Validation.java

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package ngmf.util;

import java.io.File;
import java.io.FileInputStream;
import java.security.MessageDigest;

/**
 * 
 * @author od
 */
public class Validation {

    /**
     * Creates a hex encoded sha-256 hash of all files.
     *
     * @param algorithm the algorithm to be used.
     * @param files the files to include for this hash.
     * @return the hexadecimal digest (length 64byte for sha-256).
     */
    public static String hexDigest(String algorithm, File[] files) {
        try {
            MessageDigest md = MessageDigest.getInstance(algorithm);
            byte[] buf = new byte[4096];
            for (File f : files) {
                FileInputStream in = new FileInputStream(f);
                int nread = in.read(buf);
                while (nread > 0) {
                    md.update(buf, 0, nread);
                    nread = in.read(buf);
                }
                in.close();
            }
            return toHex(md.digest(buf));
        } catch (Exception ex) {
            ex.printStackTrace(System.out);
        }
        return "<error>";
    }

    private static String toHex(byte[] data) {
        StringBuffer buf = new StringBuffer();
        for (int i = 0; i < data.length; i++) {
            int halfbyte = (data[i] >>> 4) & 0x0F;
            int two_halfs = 0;
            do {
                if ((0 <= halfbyte) && (halfbyte <= 9)) {
                    buf.append((char) ('0' + halfbyte));
                } else {
                    buf.append((char) ('a' + (halfbyte - 10)));
                }
                halfbyte = data[i] & 0x0F;
            } while (two_halfs++ < 1);
        }
        return buf.toString();
    }
}