CsvHighlightingRule.cs

public class CsvHighlightingRule : IHighlightingRule
{
	private readonly int _headerRowsCount;
	private int _counter;
	private SimpleHighlightingBrush[] _colors;
	private int _currentLineNumber;
	private List<RuleMatch> _matchesForCurrentLine = new List<RuleMatch>();

	public CsvHighlightingRule(
		string delimiter,
		int headerRowsCount
		)
	{
		Delimiter = delimiter;
		_headerRowsCount = headerRowsCount;

		var themeBrushes = ThemeService.Current.ActualCurrentTheme == ThemeService.Theme.Dark
			? ThemeColors.DarkThemeFontBrushes
			: ThemeColors.LightThemeFontBrushes;
		_colors = new SimpleHighlightingBrush[themeBrushes.Length];
		for (int i = 0; i < themeBrushes.Length; i++)
		{
			_colors[i] = new SimpleHighlightingBrush(themeBrushes[i].Color);
		}
	}

	public HighlightingColor Color
	{
		get
		{
			var highlightInfo = new HighlightingColor();
			highlightInfo.Foreground = _colors[_counter++ % _colors.Length];

			if (_currentLineNumber <= _headerRowsCount)
			{
				highlightInfo.FontWeight = FontWeights.Bold;
				highlightInfo.Underline = _currentLineNumber == _headerRowsCount;
			}

			return highlightInfo;
		}
	}

	public string RuleInfo => $"Delimiter: \"{Delimiter}\"";
	public bool CombineConsecutiveDelimiters { get; set; }
	public string Delimiter { get; }

	public RuleMatch? GetMatch(string input, int beginning, int length, int lineNumber)
	{
		if (lineNumber != _currentLineNumber)
		{
			_counter = 0;
			_currentLineNumber = lineNumber;
			_matchesForCurrentLine.Clear();
			if (DsvConverter.TryGetDelimiterMatches(input, out var result, Delimiter)) // Custom code that returns list of locations of the delimiter instances in the line
			{
				var matches = result.Value.matches;
				for (int i = 0; i < matches.Count; i++)
				{
					var valueStartIndex = i == 0 ? 0 : matches[i - 1].Index + matches[i - 1].Length;
					var value = new RuleMatch()
					{
						Success = true,
						Index = valueStartIndex,
						Length = matches[i].Index - valueStartIndex + matches[i].Length,
					};
					_matchesForCurrentLine.Add(value);
				}
				// Add the final value, which occurs after the final delimiter (or, if there is only one value, that value)
				var finalValue = new RuleMatch()
				{
					Success = true,
					Index = matches[^1].Index + matches[^1].Length,
					Length = input.Length - matches[^1].Index - matches[^1].Length,
				};
				_matchesForCurrentLine.Add(finalValue);
			}
		}

		return _matchesForCurrentLine.FirstOrDefault(x => x.Index == beginning) ?? new RuleMatch();
	}
}