// AnsiX923Padding.cs
// SPDX-License-Identifier: GPL-3.0-or-later
// SPDX-FileCopyrightText: 2025 Catswords OSS and WelsonJS Contributors
// https://github.com/gnh1201/welsonjs
// 
using System;

namespace WelsonJS.Cryptography
{
    class AnsiX923Padding
    {
        /// <summary>
        /// Add ANSI X.923 padding to the input data to make it a multiple of the block size.
        /// </summary>
        /// <param name="data">The data to be padded.</param>
        /// <param name="blockSize">The block size to pad to.</param>
        /// <returns>Padded data with ANSI X.923 padding.</returns>
        public static byte[] AddPadding(byte[] data, int blockSize)
        {
            int paddingLength = blockSize - (data.Length % blockSize);

            // If the data is already a multiple of the block size, no padding is needed
            if (paddingLength == blockSize)
            {
                return data;
            }

            byte[] paddedData = new byte[data.Length + paddingLength];

            // Copy original data into the padded array
            Array.Copy(data, paddedData, data.Length);

            // Fill with 0x00 bytes, and the last byte is the padding length
            for (int i = data.Length; i < paddedData.Length - 1; i++)
            {
                paddedData[i] = 0x00;
            }

            // Last byte is the padding length
            paddedData[paddedData.Length - 1] = (byte)paddingLength;

            return paddedData;
        }

        /// <summary>
        /// Removes ANSI X.923 padding from the given data.
        /// </summary>
        /// <param name="data">The input data, including padding.</param>
        /// <param name="blockSize">The block size used for padding.</param>
        /// <param name="ignoreErrors">If true, ignores errors and attempts to process the input data as-is.</param>
        /// <returns>The unpadded data as a byte array.</returns>
        /// <exception cref="ArgumentException">Thrown if the input data or padding is invalid and ignoreErrors is false.</exception>
        public static byte[] RemovePadding(byte[] data, int blockSize, bool ignoreErrors = false)
        {
            // Check for null or empty data
            if (data == null || data.Length == 0)
            {
                if (ignoreErrors)
                {
                    return new byte[] { };
                }
                throw new ArgumentException("Input data cannot be null or empty.");
            }

            // Ensure the data length is a multiple of the block size
            if (data.Length % blockSize != 0)
            {
                if (ignoreErrors)
                {
                    // Return the original data if errors are ignored
                    return data;
                }
                throw new ArgumentException("Input data length must be a multiple of the block size.");
            }

            // Retrieve the padding length from the last byte
            int paddingLength = data[data.Length - 1];

            // Validate padding length
            if (paddingLength <= 0 || paddingLength > blockSize)
            {
                if (!ignoreErrors)
                {
                    throw new ArgumentException($"Invalid padding length: {paddingLength}. Must be between 1 and {blockSize}.");
                }

                // Treat padding length as 0 and return the full data
                return data;
            }

            // Validate the padding region (last paddingLength - 1 bytes must be 0x00)
            for (int i = data.Length - paddingLength; i < data.Length - 1; i++)
            {
                if (data[i] != 0x00)
                {
                    if (!ignoreErrors)
                    {
                        throw new ArgumentException("Invalid padding detected. Expected padding bytes to be 0x00.");
                    }
                }
            }

            // Extract unpadded data
            byte[] unpaddedData = new byte[data.Length - paddingLength];
            Array.Copy(data, unpaddedData, unpaddedData.Length);

            return unpaddedData;
        }
    }
}

/* References:
 * [1] GitHub - 표준프레임워크의 실행환경 (eGovFrame/egovframework.rte.root)
 *     https://github.com/eGovFrame/egovframework.rte.root/blob/master/Foundation/egovframework.rte.fdl.crypto/src/main/java/egovframework/rte/fdl/cryptography/impl/aria/AnsiX923Padding.java
 * [2] ChatGPT - The prompt "AnsiX923Padding with C#"
 * [3] ChatGPT - The prompt "AnsiX923Padding with C#, Add a flag to decide how to handle possible errors when removing padding."
 */