PluralKit/PluralKit.Bot/CommandSystem/Parameters.cs

using System;
using System.Collections.Generic;

namespace PluralKit.Bot
{
    public class Parameters
    {
        // Dictionary of (left, right) quote pairs
        // Each char in the string is an individual quote, multi-char strings imply "one of the following chars"
        private static readonly Dictionary<string, string> _quotePairs = new Dictionary<string, string>
        {
            // Basic
            {"'", "'"}, // ASCII single quotes
            {"\"", "\""}, // ASCII double quotes
            
            // "Smart quotes"
            // Specifically ignore the left/right status of the quotes and match any combination of them
            // Left string also includes "low" quotes to allow for the low-high style used in some locales
            {"\u201C\u201D\u201F\u201E", "\u201C\u201D\u201F"}, // double quotes
            {"\u2018\u2019\u201B\u201A", "\u2018\u2019\u201B"}, // single quotes
            
            // Chevrons (normal and "fullwidth" variants)
            {"\u00AB\u300A", "\u00BB\u300B"}, // double chevrons, pointing away (<<text>>)
            {"\u00BB\u300B", "\u00AA\u300A"}, // double chevrons, pointing together (>>text<<)
            {"\u2039\u3008", "\u203A\u3009"}, // single chevrons, pointing away (<text>)
            {"\u203A\u3009", "\u2039\u3008"}, // single chevrons, pointing together (>text<)
            
            // Other
            {"\u300C\u300E", "\u300D\u300F"}, // corner brackets (Japanese/Chinese)
        };

        private readonly string _cmd;
        private int _ptr;
        private ISet<string> _flags = null; // Only parsed when requested first time

        private struct WordPosition
        {
            // Start of the word
            internal int startPos;
            
            // End of the word
            internal int endPos;
            
            // How much to advance word pointer afterwards to point at the start of the *next* word
            internal int advanceAfterWord;

            internal bool wasQuoted;

            public WordPosition(int startPos, int endPos, int advanceAfterWord, bool wasQuoted)
            {
                this.startPos = startPos;
                this.endPos = endPos;
                this.advanceAfterWord = advanceAfterWord;
                this.wasQuoted = wasQuoted;
            }
        }

        public Parameters(string cmd)
        {
            // This is a SUPER dirty hack to avoid having to match both spaces and newlines in the word detection below
            // Instead, we just add a space before every newline (which then gets stripped out later).
            _cmd = cmd.Replace("\n", " \n");
            _ptr = 0;
        }

        private void ParseFlags()
        {
            _flags = new HashSet<string>();
            
            var ptr = 0;
            while (NextWordPosition(ptr) is { } wp)
            {
                ptr = wp.endPos + wp.advanceAfterWord;
                
                // Is this word a *flag* (as in, starts with a - AND is not quoted)
                if (_cmd[wp.startPos] != '-' || wp.wasQuoted) continue; // (if not, carry on w/ next word)
                
                // Find the *end* of the flag start (technically allowing arbitrary amounts of dashes)
                var flagNameStart = wp.startPos;
                while (flagNameStart < _cmd.Length && _cmd[flagNameStart] == '-')
                    flagNameStart++;

                // Then add the word to the flag set
                var word = _cmd.Substring(flagNameStart, wp.endPos - flagNameStart).Trim();
                if (word.Length > 0)
                    _flags.Add(word.ToLowerInvariant());
            }
        }

        public string Pop()
        {
            // Loop to ignore and skip past flags
            while (NextWordPosition(_ptr) is { } pos)
            {
                _ptr = pos.endPos + pos.advanceAfterWord;
                if (_cmd[pos.startPos] == '-' && !pos.wasQuoted) continue;
                return _cmd.Substring(pos.startPos, pos.endPos - pos.startPos).Trim();
            }

            return "";
        }

        public string Peek()
        {
            // Loop to ignore and skip past flags, temp ptr so we don't move the real ptr
            var ptr = _ptr;
            while (NextWordPosition(ptr) is { } pos)
            {
                ptr = pos.endPos + pos.advanceAfterWord;
                if (_cmd[pos.startPos] == '-' && !pos.wasQuoted) continue;
                return _cmd.Substring(pos.startPos, pos.endPos - pos.startPos).Trim();
            }

            return "";
        }

        public ISet<string> Flags()
        {
            if (_flags == null) ParseFlags();
            return _flags;
        }

        public string Remainder(bool skipFlags = true)
        {
            if (skipFlags)
            {
                // Skip all *leading* flags when taking the remainder
                while (NextWordPosition(_ptr) is {} wp)
                {
                    if (_cmd[wp.startPos] != '-' || wp.wasQuoted) break;
                    _ptr = wp.endPos + wp.advanceAfterWord;
                }
            }

            // *Then* get the remainder
            return _cmd.Substring(Math.Min(_ptr, _cmd.Length)).Trim();
        }
        
        public string FullCommand => _cmd;

        private WordPosition? NextWordPosition(int position)
        {
            // Skip leading spaces before actual content
            while (position < _cmd.Length && _cmd[position] == ' ') position++;

            // Is this the end of the string?
            if (_cmd.Length <= position) return null;

            // Is this a quoted word?
            if (TryCheckQuote(_cmd[position], out var endQuotes))
            {
                // We found a quoted word - find an instance of one of the corresponding end quotes
                var endQuotePosition = -1;
                for (var i = position + 1; i < _cmd.Length; i++)
                    if (endQuotePosition == -1 && endQuotes.Contains(_cmd[i]))
                        endQuotePosition = i; // need a break; don't feel like brackets tho lol

                // Position after the end quote should be EOL or a space
                // Otherwise we fallthrough to the unquoted word handler below
                if (_cmd.Length == endQuotePosition + 1 || _cmd[endQuotePosition + 1] == ' ')
                    return new WordPosition(position + 1, endQuotePosition, 2, true);
            }

            // Not a quoted word, just find the next space and return if it's the end of the command
            var wordEnd = _cmd.IndexOf(' ', position + 1);
            
            return wordEnd == -1
                ? new WordPosition(position, _cmd.Length, 0, false)
                : new WordPosition(position, wordEnd, 1, false);
        }

        private bool TryCheckQuote(char potentialLeftQuote, out string correspondingRightQuotes)
        {
            foreach (var (left, right) in _quotePairs)
            {
                if (left.Contains(potentialLeftQuote))
                {
                    correspondingRightQuotes = right;
                    return true;
                }
            }
            
            correspondingRightQuotes = null;
            return false;
        }
    }
}