good good study, day day up!

vendor/gopl.io/ch2/popcount/main.go

@@ -0,0 +1,31 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 45.
+
+// (Package doc comment intentionally malformed to demonstrate golint.)
+//!+
+package popcount
+
+// pc[i] is the population count of i.
+var pc [256]byte
+
+func init() {
+	for i := range pc {
+		pc[i] = pc[i/2] + byte(i&1)
+	}
+}
+
+// PopCount returns the population count (number of set bits) of x.
+func PopCount(x uint64) int {
+	return int(pc[byte(x>>(0*8))] +
+		pc[byte(x>>(1*8))] +
+		pc[byte(x>>(2*8))] +
+		pc[byte(x>>(3*8))] +
+		pc[byte(x>>(4*8))] +
+		pc[byte(x>>(5*8))] +
+		pc[byte(x>>(6*8))] +
+		pc[byte(x>>(7*8))])
+}
+
+//!-