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

/// <summary>
/// Lazily reads a log4net style log file to parse entries
/// </summary>
/// <remarks>
/// Handles multi-line entries so that entries containing messages
/// or a stack trace with newline characters aren't dropped
/// or picked out as a separate log entry the way that
/// simply splitting the file on newline characters would.
/// </remarks>
public class LogReader
{
    const int TIMESTAMP_LENGTH = 23;
    const string TIMESTAMP_FORMAT_STRING = "yyyy-MM-dd HH:mm:ss,fff";

    public IEnumerable<string> Read(string filePath, Func<string, bool> newEntryPredicate = null)
    {
        // Use the provided function to determine if the string
        // is the first line in a log entry otherwise default it
        // to using the internal timestamp logic
        Func<string, bool> isNewEntry = null;
        if(newEntryPredicate == null)
        {
            isNewEntry = IsNewEntryUsingTimestamp;
        }

        using (FileStream fStream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
        using(StreamReader stReader = new StreamReader(fStream))
        {
            StringBuilder entryBuilder = new StringBuilder();
            string line = stReader.ReadLine();
            while(line != null)
            {
                // New log entry
                if(isNewEntry(line))
                {
                    // If the previous line included the end of an entry, remove the trailing newline and yield the value
                    if (entryBuilder.Length > 0)
                    {
                        yield return entryBuilder.ToString().Trim();
                        entryBuilder.Clear();
                    }
                }
                entryBuilder.AppendLine(line);
                line = stReader.ReadLine();
            }
        }
    }

    private bool IsNewEntryUsingTimestamp(string line)
    {
        DateTime dt;
        return line.Length >= 23 && DateTime.TryParseExact((line.Substring(0, TIMESTAMP_LENGTH)), TIMESTAMP_FORMAT_STRING, CultureInfo.InvariantCulture, DateTimeStyles.None, out dt);
    }
}