// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Text;

namespace System.Net.Mail
{
    internal sealed class BufferBuilder
    {
        private byte[] _buffer;
        private int _offset;

        internal BufferBuilder() : this(256) { }

        internal BufferBuilder(int initialSize)
        {
            _buffer = new byte[initialSize];
        }

        private void EnsureBuffer(int count)
        {
            if (count > _buffer.Length - _offset)
            {
                byte[] newBuffer = new byte[((_buffer.Length * 2) > (_buffer.Length + count)) ? (_buffer.Length * 2) : (_buffer.Length + count)];
                Buffer.BlockCopy(_buffer, 0, newBuffer, 0, _offset);
                _buffer = newBuffer;
            }
        }

        internal void Append(byte value)
        {
            EnsureBuffer(1);
            _buffer[_offset++] = value;
        }

        internal void Append(ReadOnlyMemory<byte> value)
        {
            EnsureBuffer(value.Length);
            value.Span.CopyTo(_buffer.AsSpan(_offset));
            _offset += value.Length;
        }

        internal void Append(ReadOnlySpan<byte> value)
        {
            EnsureBuffer(value.Length);
            value.CopyTo(_buffer.AsSpan(_offset));
            _offset += value.Length;
        }

        internal void Append(byte[] value, int offset, int count)
        {
            EnsureBuffer(count);
            Buffer.BlockCopy(value, offset, _buffer, _offset, count);
            _offset += count;
        }

        internal void Append(string? value)
        {
            Append(value, false);
        }

        internal void Append(string? value, bool allowUnicode)
        {
            if (string.IsNullOrEmpty(value))
            {
                return;
            }

            Append(value, 0, value.Length, allowUnicode);
        }

        internal void Append(string value, int offset, int count, bool allowUnicode)
        {
            if (allowUnicode)
            {
                int byteCount = Encoding.UTF8.GetByteCount(value, offset, count);
                EnsureBuffer(byteCount);
                Encoding.UTF8.GetBytes(value, offset, count, _buffer, _offset);
                _offset += byteCount;
            }
            else
            {
                Append(value, offset, count);
            }
        }

        // Does not allow unicode, only ANSI
        internal void Append(string value, int offset, int count)
        {
            EnsureBuffer(count);
            for (int i = 0; i < count; i++)
            {
                char c = value[offset + i];
                if ((ushort)c > 0xFF)
                {
                    throw new FormatException(SR.Format(SR.MailHeaderFieldInvalidCharacter, c));
                }

                _buffer[_offset + i] = (byte)c;
            }

            _offset += count;
        }

        internal int Length => _offset;
        internal byte[] GetBuffer() => _buffer;
        internal void Reset() { _offset = 0; }
    }
}