File content as of revision 17:82aa379e427e:

/* SD/MMC File System Library
 * Copyright (c) 2014 Neil Thiessen
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "CRC7.h"

namespace
{
const char m_CRC7Table[] = {
    0x00, 0x09, 0x12, 0x1B, 0x24, 0x2D, 0x36, 0x3F,
    0x48, 0x41, 0x5A, 0x53, 0x6C, 0x65, 0x7E, 0x77,
    0x19, 0x10, 0x0B, 0x02, 0x3D, 0x34, 0x2F, 0x26,
    0x51, 0x58, 0x43, 0x4A, 0x75, 0x7C, 0x67, 0x6E,
    0x32, 0x3B, 0x20, 0x29, 0x16, 0x1F, 0x04, 0x0D,
    0x7A, 0x73, 0x68, 0x61, 0x5E, 0x57, 0x4C, 0x45,
    0x2B, 0x22, 0x39, 0x30, 0x0F, 0x06, 0x1D, 0x14,
    0x63, 0x6A, 0x71, 0x78, 0x47, 0x4E, 0x55, 0x5C,
    0x64, 0x6D, 0x76, 0x7F, 0x40, 0x49, 0x52, 0x5B,
    0x2C, 0x25, 0x3E, 0x37, 0x08, 0x01, 0x1A, 0x13,
    0x7D, 0x74, 0x6F, 0x66, 0x59, 0x50, 0x4B, 0x42,
    0x35, 0x3C, 0x27, 0x2E, 0x11, 0x18, 0x03, 0x0A,
    0x56, 0x5F, 0x44, 0x4D, 0x72, 0x7B, 0x60, 0x69,
    0x1E, 0x17, 0x0C, 0x05, 0x3A, 0x33, 0x28, 0x21,
    0x4F, 0x46, 0x5D, 0x54, 0x6B, 0x62, 0x79, 0x70,
    0x07, 0x0E, 0x15, 0x1C, 0x23, 0x2A, 0x31, 0x38,
    0x41, 0x48, 0x53, 0x5A, 0x65, 0x6C, 0x77, 0x7E,
    0x09, 0x00, 0x1B, 0x12, 0x2D, 0x24, 0x3F, 0x36,
    0x58, 0x51, 0x4A, 0x43, 0x7C, 0x75, 0x6E, 0x67,
    0x10, 0x19, 0x02, 0x0B, 0x34, 0x3D, 0x26, 0x2F,
    0x73, 0x7A, 0x61, 0x68, 0x57, 0x5E, 0x45, 0x4C,
    0x3B, 0x32, 0x29, 0x20, 0x1F, 0x16, 0x0D, 0x04,
    0x6A, 0x63, 0x78, 0x71, 0x4E, 0x47, 0x5C, 0x55,
    0x22, 0x2B, 0x30, 0x39, 0x06, 0x0F, 0x14, 0x1D,
    0x25, 0x2C, 0x37, 0x3E, 0x01, 0x08, 0x13, 0x1A,
    0x6D, 0x64, 0x7F, 0x76, 0x49, 0x40, 0x5B, 0x52,
    0x3C, 0x35, 0x2E, 0x27, 0x18, 0x11, 0x0A, 0x03,
    0x74, 0x7D, 0x66, 0x6F, 0x50, 0x59, 0x42, 0x4B,
    0x17, 0x1E, 0x05, 0x0C, 0x33, 0x3A, 0x21, 0x28,
    0x5F, 0x56, 0x4D, 0x44, 0x7B, 0x72, 0x69, 0x60,
    0x0E, 0x07, 0x1C, 0x15, 0x2A, 0x23, 0x38, 0x31,
    0x46, 0x4F, 0x54, 0x5D, 0x62, 0x6B, 0x70, 0x79
};
}

char CRC7(const char* data, int length)
{
    //Calculate the CRC7 checksum for the specified data block
    char crc = 0;
    for (int i = 0; i < length; i++) {
        crc = m_CRC7Table[(crc << 1) ^ data[i]];
    }

    //Return the calculated checksum
    return crc;
}