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
{
    /// <summary>
    /// Array of all month names in order (January through December).
    /// This is the single source of truth for month names used throughout the parser.
    /// </summary>
    public static readonly string[] MonthNames =
    [
        "January", "February", "March", "April", "May", "June",
        "July", "August", "September", "October", "November", "December"
    ];

    // Build month parsers dynamically from MonthNames array
    private static readonly Parser<string>[] MonthParsers = MonthNames
        .Select(month => Parse.String(month).Text().Token())
        .ToArray();

    /// <summary>
    /// Parser for month names (January through December).
    /// Built dynamically from MonthNames array.
    /// </summary>
    public static readonly Parser<string> Month = MonthParsers
        .Aggregate((current, next) => current.Or(next));

    /// <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 character.
    /// Note: Input is assumed to be normalized (en-dash and em-dash converted to regular hyphen) via SanitizeInput.
    /// </summary>
    public static readonly Parser<char> Hyphen = 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).
    /// Note: Input is assumed to be normalized (hyphens normalized) via SanitizeInput.
    /// </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 (Sprache.ParseException)
        {
            // Expected - line is not a section header, return null
            return null;
        }
        catch
        {
            // Unexpected exception, return null to continue parsing
            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 (Sprache.ParseException)
        {
            // Expected - line is not a General Schedule header
            return false;
        }
        catch
        {
            // Unexpected exception, return false to continue parsing
            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);
    }

    /// <summary>
    /// Attempts to parse an occurrence line from the given text.
    /// Returns null if parsing fails.
    /// Strategy: Find the first month name in the line, then parse from there.
    /// </summary>
    public static (string Name, string Month, int Day, string TimeAndLocation)? TryParseOccurrenceLine(string line)
    {
        // Find the first occurrence of any month name (using normalized MonthNames array)
        int monthIndex = -1;
        string foundMonth = string.Empty;
        
        foreach (var month in MonthNames)
        {
            var index = line.IndexOf(month, StringComparison.OrdinalIgnoreCase);
            if (index >= 0 && (monthIndex < 0 || index < monthIndex))
            {
                monthIndex = index;
                foundMonth = month;
            }
        }
        
        if (monthIndex < 0)
            return null;
        
        // Extract name (everything before the month)
        var name = line.Substring(0, monthIndex).Trim();
        
        // Parse from the month onwards
        var restOfLine = line.Substring(monthIndex);
        try
        {
            var monthParser = Parse.String(foundMonth).Text().Token();
            var result = from month in monthParser
                        from day in DayOfMonth.Token()
                        from timeAndLocation in Parse.AnyChar.Many().Text()
                        select (name, month, day, timeAndLocation.Trim());
            
            var parsed = result.Parse(restOfLine);
            return parsed;
        }
        catch (Sprache.ParseException)
        {
            // Expected - line is not a valid occurrence line, return null
            return null;
        }
        catch
        {
            // Unexpected exception, return null to continue parsing
            return null;
        }
    }
}