159.341_Assignment_1/Assignment 1/Program.cs

/* Dempsey-Jensen, Brychan, 14299890, Assignment 1, 159.341 */
/* Parses and interprets a simple programming language line-by-line */

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading;

namespace Assignment_1
{
    class Program
    {
        /// <summary>
        /// Flags to set symbol properties
        /// </summary>
        [Flags]
        enum VariableFlags
        {
            Empty = 0,
            Reserved = 1,
            NoPrint = 2,
            Static = 4
        }

        // Max display width for tables etc.
        static readonly int ConsoleWidthLimit = 80;

        static void Main(string[] args)
        {
            Console.WriteLine(CenterString("┌──────────────────────────────────────────┐", ConsoleWidthLimit));
            Console.WriteLine(CenterString("│  159.341 2021 Semester 1, Assignment 1   │", ConsoleWidthLimit));
            Console.WriteLine(CenterString("│  Submitted by Brychan Dempsey, 14299890  │", ConsoleWidthLimit));
            Console.WriteLine(CenterString("└──────────────────────────────────────────┘", ConsoleWidthLimit));
            bool loadedFromFile = false;
            bool exit = false;
            while (!exit)
            {
                MemoryStream sourceStream = new MemoryStream(1024);
                Parser parser = new Parser();
                bool dynamicInput = false;
                // From https://stackoverflow.com/questions/3453220/how-to-detect-if-console-in-stdin-has-been-redirected
                // Reading from pipes is equivalent to reading user input, but the input is redirected
                if (Console.IsInputRedirected || loadedFromFile)
                {
                    // To simplify reading, we read all input bytes from the piped input to the stream.
                    // Not the best way to do it; we don't need to keep any data that has already been read and parsed successfully,
                    // and we could avoid copying the supplied stream. But keeps interactivity with the console simple
                    sourceStream.Write(Encoding.UTF8.GetBytes(Console.In.ReadToEnd()));
                    Console.In.Dispose();
                    Console.SetIn(new StreamReader(Console.OpenStandardInput()));
                    Console.OpenStandardInput();
                    sourceStream.Position = 0;
                }
                else
                {
                    sourceStream.Position = 0;
                    dynamicInput = true;
                }
                parser.StartParsing(sourceStream, dynamicInput);
                Console.WriteLine(Environment.NewLine + new string('─', 40));
                Console.WriteLine("\nProgram Parsed Successfully!");

                if (Console.IsInputRedirected)
                {
                    Thread.Sleep(3000);
                    Environment.Exit(0);
                }
                ConsoleKeyInfo ck = new ConsoleKeyInfo();
                while (ck.Key != ConsoleKey.Y && ck.Key != ConsoleKey.N)
                {
                    Console.WriteLine("\nWould you like to parse another program? Y/n:");
                    ck = Console.ReadKey(true);
                }
                if (ck.Key == ConsoleKey.N)
                {
                    exit = true;
                }
                else
                {
                    ck = new ConsoleKeyInfo();
                    while (ck.Key != ConsoleKey.Y && ck.Key != ConsoleKey.N)
                    {
                        Console.WriteLine("\nWould you like to pipe data from a source file? Y/n:");
                        ck = Console.ReadKey(true);
                    }
                    if (ck.Key == ConsoleKey.N)
                    {
                        // Set the input to standard input stream
                        Console.SetIn(Console.In);
                        loadedFromFile = false;
                    }
                    else
                    {
                        Console.WriteLine("Enter the source path:");
                        string sourcePath = Console.ReadLine();
                        if (File.Exists(sourcePath))
                        {
                            try
                            {
                                Console.SetIn(File.OpenText(sourcePath));
                                loadedFromFile = true;
                            }
                            catch (Exception e)
                            {
                                Console.WriteLine("Encountered an error opening the source file: " + e.Message);
                            }
                        }
                    }
                }
            }
        }

        public class Parser
        {
            Dictionary<string, Tuple<string, VariableFlags>> Symbols = new Dictionary<string, Tuple<string, VariableFlags>>()
            {
                { "SPACE", new Tuple<string, VariableFlags>(" ", VariableFlags.Reserved | VariableFlags.NoPrint) },
                { "TAB", new Tuple<string, VariableFlags>("\t", VariableFlags.Reserved | VariableFlags.NoPrint) },
                { "NEWLINE", new Tuple<string, VariableFlags>("\n", VariableFlags.Reserved | VariableFlags.NoPrint) },
                { "CARRIAGE_RETURN", new Tuple<string, VariableFlags>("\r", VariableFlags.Reserved | VariableFlags.NoPrint) }
            };

            public enum Statements
            {
                exit,
                append,
                list,
                print,
                printlength,
                printwords,
                printwordcount,
                set,
                reverse,
                h,
            }
            public void StartParsing(Stream source, bool dynamicInput = false)
            {
                long initSourceLength = source.Length;
                long lastLinePos = 0;
                long initPos = 0;
                bool cont = false;
                bool isLineFinished = true;
                while (true)
                {
                    if (dynamicInput && isLineFinished)
                    {
                        lastLinePos = source.Position;
                        if (!cont)
                        {
                            Console.WriteLine("Enter a command: ");
                        }
                        string s = Console.ReadLine();
                        long pos = source.Position;
                        source.Write(Encoding.UTF8.GetBytes(s));
                        source.Position = pos;

                    }
                    if (!cont)
                    {
                        initPos = source.Position;
                    }
                    else
                    {
                        source.Position = initPos;
                    }
                    SkipWhitespace(source);
                    long position = FindNextWord(source, out string word);
                    try
                    {
                        if (Enum.TryParse(typeof(Statements), word, out object statementType))
                        {
                            // By turning the result of the command into an action,
                            // we can defer processing the final result until the end of this control flow
                            Action result = () => { };
                            source.Position = position;
                            switch ((Statements)statementType)
                            {
                                case Statements.exit:
                                    result = Exit(source, initSourceLength, dynamicInput);
                                    break;
                                case Statements.append:
                                    result = AppendSet(source);
                                    break;
                                case Statements.list:
                                    long pos = FindNextWord(source, out string nextWord);
                                    if (nextWord == "all")
                                    {
                                        source.Position = pos;
                                        result = List(true);
                                    }
                                    else
                                    {
                                        result = List();
                                    }
                                    break;
                                case Statements.print:
                                    result = Print(source, 0);
                                    break;
                                case Statements.printlength:
                                    result = Print(source, 1);
                                    break;
                                case Statements.printwords:
                                    result = Print(source, 2);
                                    break;
                                case Statements.printwordcount:
                                    result = Print(source, 3);
                                    break;
                                case Statements.set:
                                    result = AppendSet(source, false);
                                    break;
                                case Statements.reverse:
                                    result = Reverse(source);
                                    break;
                                case Statements.h:
                                    Console.WriteLine("Commands are: ");
                                    foreach (var item in Enum.GetValues(typeof(Statements)))
                                    {
                                        Console.WriteLine("\t{0}", ((Statements)item).ToString());
                                    }
                                    // Stream ignores this command
                                    source.Position = initPos;
                                    source.SetLength(initPos);
                                    break;
                            }
                            if (IsNextEoS(source))
                            {
                                cont = false;
                                source.Position++;
                                SkipWhitespace(source);
                                if (source.Position == source.Length) isLineFinished = true;
                                else isLineFinished = false;
                                if (dynamicInput && isLineFinished)
                                {
                                    source.WriteByte((byte)'\n'); // put the next statement on a newline so the exported list of commands is clean
                                }
                                result();
                                if (((Statements)statementType).Equals(Statements.exit))
                                {
                                    return;
                                }
                            }
                            else if (source.Position != lastLinePos)
                            {
                                // If the semicolon is missing *once*, assume we need more data.
                                lastLinePos = source.Position;
                                cont = true;
                                Console.Write(">");
                            }
                            else
                            {
                                if ((Statements)statementType != Statements.h)
                                {
                                    throw new ParserException("expected a semi-colon", 0, source.Position);
                                }
                            }
                        }
                        else
                        {
                            throw new ParserException("Failed parsing statement", 0, source.Position);
                        }
                    }
                    // Throwing a ParserException will return us to this point immediately. From here, the line is automatically restored,
                    // and the excepion printed to the console window.
                    // This means that each function does not need to keep track of our current position in the stream
                    catch (ParserException e)
                    {
                        isLineFinished = true;
                        if (e.Importance > 3)
                        {
                            throw new ApplicationException("A critical error occurred.");
                        }
                        if (e.LinePosition > 0)
                        {
                            WriteDebugLine(initPos, e.LinePosition, e.Message, source);
                        }
                        else
                        {
                            WriteDebugLine(0, 0, e.Message, source);
                        }
                        if (!dynamicInput)
                        {
                            Environment.Exit(-1);
                        }
                    }
                }
            }

            /// <summary>
            /// Checks if the next expression in the source meets the requirements of being a key,
            /// and optionally verify that key exists. Acoids overidding the value of constants, and
            /// ensures all characters are valid
            /// </summary>
            private string ValidateKey(Stream source, bool checkExist)
            {
                long keyEndPos = FindIdentifier(source, out string key);
                if (keyEndPos < 0 || key.Length == 0)
                {
                    throw new ParserException("Could not identify object", 0, source.Position);
                }
                else if (checkExist && !Symbols.ContainsKey(key))
                {
                    throw new ParserException("Key not found", 0, source.Position);
                }
                else if (Symbols.ContainsKey(key) && Symbols[key].Item2.HasFlag(VariableFlags.Reserved))
                {
                    throw new ParserException("Cannot assign a value to a reserved constant", 0, keyEndPos - (key.Length + 1));
                }
                else
                {
                    int indx = Array.FindIndex(key.ToCharArray(), (c) => (c > 122 || c > 90 && c < 97 && c != '_' || c > 57 && c < 65 || c < 48));
                    if (indx > -1)
                    {
                        throw new ParserException(string.Format("Character \'{0}\' is not valid for an identifier",key[indx]), 0, keyEndPos-key.Length + indx);
                    }
                    source.Position = keyEndPos;
                }
                return key;
            }

            /// <summary>
            /// Checks if the next expression meets the requirements of being a value
            /// </summary>
            private string ValidateValue(Stream source)
            {
                long valuePos = FindExpression(source, out string value);
                if (valuePos < 0)
                {
                    throw new ParserException("Could not evaluate expression", 0, source.Position);
                }
                else
                {
                    source.Position = valuePos;
                }
                return value;
            }

            /// <summary>
            /// Handles the 'append x y [ + z];' case <br />
            /// And the 'set x y [ + z];' case
            /// </summary>
            Action AppendSet(Stream source, bool appendMode = true)
            {
                string key = ValidateKey(source, appendMode);
                string value = ValidateValue(source);
                if (appendMode)
                {
                    return () => Symbols[key] = new Tuple<string, VariableFlags>(Symbols[key].Item1 + value, Symbols[key].Item2);
                }
                else
                {
                    if (Symbols.ContainsKey(key))
                    {
                        return () => Symbols[key] = new Tuple<string, VariableFlags>(value, Symbols[key].Item2);
                    }
                    else
                    {
                        return () => Symbols.Add(key, new Tuple<string, VariableFlags>(value, VariableFlags.Empty));
                    }
                }
            }

            /// <summary>
            /// Creates and prints a nicely formatted table of all values
            /// </summary>
            /// <param name="printUnprint">List values normally excluded from printing</param>
            Action List(bool printUnprint = false)
            {
                int flagWidth = Math.Max(Enum.GetNames(typeof(VariableFlags)).Length, "Flags".Length);
                int keyWidth = (int)((ConsoleWidthLimit - flagWidth) * 0.2); // 20% - flag width
                int valueWidth = (int)((ConsoleWidthLimit - flagWidth) * 0.8); // 80% - flag width
                
                StringBuilder consoleOutput = new StringBuilder();
                consoleOutput.Append(string.Format("┌" + new string('─', keyWidth) + "┬" + new string('─', valueWidth) + "┬" + new string('─', flagWidth) + "┐\n"));
                consoleOutput.Append(string.Format("│{0}│{1}│{2}│\n", CenterString("Symbol", keyWidth), CenterString("Value", valueWidth), CenterString("Flags", flagWidth)));
                List<string> eligibleKeys = new List<string>(Symbols.Count);
                foreach (var item in Symbols.Keys)
                {
                    if (!Symbols[item].Item2.HasFlag(VariableFlags.NoPrint) || (Symbols[item].Item2.HasFlag(VariableFlags.NoPrint) && printUnprint))
                    {
                        eligibleKeys.Add(item);
                    }
                }
                if (eligibleKeys.Count > 0)
                {
                    consoleOutput.Append(string.Format("├" + new string('─', keyWidth) + "┼" + new string('─', valueWidth) + "┼" + new string('─', flagWidth) + "┤\n"));
                    for (int i = 0; i < eligibleKeys.Count; i++)
                    {
                        string entryFormat = "│{0," + -1 * keyWidth + "}│{1," + -1 * valueWidth + "}│{2," + -1 * flagWidth + "}│\n";
                        List<string> keyLines = GetStringLines(eligibleKeys[i], keyWidth);
                        List<string> valueLines = GetStringLines(Symbols[eligibleKeys[i]].Item1.Replace("\r", "\\r").Replace("\n", "\\n").Replace("\t", "\\t"), valueWidth);

                        for (int j = 0; j < (keyLines.Count > valueLines.Count ? keyLines.Count : valueLines.Count); j++)
                        {
                            consoleOutput.Append(string.Format(entryFormat, j >= keyLines.Count ? "" : keyLines[j], j >= valueLines.Count ? "" : valueLines[j], j == 0 ? Convert.ToString((byte)Symbols[eligibleKeys[i]].Item2, 2).PadLeft(flagWidth, '0'): ""));
                        }
                        if (i + 1 < eligibleKeys.Count)
                        {
                            consoleOutput.Append(string.Format("├" + new string('─', keyWidth) + "┼" + new string('─', valueWidth) + "┼" + new string('─', flagWidth) + "┤\n"));
                        }
                    }
                }
                consoleOutput.Append(string.Format("└" + new string('─', keyWidth) + "┴" + new string('─', valueWidth) + "┴" + new string('─', flagWidth) + "┘\n"));
                return () => Console.WriteLine(consoleOutput.ToString());
            }
            /// <summary>
            /// Exit Application logic
            /// </summary>
            Action Exit(Stream source, long initialStreamLength, bool isDynamicInput=false)
            {
                void exitAction()
                {
                    if (source.Length != initialStreamLength && isDynamicInput)
                    {
                        Console.WriteLine("Commands list has been modified; would you like to save it to a file?");
                        string commandState = "";
                        while (commandState.ToLower() != "y" && commandState.ToLower() != "n")
                        {
                            Console.Write("Y/n: ");
                            commandState = Console.ReadLine();
                        }
                        if (commandState.ToLower() == "y")
                        {
                            Console.WriteLine("Enter an output file (default {0}):", Environment.CurrentDirectory);
                            string path = Console.ReadLine();
                            if (path != "")
                            {
                                path = Path.Combine(Environment.CurrentDirectory, path);
                                source.Position = 0;
                                using (FileStream fs = File.OpenWrite(path))
                                {
                                    source.CopyTo(fs);
                                }
                                source.Close();
                            }
                        }
                    }
                }
                return exitAction;
            }

            /// <summary>
            /// Prints the expression to the console:<br />
            ///     0: print the value <br />
            ///     1: print the length <br />
            ///     2: print the word count <br />
            ///     3: print the words in the value <br />
            /// </summary>
            Action Print(Stream source, int mode = 0)
            {
                StringBuilder outputString = new StringBuilder();
                string expression = ValidateValue(source);
                if (mode == 0)
                {
                    outputString.Append(expression + Environment.NewLine);
                }
                else if (mode == 1)
                {
                    outputString.Append("Length of the expression is: ");
                    outputString.Append(expression.Length + Environment.NewLine);
                }
                else if (mode >= 2)
                {

                    string[] words = expression.Split(' ');
                    if (mode == 3)
                    {
                        outputString.Append("Wordcount is: ");
                        outputString.Append(words.Length + Environment.NewLine);
                    }
                    else
                    {
                        Console.WriteLine("Words are:");
                        foreach (string word in words)
                        {
                            outputString.Append(word + Environment.NewLine);
                        }
                    }
                }
                return () => Console.WriteLine(outputString.ToString());
            }
            /// <summary>
            /// Reverses the word-order of the symbol (in-place).
            /// </summary>
            Action Reverse(Stream source)
            {
                string key = ValidateKey(source, true);
                string ToReverse = Symbols[key].Item1;
                string[] words = ToReverse.Split(' ');
                StringBuilder reversed = new StringBuilder();
                for (int i = words.Length - 1; i >= 0; i--)
                {
                    reversed.Append(words[i]);
                    reversed.Append(' ');
                }

                return () => Symbols[key] = new Tuple<string, VariableFlags>(reversed.ToString(), Symbols[key].Item2);
            }

            /// <summary>
            /// Writes the debug info to the screen in the form: <br/>
            /// line read from stream (lineStart) to line end    <br/>
            /// &lt;whitespace@caratPos&gt; ^ &lt;errorMessage&gt;
            /// </summary>
            static void WriteDebugLine(long lineStart, long caratPos, string errorMessage, Stream source)
            {
                source.Position = lineStart;
                string fullLine = GetNextLine(source);
                string errorMSG = new string(' ', (caratPos - lineStart) >= 0 ? (int)(caratPos - lineStart) : 0) + "^ " + errorMessage;
                Console.WriteLine(fullLine);
                Console.WriteLine(errorMSG);
                source.Position = lineStart;
                source.SetLength(source.Position);
            }

            /// <summary>
            /// Parses & evaluates the expression from the stream, moving the stream to the end of the last value
            /// </summary>
            long FindExpression(Stream s, out string expression)
            {
                string result = "";
                bool IsAppendSet = true;
                while (s.Position < s.Length && !IsNextEoS(s))
                {
                    if (IsNextEoS(s, '+'))
                    {
                        s.Position = FindNextWord(s, out _);
                        IsAppendSet = true;
                    }
                    else
                    {
                        long val = FindValue(s, out string value);
                        if (val == -1)
                        {
                            Console.WriteLine("Could not parse value");
                        }
                        if (IsAppendSet)
                        {
                            s.Position = val;
                            result += value;
                            IsAppendSet = false;
                        }
                        else
                        {
                            throw new ParserException("Append operator not set", 0, s.Position);
                        }
                    }
                }
                expression = result;
                return s.Position;
            }
            /// <summary>
            /// Checks ahead to see if the next non-whitespace character is the EoS indicator
            /// </summary>
            /// <returns>true if the next char is <paramref name="EoSChar"/>, else false</returns>
            static bool IsNextEoS(Stream s, char EoSChar = ';')
            {
                long pos = s.Position;
                char readChar = PeekChar(s);
                while (readChar != 0 && char.IsWhiteSpace(readChar))
                {
                    readChar = ReadChar(s);
                }
                s.Position = pos;
                if (readChar == EoSChar) return true;
                else return false;
            }

            /// <summary>
            /// Finds the next value expression in the stream
            /// </summary>
            long FindValue(Stream s, out string returnedValue)
            {
                SkipWhitespace(s);
                char result = PeekChar(s);
                if (result == '\"')
                {
                    // The first char is a ", i.e. the start of a literal - search as if it were a literal.
                    return FindLiteral(s, out returnedValue);
                }
                else
                {
                    long t = FindExistingIdentifier(s, out string keyValue);
                    if (!Symbols.ContainsKey(keyValue))
                    {
                        throw new ParserException("Could not find key: " + keyValue, 0, s.Position);
                    }
                    returnedValue = Symbols[keyValue].Item1;
                    return t;
                }
            }

            static long FindIdentifier(Stream s, out string returnedKey)
            {
                long wordEnd = FindNextWord(s, out returnedKey);
                return wordEnd;
            }
            static long FindExistingIdentifier(Stream s, out string returnedKey)
            {
                long wordEnd = FindNextWord(s, out string identifier);
                if (identifier.Length > 1 && identifier.EndsWith(';'))
                {
                    // Remove the trailing semicolon from the parse & backtrack the identifier length one spot
                    identifier = identifier.TrimEnd(';');
                    wordEnd--;
                    s.Position--;
                }
                returnedKey = identifier;
                return wordEnd;
            }

            /// <summary>
            /// Finds the end of the complete literal definition, returning the stream to the original position
            /// </summary>
            static long FindLiteral(Stream s, out string returnedLiteral)
            {
                long pos = s.Position;
                // Is a literal. Now we must parse until we find the end of the literal
                // Remove the first char, if it is a literal definition.
                if (PeekChar(s) == '\"') ReadChar(s);
                long resultPosition = FindNextOccurance(s, (c, s) =>
                {
                    if (c == '\"')
                    {
                        s.Position--;
                        if (PreviousChar(s) == '\\')
                        {
                            // TODO: handle the \\ escape
                            s.Position++;
                            return false;
                        }
                        else
                        {
                            s.Position++;
                            return true;
                        }
                    }
                    return false;
                }, out string resultLiteral);
                if (resultPosition > -1)
                {
                    returnedLiteral = resultLiteral.Replace("\\\"", "\"");
                }
                else
                {
                    throw new ParserException("Could not parse the literal", 0, s.Position);
                }
                s.Position = pos;
                return resultPosition;
            }
        }

        /// <summary>
        /// Reads the memory stream as a UTF-8 encoded string until the next occurance of '\n' or '\r\n' (consuming, and excluded)
        /// </summary>
        static string GetNextLine(Stream s)
        {
            FindNextOccurance(s, '\n', out string nextLine);
            return nextLine;
        }

        /// <summary>
        /// Finds the end-boundary of the next word in the stream, and returns the stream to the original position
        /// </summary>
        static long FindNextWord(Stream s, out string nextWord)
        {
            StringBuilder newWord = new StringBuilder();
            long start = s.Position;
            char currentChar = ReadChar(s);
            while (s.Position < s.Length && char.IsWhiteSpace(currentChar))
            {
                currentChar = ReadChar(s);
            }
            newWord.Append(currentChar);
            // Start a second loop, this time checking we're not a whitespace char
            while (s.Position < s.Length)
            {
                currentChar = ReadChar(s);
                if (char.IsWhiteSpace(currentChar) || currentChar == ';')
                {
                    s.Position--;
                    break;
                }
                else
                {
                    newWord.Append(currentChar);
                }
            }
            nextWord = newWord.ToString();

            long endPos = s.Position;
            s.Position = start;
            return endPos;
        }


        /// <summary>
        /// Finds and returns the position of the next occurance of the Func returning true.
        /// </summary>
        static long FindNextOccurance(Stream s, Func<char, Stream, bool> p, out string result)
        {
            long start = s.Position;
            StringBuilder sb = new StringBuilder();
            bool charFound = false;
            while (s.Position < s.Length && !charFound)
            {
                char nextChar = ReadChar(s);
                if (nextChar == 0)
                {
                    charFound = true;
                }
                else if (p(nextChar, s))
                {
                    charFound = true;
                }
                else
                {
                    sb.Append(nextChar);
                }
            }

            result = sb.ToString();
            long newPosition = s.Position;
            s.Position = start;
            return newPosition--;
        }

        /// <summary>
        /// Finds the next position of the supplied character
        /// </summary>
        static long FindNextOccurance(Stream s, char c, out string result)
        {
            return FindNextOccurance(s, (streamChar, s) => streamChar == c, out result);
        }

        /// <summary>
        /// Reads the next UTF-8 encoded character in the stream, and advances the stream by the amount of characters read
        /// </summary>
        static char ReadChar(Stream s)
        {
            // As UTF-8 allows codepoints to span multiple bytes, reading a single byte as a character will not always give the expected
            // value. 
            // Fortunately, the standard ASCII table is 7-bits long. The 8th bit is used to determine the character size
            int readAmount = 0;
            int firstChar = s.ReadByte();
            if (firstChar == -1)
            {
                return (char)0;
            }
            if ((firstChar >> 3) == 0x1E) // 11110xxx implies a 4-byte length character
            {
                readAmount = 3;
            }
            else if ((firstChar >> 4) == 0xE) // 1110xxxx, 3-byte
            {
                readAmount = 2;
            }
            else if ((firstChar >> 5) == 0x6) // 110xxxxx, 2-byte
            {
                readAmount = 1;
            }

            byte[] charBytes = new byte[readAmount + 1];
            charBytes[0] = (byte)firstChar;
            for (int i = 1; i < readAmount; i++)
            {
                int nextChar = s.ReadByte();
                if (nextChar >> 6 != 2) throw new Exception("Character is not a valid UTF-8 code point!");
                charBytes[i] = (byte)nextChar;
            }
            s.Position += readAmount;
            string converted = Encoding.UTF8.GetString(charBytes);
            return converted[0];
        }

        /// <summary>
        /// Reads the next character in the stream, and returns the position to the original position
        /// </summary>
        static char PeekChar(Stream s)
        {
            long curr = s.Position;
            char c = ReadChar(s);
            s.Position = curr;
            return c;
        }

        /// <summary>
        /// Reads the previous char
        /// </summary>
        static char PreviousChar(Stream s)
        {
            Stack<byte> charBytes = new Stack<byte>(4);
            for (int i = 0; i < 4; i++)
            {
                if (s.Position == 0)
                {
                    break;
                }
                s.Position--;
                byte read = (byte)s.ReadByte();
                charBytes.Push(read);
                // No longer an UTF-8 extension, last byte is the final
                if (read >> 6 != 2) break;
            }
            string converted = Encoding.UTF8.GetString(charBytes.ToArray());
            return converted[0];
        }

        /// <summary>
        /// Skips whitespace characters
        /// </summary>
        static void SkipWhitespace(Stream s)
        {
            char c = PeekChar(s);
            while (s.Position < s.Length && char.IsWhiteSpace(c))
            {
                ReadChar(s);
                c = PeekChar(s);
            }
        }

        static string CenterString(string source, int totalPadding, char paddingChar = ' ')
        {
            if (source.Length >= totalPadding) return source;
            int rightHalf = (int)Math.Ceiling(source.Length / 2.0);
            int leftHalfPad = (int)Math.Floor(totalPadding / 2.0);
            int rightHalfPad = (int)Math.Ceiling(totalPadding / 2.0);
            string t = "{0," + leftHalfPad + "}{1," + -1 * rightHalfPad + "}";
            string result = string.Format(t, source[..rightHalf], source[rightHalf..]);
            return result;
        }

        static List<string> GetStringLines(string source, int maxWidth)
        {
            List<string> lines = new List<string>();
            int j = 0;
            while (j < source.Length)
            {
                int max = j + maxWidth <= source.Length ? j + maxWidth : source.Length;
                lines.Add(source[j..max]);
                j = max;
            }
            return lines;
        }

        public class ParserException : Exception
        {
            /// <summary>
            /// Importance is used to signify how the parser should respond to the error.
            /// A code of 3 or greater is a critical error; the application will throw the error further up the call and exit.
            /// 0 implies the line may be retried.
            /// 1 should imply the current block is not valid and should be retried.
            /// </summary>
            public int Importance = 0;
            public long LinePosition = -1;
            public ParserException(string message, int importance, long linePos) : base(message)
            {
                Importance = importance;
                LinePosition = linePos;
            }
        }
    }
}