// Copyright (C) 2024, The Duplicati Team
// https://duplicati.com, hello@duplicati.com
// 
// Permission is hereby granted, free of charge, to any person obtaining a 
// copy of this software and associated documentation files (the "Software"), 
// to deal in the Software without restriction, including without limitation 
// the rights to use, copy, modify, merge, publish, distribute, sublicense, 
// and/or sell copies of the Software, and to permit persons to whom the 
// Software is furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in 
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS 
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER 
// DEALINGS IN THE SOFTWARE.

using System;
using System.Collections.Generic;
using System.IO;

namespace Duplicati.Library.Utility
{
    /// <summary>
    /// Represents an enumerable list that can be appended to.
    /// If the use of the list exceeds the threshold, the list will
    /// switch from memory based to storage to file based storage.
    /// Typical usage of this list is for storing log messages,
    /// that occasionally grows and produces out-of-memory errors
    /// </summary>
    public abstract class FileBackedList<T> : IEnumerable<T>, IDisposable
    {
        private class StreamEnumerator : IEnumerator<T>, System.Collections.IEnumerator
        {
            private Stream m_stream;
            private readonly Func<Stream, long, T> m_deserialize;
            private long m_position;
            private readonly byte[] m_sizebuffer;
            private readonly long m_expectedCount;
            private readonly FileBackedList<T> m_parent;
            private T m_current;

            public StreamEnumerator(Stream stream, Func<Stream, long, T> deserialize, FileBackedList<T> parent)
            {
                m_stream = stream;
                m_deserialize = deserialize;
                m_sizebuffer = new byte[8];
                m_parent = parent;
                m_expectedCount = m_parent.Count;
                this.Reset();
            }

            public T Current
            {
                get
                {
                    if (m_expectedCount != m_parent.Count)
                        throw new Exception("Collection modified");
                    return m_current;
                }
            }

            public void Dispose()
            {
                m_stream = null;
            }

            object System.Collections.IEnumerator.Current
            {
                get { return this.Current; }
            }

            public bool MoveNext()
            {
                if (m_position >= m_stream.Length)
                {
                    m_current = default(T);
                    return false;
                }
                    
                if (m_expectedCount != m_parent.Count)
                    throw new Exception("Collection modified");
                    
                m_stream.Position = m_position;
                if (Utility.ForceStreamRead(m_stream, m_sizebuffer, m_sizebuffer.Length) != m_sizebuffer.Length)
                    throw new IOException("Unexpected EOS");
                var len = BitConverter.ToInt64(m_sizebuffer, 0);
                m_current = m_deserialize(m_stream, len);
                m_position += m_sizebuffer.Length + len;
                return true;
            }

            public void Reset()
            {
                m_position = 0;
                m_current = default(T);
            }
        }

        private Library.Utility.TempFile m_file;
        private Stream m_stream;
        private long m_count;
                
        public bool IsFileBacked { get { return !(m_stream is MemoryStream); } }
        public long SwitchToFileLimit { get; set; }
        
        protected FileBackedList()
        {
            m_file = null;
            m_stream = new MemoryStream();
            m_count = 0;
                        
            this.SwitchToFileLimit = 2 * 1024 * 1024;
        }

        public void Add(T value)
        {
            long size = GetSize(value);
            if (m_stream is MemoryStream && (m_stream.Length + size) > this.SwitchToFileLimit)
            {
                m_file = new Library.Utility.TempFile();
                using(var oldstream = m_stream)
                {
                    m_stream = System.IO.File.Open(m_file, FileMode.Create, FileAccess.ReadWrite, FileShare.None);
                    oldstream.Position = 0;
                    oldstream.CopyTo(m_stream);
                }
            }

            m_stream.Position = m_stream.Length;
            m_stream.Write(BitConverter.GetBytes(size), 0, 8);
            var pos = m_stream.Position;
            Serialize(value, m_stream);
            if (m_stream.Position - pos != size)
                throw new Exception(string.Format("Stream serializer wrote a different set of bytes than it was supposed to. Expected {0} bytes, but wrote {1} bytes", m_stream.Position - pos, size));

            m_count++;
        }

        public long Count { get { return m_count; } }

        public void Dispose()
        {
            Dispose(true);
        }

        ~FileBackedList()
        {
            Dispose(false);
        }

        protected void Dispose(bool disposing)
        {
            if (disposing)
                GC.SuppressFinalize(this);

            if (m_stream != null)
            {
                m_stream.Dispose();
                m_stream = null;
            }

            if (m_file != null)
            {
                m_file.Dispose();
                m_file = null;
            }
        }
        
        protected abstract long GetSize(T value);
        protected abstract void Serialize(T value, Stream stream);
        protected abstract T Deserialize(Stream stream, long length);
        
        #region IEnumerable implementation

        public IEnumerator<T> GetEnumerator()
        {
            return new StreamEnumerator(m_stream, this.Deserialize, this);
        }

        #endregion

        #region IEnumerable implementation

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            return this.GetEnumerator();
        }

        #endregion
    }
    
    /// <summary>
    /// Represents an enumerable list that can be appended to.
    /// If the use of the list exceeds the threshold, the list will
    /// switch from memory based to storage to file based storage.
    /// Typical usage of this list is for storing log messages,
    /// that occasionally grows and produces out-of-memory errors
    /// </summary>
    public class FileBackedStringList : FileBackedList<string>
    {
        private byte[] m_buf;
        public readonly System.Text.Encoding m_encoding;
        private const int MIN_BUF_SIZE = 1024;
        private const int BUFFER_OVERSHOOT = 32;

        /// <summary>
        /// Initializes a new instance of the <see cref="Duplicati.Library.Utility.FileBackedStringList"/> class.
        /// </summary>
        /// <param name="encoding">The text encoding to use, defaults to UTF8</param>
        public FileBackedStringList(System.Text.Encoding encoding = null)
        {
            m_encoding = encoding ?? System.Text.Encoding.UTF8;
        }
                            
        protected override long GetSize(string value)
        {
            var size = m_encoding.GetByteCount(value);
            if (m_buf == null || m_buf.Length < size)
                m_buf = new byte[Math.Max(MIN_BUF_SIZE, size + BUFFER_OVERSHOOT)];
            return size;
        }
        
        protected override void Serialize(string value, Stream stream)
        {
            var size = m_encoding.GetBytes(value, 0, value.Length, m_buf, 0);            
            stream.Write(m_buf, 0, size);
        }
        
        protected override string Deserialize(Stream stream, long length)
        {
            if (m_buf == null || m_buf.Length < length)
                m_buf = new byte[Math.Max(MIN_BUF_SIZE, length + BUFFER_OVERSHOOT)];
            Utility.ForceStreamRead(stream, m_buf, (int)length);
            return (m_encoding ?? System.Text.Encoding.UTF8).GetString(m_buf, 0, (int)length);
        }            
            
    }
}