nf-core_modules/lib/checksum.groovy

import java.security.MessageDigest
private static String getMD5(File file) throws IOException
{
    // https://howtodoinjava.com/java/io/how-to-generate-sha-or-md5-file-checksum-hash-in-java/
    //Get file input stream for reading the file content
    FileInputStream fis = new FileInputStream(file);

    //Create byte array to read data in chunks
    byte[] byteArray = new byte[1024];
    int bytesCount = 0;

    //Read file data and update in message digest
    def digest = MessageDigest.getInstance("MD5")
    while ((bytesCount = fis.read(byteArray)) != -1) {
        digest.update(byteArray, 0, bytesCount);
    };

    //close the stream; We don't need it now.
    fis.close();

    //Get the hash's bytes
    byte[] bytes = digest.digest();

    //This bytes[] has bytes in decimal format;
    //Convert it to hexadecimal format
    StringBuilder sb = new StringBuilder();
    for(int i=0; i< bytes.length ;i++)
    {
        sb.append(Integer.toString((bytes[i] & 0xff) + 0x100, 16).substring(1));
    }

    //return complete hash
    return sb.toString();
}