45 lines
2.4 KiB
Plaintext
45 lines
2.4 KiB
Plaintext
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From http://www.exploringbinary.com/ten-ways-to-check-if-an-integer-is-a-power-of-two-in-c/
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8. Shift Right
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This function is the equivalent of the Divide by Two algorithm. Dividing a binary integer by 2 is the same as shifting it right one bit, and checking whether a binary integer is odd is the same as checking if its least significant bit is 1.
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int isPowerOfTwo (unsigned int x)
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{
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while (((x & 1) == 0) && x > 1) /* While x is even and > 1 */
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x >>= 1;
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return (x == 1);
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}
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The only way the statement “x == 1” becomes true is when there is only one 1 bit in x to start with.
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9. Decrement and Compare
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This function is a common one-liner you’ll find on the Web, and is how the check is implemented in malloc.c in the GNU C Library (glibc).
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int isPowerOfTwo (unsigned int x)
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{
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return ((x != 0) && !(x & (x - 1)));
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}
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There are two halves to the expression: x != 0 and !(x & (x – 1)). The first half makes 0 a special case, since the second half only works correctly for positive numbers. The second half — the interesting part of the expression — is true when x is a power of two and false when x is not. Let’s see why.
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Let n be the position of the leftmost 1 bit if x. If x is a power of two, its lone 1 bit is in position n. This means x – 1 has a 0 in position n. To see why, recall how binary subtraction works. When subtracting 1 from x, the borrow propagates all the way to position n; bit n becomes 0 and all lower bits become 1. Now, since x has no 1 bits in common with x – 1, x & (x – 1) is 0, and !(x & (x – 1)) is true.
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Here are some examples (I’ll use 8-bit unsigned integers to keep things manageable):
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Decrement and Compare, when x is a power of two
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x x – 1 x & (x – 1)
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00000001 00000000 00000000
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00000100 00000011 00000000
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00010000 00001111 00000000
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If x is not a power of two, x – 1 has a 1 in position n. This is because the borrow will not propagate to position n. Subtraction borrows from the lowest 1 bit, which by virtue of x not being a power of two, is before position n. The lowest 1 bit is like a firewall that prevents the borrow from reaching position n. Since x and x – 1 have at least one 1 bit in common — at position n — x & (x – 1) is nonzero, and !(x & (x – 1)) is false.
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Here are some examples:
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Decrement and Compare, when x is a NOT power of two
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x x – 1 x & (x – 1)
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00000011 00000010 00000010
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00000110 00000101 00000100
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00001011 00001010 00001010
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00011100 00011011 00011000
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