source: TFP-WebServer/WebServer/src/LogBuffer.cs@ 440

using System;
using System.Collections.Generic;
using UnityEngine;

namespace Webserver {
        public class LogBuffer {
                private const int maxEntries = 3000;
                
                private static LogBuffer instance;

                private readonly List<LogEntry> logEntries = new List<LogEntry> ();

                private int listOffset;

                public static void Init () {
                        instance ??= new LogBuffer ();
                }

                private LogBuffer () {
                        Log.LogCallbacksExtended += LogCallback;
                }

                public static LogBuffer Instance => instance ??= new LogBuffer ();

                public int OldestLine {
                        get {
                                lock (logEntries) {
                                        return listOffset;
                                }
                        }
                }

                public int LatestLine {
                        get {
                                lock (logEntries) {
                                        return listOffset + logEntries.Count - 1;
                                }
                        }
                }

                public int StoredLines {
                        get {
                                lock (logEntries) {
                                        return logEntries.Count;
                                }
                        }
                }

                public LogEntry this [int _index] {
                        get {
                                lock (logEntries) {
                                        if (_index >= listOffset && _index < listOffset + logEntries.Count) {
                                                return logEntries [_index];
                                        }
                                }

                                return null;
                        }
                }

                private void LogCallback (string _formattedMsg, string _plainMsg, string _trace, LogType _type, DateTime _timestamp, long _uptime) {
                        LogEntry le = new LogEntry (_timestamp, _plainMsg, _trace, _type, _uptime);

                        lock (logEntries) {
                                logEntries.Add (le);
                                if (logEntries.Count <= maxEntries) {
                                        return;
                                }

                                listOffset += logEntries.Count - maxEntries;
                                logEntries.RemoveRange (0, logEntries.Count - maxEntries);
                        }
                }

                private readonly List<LogEntry> emptyList = new List<LogEntry> ();

                public List<LogEntry> GetRange (ref int _start, int _count, out int _end) {
                        lock (logEntries) {
                                int index;
                                
                                if (_count < 0) {
                                        _count = -_count;
                                        
                                        if (_start >= listOffset + logEntries.Count) {
                                                _start = listOffset + logEntries.Count - 1;
                                        }

                                        _end = _start;

                                        if (_start < listOffset) {
                                                return emptyList;
                                        }
                                        
                                        _start -= _count - 1;

                                        if (_start < listOffset) {
                                                _start = listOffset;
                                        }

                                        index = _start - listOffset;
                                        _end += 1;
                                        _count = _end - _start;
                                } else {
                                        if (_start < listOffset) {
                                                _start = listOffset;
                                        }

                                        if (_start >= listOffset + logEntries.Count) {
                                                _end = _start;
                                                return emptyList;
                                        }

                                        index = _start - listOffset;

                                        if (index + _count > logEntries.Count) {
                                                _count = logEntries.Count - index;
                                        }

                                        _end = _start + _count;
                                }

                                return logEntries.GetRange (index, _count);
                        }
                }


                public class LogEntry {
                        public readonly DateTime Timestamp;
                        public readonly string IsoTime;
                        public readonly string Message;
                        public readonly string Trace;
                        public readonly LogType Type;
                        public readonly long Uptime;

                        public LogEntry (DateTime _timestamp, string _message, string _trace, LogType _type, long _uptime) {
                                Timestamp = _timestamp;
                                IsoTime = _timestamp.ToString ("o");

                                Message = _message;
                                Trace = _trace;
                                Type = _type;
                                Uptime = _uptime;
                        }
                }
        }
}