1 files changed, 43 insertions, 0 deletions
diff --git a/ch2/2-09_bitcount.c b/ch2/2-09_bitcount.c
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+++ b/ch2/2-09_bitcount.c
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+#include <stdio.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 2-9: In a two's complement number system, x &= (x-1) deletes
+ * the rightmost 1-bit in x. Explain why. Use this observation to write a
+ * faster version of bitcount.
+ *
+ * Answer: Subtracting 1 from a number reverses the rightmost 1 bit and
+ * replaces all lower-order bits to 0. So for example:
+ *
+ * 110100 (52) minus 1 is
+ * 110011 (51)
+ *
+ * The & operator only masks bits off (makes them zero) if that bit in the
+ * second operand is also zero. That means...
+ *
+ * 110100 (52) &
+ * 110011 (51) is
+ * 110000
+ *
+ * Why? Because the 2nd operand (51) is only masking off two fields, the 3rd
+ * and 4th (which are zeros). The rightmost 1-bit in the 1st operand (52) is
+ * in that mask, so it goes poof.
+ */
+
+unsigned bitcount(unsigned x) {
+	int count = 0;
+	while (x != 0) {
+		x &= (x - 1);
+		count++;
+	}
+	return count;
+}
+
+int main() {
+	int i;
+	unsigned test[4] = { 51, 65535, 256, 10834 };
+	for (i = 0; i < 4; i++) {
+		printf("%6u has %2d ones in its binary representation.\n", test[i], bitcount(test[i]));
+	}
+	return 0;
+}