SecureHashable.java

package net.morimekta.collect;

import java.util.Collection;
import java.util.Map;
import java.util.Set;

/**
 * A type that can be hashed to a secure 64 bit (long) hash.
 */
public interface SecureHashable {
    /**
     * Get a secure hash code for the value. This is meant to be used on
     * values that can have pretty similar values, not only to ensure unique
     * hash codes, but also numerical distribution among the 64-bit integers.
     * <p>
     * The
     *
     * @return A secure hash code.
     */
    long secureHashCode();

    /**
     * Make a secure hash code from an object.
     *
     * @param o The object to hash.
     * @return The secure hashcode.
     */
    static long secureHash(Object o) {
        if (o instanceof SecureHashable) {
            return ((SecureHashable) o).secureHashCode();
        } else if (o instanceof CharSequence) {
            return secureHash((CharSequence) o);
        } else if (o instanceof byte[]) {
            return secureHash((byte[]) o);
        } else if (o instanceof Set) {
            long result = -1466376081728027815L;
            result += (5345617L + ((Set<?>) o).size()) * 2993991083002160833L;
            for (var i : (Set<?>) o) {
                // The order does not matter in sets.
                result += secureHash(i) * 8778220231891916799L;
            }
            return result;
        } else if (o instanceof Collection) {
            long result = -1466376081728027815L;
            for (var i : (Collection<?>) o) {
                // The order matters in other collections.
                result *= -2303118536797078809L;
                result += secureHash(i) * 8778220231891916799L;
            }
            return result;
        } else if (o instanceof Map) {
            long result = 2993991083002160833L;
            result += (5345617L + ((Map<?, ?>) o).size()) * -1466376081728027815L;
            for (var e : ((Map<?, ?>) o).entrySet()) {
                result += (7675589L + secureHash(e.getKey())) *
                          (96545101L + secureHash(e.getValue())) *
                          150090999566823819L;
            }
            return result;
        }
        return 2113876914436862725L * (5345617L + o.hashCode());
    }

    /**
     * Make a secure hash code from a char sequence or string.
     *
     * @param sequence The string to hash.
     * @return The secure hashcode.
     */
    static long secureHash(CharSequence sequence) {
        long result = 6965299255647741657L;
        for (int pos = 0; pos < sequence.length(); ++pos) {
            result *= -2113876914436862725L;
            result += (5345617L + sequence.charAt(pos)) * -7735678928159363275L;
        }
        return result;
    }

    /**
     * Make a secure hash code from a byte array.
     *
     * @param array The array to hash.
     * @return The secure hashcode.
     */
    static long secureHash(byte[] array) {
        return secureHash(array, 0, array.length);
    }

    /**
     * Make a secure hash code from a byte array.
     *
     * @param array The array to hash.
     * @param offset Offset to start hashing at.
     * @param length Number of bytes to hash.
     * @return The secure hashcode.
     */
    static long secureHash(byte[] array, int offset, int length) {
        long result = 8812079937318930253L;
        for (int i = 0; i < length; ++i) {
            result *= 3553555017746745835L;
            result += (344629L + array[i + offset]) * 8670115223866369197L;
        }
        return result;
    }
}