Custom uint16_t reverse function returns 0x8000

Question

    uint16_t ReverseInt16(uint16_t nonreversed) {
    uint16_t reversed = 0;
    reversed |= (nonreversed & 1 << 15) << 0; //check if bit 15 of nonreversed int is 1, if yes, write 1 to position 0, else write 0 to position 0
    reversed |= (nonreversed & 1 << 14) << 1;
    reversed |= (nonreversed & 1 << 13) << 2;
    reversed |= (nonreversed & 1 << 12) << 3;
    reversed |= (nonreversed & 1 << 11) << 4;
    reversed |= (nonreversed & 1 << 10) << 5;
    reversed |= (nonreversed & 1 << 9) << 6;
    reversed |= (nonreversed & 1 << 8) << 7;
    reversed |= (nonreversed & 1 << 7) << 8;
    reversed |= (nonreversed & 1 << 6) << 9;
    reversed |= (nonreversed & 1 << 5) << 10;
    reversed |= (nonreversed & 1 << 4) << 11;
    reversed |= (nonreversed & 1 << 3) << 12;
    reversed |= (nonreversed & 1 << 2) << 13;
    reversed |= (nonreversed & 1 << 1) << 14;
    reversed |= (nonreversed & 1 << 0) << 15;
    return reversed;
}

I need to bit-reverse uint16_t, so I wrote this function that reads bit by bit the original uint_t, and writes them to another uint, but in reverse bit position. The problem is, when I pass integer larger than 0x00 (0), the function returns maximum value of uint16_t.

I'm beginner in c bitwise operations.

Thank you for replies.

EDIT: The "Best Algorithm for Bit Reversal ( from MSB->LSB to LSB->MSB) in C" uses 32bit ints!

Answer 1

I think you mean the following :)

#include <stdio.h>
#include <stdint.h>

uint16_t ReverseInt16( uint16_t nonreversed ) 
{
    uint16_t reversed = 0;

    for ( uint16_t i = 0; i < 16; i++ )
    {
        reversed |= ( nonreversed >> ( 16 - i - 1 ) & 1 ) << i;
    }        

    return reversed;
}

int main( void )
{
    uint16_t x = 0x1234;

    printf( "%x\n", x );
    printf( "%x\n", ReverseInt16( x ) );

}    

The program output is

1234
2c48

As for your code then it does not reverse bits. For example, consider statement

reversed |= (nonreversed & 1 << 15) << 0;

1 << 15 gives in hexadecimal notation 8000

then for example if the MSB is set then (nonreversed & 1 << 15) gives also 8000. You need to shift the result right

reversed |= (nonreversed & 1 << 15) >> 15;

but you are trying to shift it left

reversed |= (nonreversed & 1 << 15) << 0;