Collect and Compare Log Statistics using LogJoin

/*
 * Copyright (c) 2013, Yuriy Nelipovich
 * 
 * If you find this code useful or in case of any questions, suggestions
 * bug reports, donation, please email me: dev.yuriy.n@gmail.com
 */

using System;
using System.Collections.Generic;
using System.Text;

namespace LogJoin
{
    /// <summary>
    /// Sequence of text portions, each portion is composed of a queue of text lines (called frame).
    /// When moving forward, the frame is shifted by one line.
    /// </summary>
    public class MultilineInput : Input
    {
        private readonly int _frameSize;

        private readonly Queue<string> _lines;

        /// <summary>
        /// Constructs new input
        /// </summary>
        /// <param name="filePaths">Sequence of files to read</param>
        /// <param name="frameSize">Maximum size of text frame (i.e. maximum count of lines)</param>
        public MultilineInput(string[] filePaths, int frameSize) : base(filePaths)
        {
            if (frameSize <= 0)
                throw new ArgumentOutOfRangeException("frameSize",
                                                      string.Format("Lines count {0} is invalid", frameSize));
            this._frameSize = frameSize;
            this._lines = new Queue<string>(frameSize);
        }

        /// <summary>
        /// Moves the input to the start position
        /// </summary>
        public override void ResetPosition()
        {
            base.ResetPosition();
            this._lines.Clear();
        }

        /// <summary>
        /// Rads the next text line, pushes it to the frame and moves to the next position
        /// </summary>
        /// <returns>Frame contents</returns>
        public override string ReadTextPortion()
        {
            if (this._lines.Count == 0)
                this.FillFrameWithInitialText();
            else
                this.ReadNextLineAndRoll();
            if (this._lines.Count == 0)
                return null;
            var result = new StringBuilder();
            foreach (var line in this._lines)
            {
                result.AppendLine(line);
            }
            return result.ToString();
        }

        /// <summary>
        /// Fills the frame with initial text
        /// </summary>
        private void FillFrameWithInitialText()
        {
            string nextLine;
            do
            {
                nextLine = base.ReadTextPortion();
                if (nextLine != null)
                    this._lines.Enqueue(nextLine);
            }
            while (nextLine != null && this._lines.Count < this._frameSize);
        }

        /// <summary>
        /// Rads the next text line, pushes it to the frame
        /// and ensures that the frame size is not exceeded
        /// </summary>
        private void ReadNextLineAndRoll()
        {
            var next = base.ReadTextPortion();
            if (next != null)
                this._lines.Enqueue(next);
            if (this._lines.Count > 0)
                this._lines.Dequeue();
        }
    }
}