XFESystem

namespace XFESystem.Desktop;

/// <summary>Immutable UTF-16 line offsets; materialize only lines actually used by the view.</summary>
public sealed class TextLineIndex
{
    private readonly int[] starts;
    private readonly string?[] lines;
    private string?[]? displayed;
    public string Source { get; }
    public int Count => starts.Length;

    public TextLineIndex(string source)
    {
        ArgumentNullException.ThrowIfNull(source);
        Source = source;
        int count = 1;
        foreach (char value in source) if (value == '\n') count++;
        starts = new int[count]; lines = new string?[count];
        for (int at = 0, line = 1; at < source.Length; at++) if (source[at] == '\n') starts[line++] = at + 1;
    }

    public int Start(int line) => starts[line];
    public string RawLine(int line)
    {
        int end = line + 1 < Count ? starts[line + 1] - 1 : Source.Length;
        return lines[line] ??= Source.Substring(starts[line], end - starts[line]);
    }
    public string DisplayLine(int line)
    {
        string raw = RawLine(line);
        if (!raw.Contains('\r')) return raw;
        displayed ??= new string?[Count];
        return displayed[line] ??= raw.Replace("\r", "", StringComparison.Ordinal);
    }
}