonly Parser<string> November = Parse.String("November").Text().Token();
    private static readonly Parser<string> December = Parse.String("December").Text().Token();

    /// <summary>
    /// Parser for month names (January through December).
    /// </summary>
    public static readonly Parser<string> Month = January
        .Or(February)
        .Or(March)
        .Or(April)
        .Or(May)
        .Or(June)
        .Or(July)
        .Or(August)
        .Or(September)
        .Or(October)
        .Or(November)
        .Or(December);

    /// <summary>
    /// Parser for day of month (1-31, optional semicolon).
    /// </summary>
    public static readonly Parser<int> DayOfMonth = 
        from day in Parse.Number
        from semicolon in Parse.Char(';').Optional()
        select int.Parse(day);

    // Time parsing components
    private static readonly Parser<string> Noon = Parse.String("NOON").Text().Token();
    private static readonly Parser<string> Tbd = Parse.String("TBD").Text().Token();
    
    private static readonly Parser<string> AmPm = 
        Parse.String("a.m.").Or(Parse.String("am")).Or(Parse.String("A.M.")).Or(Parse.String("AM"))
        .Or(Parse.String("p.m.")).Or(Parse.String("pm")).Or(Parse.String("P.M.")).Or(Parse.String("PM"))
        .Text().Token();

    private static readonly Parser<string> TimeValue = 
        from hour in Parse.Number
        from colon in Parse.Char(':').Optional()
        from minute in Parse.Number.Optional()
        from ws in Parse.WhiteSpace.Many()
        from ampm in AmPm
        select $"{hour}:{(minute.IsDefined ? minute.Get() : "00")} {ampm}";

    /// <summary>
    /// Parser for hyphen characters (en-dash, hyphen, em-dash).
    /// </summary>
    public static readonly Parser<char> Hyphen = Parse.Char('–').Or(Parse.Char('-')).Or(Parse.Char('—'));

    /// <summary>
    /// Parser for time values, including ranges and special values (NOON, TBD).
    /// </summary>
    public static readonly Parser<string> Time = 
        Noon.Or(Tbd)
        .Or(
            from start in TimeValue.Or(Noon)
            from dash in Hyphen.Then(_ => Parse.WhiteSpace.Many()).Optional()
            from end in TimeValue.Or(Noon).Optional()
            select end.IsDefined 
                ? $"{start} – {end.Get()}" 
                : start
        );

    /// <summary>
    /// Parser for section headers: EventName [–-—] (MS|HS).
    /// </summary>
    public static readonly Parser<(string EventName, string SchoolLevel)> SectionHeader =
        from eventName in Parse.AnyChar.Except(Hyphen).Many().Text().Token()
        from hyphen in Hyphen.Token()
        from schoolLevel in Parse.String("MS").Or(Parse.String("HS")).Text().Token()
        select (eventName.Trim(), schoolLevel);

    /// <summary>
    /// Parser for General Schedule/Session headers.
    /// </summary>
    public static readonly Parser<string> GeneralSchedule = 
        Parse.String("General Schedule").Or(Parse.String("General Session")).Text().Token();

    /// <summary>
    /// Parser for comment lines (starting with #).
    /// </summary>
    public static readonly Parser<string> CommentLine = 
        from hash in Parse.Char('#')
        from rest in Parse.AnyChar.Many().Text()
        select rest;

    /// <summary>
    /// Attempts to parse a section header from the given line.
    /// Returns null if not a section header.
    /// </summary>
    public static (string EventName, string SchoolLevel)? TryParseSectionHeader(string line)
    {
        try
        {
            var result = SectionHeader.Parse(line);
            return result;
        }
        catch
        {
            return null;
        }
    }

    /// <summary>
    /// Attempts to parse a General Schedule/Session header from the given line.
    /// Returns null if not a General Schedule header.
    /// </summary>
    public static bool IsGeneralSchedule(string line)
    {
        try
        {
            GeneralSchedule.Parse(line);
            return true;
        }
        catch
        {
            return false;
        }
    }

    /// <summary>
    /// Attempts to parse a comment line from the given line.
    /// Returns true if the line is a comment.
    /// </summary>
    public static bool IsCommentLine(string line)
    {
        return line.TrimStart().StartsWith("#", StringComparison.Ordinal);
    }
}