desmondoray / csapp-3e Goto Github PK

This code does not work properly when x contains most significant bit set to 1, e.g. 0xFFFFFFFF.
Maybe this solution might avoid this problem:

0. Main idea is to shift right and add 0x1 to set bit
1. Save if (x == 0) to return 0 in this case: unsigned add_bit = (x == 0);
2. After setting all right bits to 1, shift right: x >>= 1;
3. Add bit: return (x + add_bit);

So the code will look like this:
int leftmost_one(unsigned x)
{
unsigned add_bit = (x != 0);
x |= (x >> 1);
x |= (x >> 2);
x |= (x >> 4);
x |= (x >> 8);
x |= (x >> 16);
x >>= 1;
return (x + add_bit); // Add 1 if x != 0, 0 otherwise
}

Thanks!

B (c)
n should be 19/63

Line 4 is wrong.
It should be $5 * 2^{-16}$ and $1 * 2^{-10}$

/**
P 3-61
long cread(long *xp) {
return (xp ? *xp : 0);
}
Explain:
The key point is to avoid * and xp come together.
*/
long cread_alt(long xp) {
long a=0;
return *(xp? xp:a);
}

failed for test case: lower_one_mask(32) == 0xFFFFFFFF

long switch_prob(long x, long n) { long result = x; switch (n) { case 60: case 62: result = 8 * x; break; case 63: result = x >> 3; break; case 64: result = 15 * x; x = result; case 65: x *= x; default: result = x + 75; break; } return result; }