CsvReader.cs

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

namespace ZapTheDingbat.IO
{
    public class CsvReader : IEnumerable<IReadOnlyList<string>>, IEnumerator<IReadOnlyList<string>>
    {
        private readonly TextReader _reader;
        private readonly IList<string> _currentRow;
        private readonly StringBuilder _currentValue;
        private IReadOnlyList<string> _current;
        private State _state;

        public CsvReader(TextReader reader)
        {
            _reader = reader;
            _state = State.DataStart;
            _currentRow = new List<string>();
            _currentValue = new StringBuilder();
        }

        private enum State
        {
            DataStart,
            DataStartQuoteStart,
            DataStartQuote,
            DataQuoted,
            DataQuotedQuoteStart,
            Data,
            DataEnd,
            NewLineStart,
        }

        public bool MoveNext()
        {
            var c = _reader.Read();
            while (c != -1)
            {
                if (MoveNextChar((char)c))
                {
                    _current = _currentRow.ToArray();
                    _currentRow.Clear();
                    return true;
                }
                c = _reader.Read();
            }
            return false;
        }

        public IReadOnlyList<string> Current => _current;

        object IEnumerator.Current => Current;
        
        private bool MoveNextChar(char c)
        {
            if (_state == State.DataStart)
            {
                _currentValue.Clear();
                switch (c)
                {
                    case '"':
                        _state = State.DataStartQuoteStart;
                        break;
                    case ',':
                        _state = State.DataStart;
                        _currentRow.Add(_currentValue.ToString());
                        _currentValue.Clear();
                        break;
                    default:
                        _state = State.Data;
                        _currentValue.Append(c);
                        break;
                }
            }
            else if (_state == State.DataStartQuoteStart)
            {
                if (c == '"')
                {
                    _state = State.DataStartQuote;
                }
                else
                {
                    _state = State.DataQuoted;
                    _currentValue.Append(c);
                }
            }
            else if (_state == State.DataStartQuote)
            {
                switch (c)
                {
                    case '"':
                        _state = State.DataQuoted;
                        _currentValue.Append(c);
                        break;
                    case ',':
                        _state = State.DataStart;
                        _currentRow.Add(_currentValue.ToString());
                        _currentValue.Clear();
                        break;
                    default:
                        _state = State.DataEnd;
                        break;
                }
            }
            else if (_state == State.DataEnd)
            {
                if (c == ',')
                {
                    _state = State.DataStart;
                    _currentRow.Add(_currentValue.ToString());
                    _currentValue.Clear();
                }
            }
            else if (_state == State.Data)
            {
                switch (c)
                {
                    case ',':
                        _state = State.DataStart;
                        _currentRow.Add(_currentValue.ToString());
                        _currentValue.Clear();
                        break;
                    case '\r':
                        _state = State.NewLineStart;
                        break;
                    default:
                        _currentValue.Append(c);
                        break;
                }
            }
            else if (_state == State.DataQuoted)
            {
                if (c == '"')
                {
                    _state = State.DataQuotedQuoteStart;
                }
                else
                {
                    _currentValue.Append(c);
                }
            }
            else if (_state == State.DataQuotedQuoteStart)
            {
                switch (c)
                {
                    case '"':
                        _state = State.DataQuoted;
                        _currentValue.Append(c);
                        break;
                    case ',':
                        _state = State.DataStart;
                        _currentRow.Add(_currentValue.ToString());
                        _currentValue.Clear();
                        break;
                    default:
                        _state = State.DataEnd;
                        break;
                }
            }
            else if (_state == State.NewLineStart)
            {
                if (c != '\n')
                {
                    throw new FormatException("Invalid NewLine seperator");
                }
                _currentRow.Add(_currentValue.ToString());
                _currentValue.Clear();
                _state = State.DataStart;
                return true;
            }
            return false;
        }

        public void Dispose()
        {
            _reader.Dispose();
        }

        public void Reset()
        {
            throw new NotSupportedException();
        }

        public IEnumerator<IReadOnlyList<string>> GetEnumerator()
        {
            return this;
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}