deploy: 06a1bdf735

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))])
+}
+
+//!-

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

@@ -0,0 +1,83 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package popcount_test
+
+import (
+	"testing"
+
+	"gopl.io/ch2/popcount"
+)
+
+// -- Alternative implementations --
+
+func BitCount(x uint64) int {
+	// Hacker's Delight, Figure 5-2.
+	x = x - ((x >> 1) & 0x5555555555555555)
+	x = (x & 0x3333333333333333) + ((x >> 2) & 0x3333333333333333)
+	x = (x + (x >> 4)) & 0x0f0f0f0f0f0f0f0f
+	x = x + (x >> 8)
+	x = x + (x >> 16)
+	x = x + (x >> 32)
+	return int(x & 0x7f)
+}
+
+func PopCountByClearing(x uint64) int {
+	n := 0
+	for x != 0 {
+		x = x & (x - 1) // clear rightmost non-zero bit
+		n++
+	}
+	return n
+}
+
+func PopCountByShifting(x uint64) int {
+	n := 0
+	for i := uint(0); i < 64; i++ {
+		if x&(1<<i) != 0 {
+			n++
+		}
+	}
+	return n
+}
+
+// -- Benchmarks --
+
+func BenchmarkPopCount(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		popcount.PopCount(0x1234567890ABCDEF)
+	}
+}
+
+func BenchmarkBitCount(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		BitCount(0x1234567890ABCDEF)
+	}
+}
+
+func BenchmarkPopCountByClearing(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		PopCountByClearing(0x1234567890ABCDEF)
+	}
+}
+
+func BenchmarkPopCountByShifting(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		PopCountByShifting(0x1234567890ABCDEF)
+	}
+}
+
+// 2.67GHz Xeon
+// $ go test -cpu=4 -bench=. gopl.io/ch2/popcount
+// BenchmarkPopCount-4                  200000000         6.30 ns/op
+// BenchmarkBitCount-4                  300000000         4.15 ns/op
+// BenchmarkPopCountByClearing-4        30000000         45.2 ns/op
+// BenchmarkPopCountByShifting-4        10000000        153 ns/op
+//
+// 2.5GHz Intel Core i5
+// $ go test -cpu=4 -bench=. gopl.io/ch2/popcount
+// testing: warning: no tests to run
+// BenchmarkPopCount-4                  200000000         7.52 ns/op
+// BenchmarkBitCount-4                  500000000         3.36 ns/op
+// BenchmarkPopCountByClearing-4        50000000         34.3 ns/op
+// BenchmarkPopCountByShifting-4        20000000        108 ns/op