ParagonIE_Sodium_Core_BLAKE2b::increment_counter

    public static function increment_counter($ctx, $inc)
    {
        if ($inc < 0) {
            throw new SodiumException('Increasing by a negative number makes no sense.');
        }
        $t = self::to64($inc);
        # S->t is $ctx[1] in our implementation

        # S->t[0] = ( uint64_t )( t >> 0 );
        $ctx[1][0] = self::add64($ctx[1][0], $t);

        # S->t[1] += ( S->t[0] < inc );
        if (self::flatten64($ctx[1][0]) < $inc) {
            $ctx[1][1] = self::add64($ctx[1][1], self::to64(1));
        }
    }