Friday, January 2, 2015

Inserting a number's bits into another 32 bit number at a specified position. [Java]

Q. You are given two 32-bit numbers, N and M, and two bit positions, i and j. Write a method to insert M into N such that M starts at bit j and ends at bit i. You can assume that the bits j through i have enough space to fit all of M. That is, if M = 10011, you can assume that there are at least 5 bits between j and i. You would not, for example, have j = 3 and i = 2, because M could not fully fit between bit 3 and bit 2.

EXAMPLE

Input:    N = 10000000000, M = 10011, i = 2, j = 6
Output:   N = 10001001100

Source:

public class Code{
    static int change(int N, int M, int i, int j){
        return ((~(-1>>>(31-(j-i))<<i))&N)|(M<<i);
    }
    
    public static void main(String[] args){  
        int N = (int)Long.parseLong("10110011001100111111001111111100", 2);
        int M = Integer.parseInt("110011", 2); 
        System.out.println(Integer.toBinaryString(change(N, M, 17, 22)));
        
        N = Integer.parseInt("10000000000", 2);
        M = Integer.parseInt("10011", 2);
        System.out.println(Integer.toBinaryString(change(N, M, 2, 6)));
    }
}

Output:

10110011011001111111001111111100
10001001100

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