StringBuilder.cs

using System;

namespace Tonari.Text
{
    public class StringBuilder : IDisposable
    {
        private char[] _buffer;
        private int _bufferLength;
        private int _position;

        public StringBuilder(int capacity)
        {
            _bufferLength = capacity;
            _buffer = new char[_bufferLength];
        }

        public void Dispose()
        {
            _buffer = null;
        }

        public void Clear()
        {
            _position = 0;
        }

        public void Append(string source)
        {
            if (source == null) return;
            var length = source.Length;
            
            if (_position + length > _bufferLength)
            {
                ExtendBuffer(length);
            }

            for (var i = 0; i < length; ++i)
            {
                _buffer[_position + i] = source[i];
            }
            
            _position += length;
        }

        public void Append(StringBuilder builder)
        {
            if (builder == null) return;
            var length = builder._position;

            if (_position + length > _bufferLength)
            {
                ExtendBuffer(length);
            }

            for (var i = 0; i < length; ++i)
            {
                _buffer[_position + i] = builder._buffer[i];
            }

            _position += length;
        }

        private void ExtendBuffer(int length)
        {
            var temp = _buffer;
            var tempLength = _bufferLength;

            do
            {
                _bufferLength *= 2;
            } while (_bufferLength < tempLength + length);

            _buffer = new char[_bufferLength];

            Buffer.BlockCopy(temp, 0, _buffer, 0, sizeof(char) * tempLength);
        }

        public override string ToString()
        {
            return new string(_buffer, 0, _position);
        }
    }
}