import java.security.*;

/**
 * SecureSFMT provides byte SFMT generate int value to hashing by SHA2.
 */
public class SecureSFMT {

    /* 256bit div 8bit eq 32byte */
    final static int DIGEST_LENGTH = 32;

    MessageDigest digest;

    SFMT sfmt;

    byte[] buffer = new byte[DIGEST_LENGTH];

    int index = DIGEST_LENGTH;

    public SecureSFMT(int ... seed) {
        try {
            digest = MessageDigest.getInstance("SHA-256");
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        }
        sfmt = new SFMT(seed);
    }

    public byte nextByte() {
        if(index == DIGEST_LENGTH) {
            init();
            index = 0;
        }
        return buffer[index++];
    }

    void init() {
        for (int i = 0; i < DIGEST_LENGTH; ) {
            int v = sfmt.nextInt();
            buffer[i++] = (byte) v;
            buffer[i++] = (byte)(v >>> 8);
            buffer[i++] = (byte)(v >>> 16);
            buffer[i++] = (byte)(v >>> 24);
        }
        digest.update(buffer);
        try {
            digest.digest(buffer, 0, DIGEST_LENGTH);
        } catch (DigestException e) {
            throw new RuntimeException(e);
        }
    }

    public int nextInt(int count) {
        int mask = 0;

        int bitCount;
        for (bitCount = 0; count > (1 << bitCount); bitCount++) {
            mask |= (1 << bitCount);
        }

        int value;
        do {
            value = 0;
            for(int shift = 0; shift < bitCount; shift += 8) {
                int v = (nextByte() & 0xff) << shift;
                value |= (v & mask);
            }
        } while(value >= count);

        return value;
    }

}