Database System Concepts
7th Edition
ISBN: 9780078022159
Author: Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher: McGraw-Hill Education
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This code is correct in reversing a 32-bit integer when looking at the binary form of the integer. But it uses 54 operations instead of 45. We are strictly limited to using only !, +, ^, &, <<, >>, |, ~. We cannot use constants that are anything other than 0x0 to 0xFF. We also cannot use loops or conditionals, in other words "straight line" code. I am unsure where I can simplify this code in order to get it into the required maximum number of operations (45).
Will leave good review!
int bitReverse(int x) {
int result, mask1, mask2, mask4, mask8, mask16, min;
min = 1 << 31;
mask16 = ((0xff << 8) | 0xff);
mask8 = (mask16 << 8) ^ mask16;
mask4 = (mask8 << 4) ^ mask8;
mask2 = (mask4 << 2) ^ mask4;
mask1 = (mask2 << 1) ^ mask2;
mask1 = ~mask1;
mask2 = ~mask2;
mask4 = ~mask4;
mask8 = ~mask8;
mask16 = ~mask16;
result = ((x << 16) & mask16) | (((x & mask16) >> 16) & (~mask16));
result = ((result << 8) & mask8) | (((result & mask8) >> 8) & (~(min >> 7)));
result = ((result << 4) & mask4) | (((result & mask4) >> 4) & (~(min >> 3)));
result = ((result << 2) & mask2) | (((result & mask2) >> 2) & (~(min >> 1)));
result = ((result << 1) & mask1) | (((result & mask1) >> 1) & (~min));
return result;
}
int result, mask1, mask2, mask4, mask8, mask16, min;
min = 1 << 31;
mask16 = ((0xff << 8) | 0xff);
mask8 = (mask16 << 8) ^ mask16;
mask4 = (mask8 << 4) ^ mask8;
mask2 = (mask4 << 2) ^ mask4;
mask1 = (mask2 << 1) ^ mask2;
mask1 = ~mask1;
mask2 = ~mask2;
mask4 = ~mask4;
mask8 = ~mask8;
mask16 = ~mask16;
result = ((result << 8) & mask8) | (((result & mask8) >> 8) & (~(min >> 7)));
result = ((result << 4) & mask4) | (((result & mask4) >> 4) & (~(min >> 3)));
result = ((result << 2) & mask2) | (((result & mask2) >> 2) & (~(min >> 1)));
result = ((result << 1) & mask1) | (((result & mask1) >> 1) & (~min));
return result;
}
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