Merge pull request #1 from yourbasic/tip

Tip

README.md

@@ -26,7 +26,7 @@ There is an online reference for the package at
 * Version numbers adhere to [semantic versioning][sv].
 
 The only accepted reason to modify the API of this package
-is to handle bug fixes that can't be resolved in any other
+is to handle issues that can't be resolved in any other
 reasonable way.
 
 Stefan Nilsson – [korthaj](https://github.com/korthaj)

bench_test.go

@@ -4,8 +4,9 @@ import (
 	"testing"
 )
 
+const n = 1000
+
 func BenchmarkNew(b *testing.B) {
-	n := 1000
 	b.StopTimer()
 	a := make([]int64, n)
 	b.StartTimer()
@@ -14,8 +15,16 @@ func BenchmarkNew(b *testing.B) {
 	}
 }
 
+func BenchmarkAppend(b *testing.B) {
+	l := new(List)
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < n; j++ {
+			l.Append(int64(j))
+		}
+	}
+}
+
 func BenchmarkSum(b *testing.B) {
-	n := 1000
 	b.StopTimer()
 	a := make([]int64, n)
 	l := New(a...)
@@ -27,18 +36,38 @@ func BenchmarkSum(b *testing.B) {
 	}
 }
 
-func BenchmarkSumSlice(b *testing.B) {
-	n := 1000
+func BenchmarkGet(b *testing.B) {
+	b.StopTimer()
+	a := make([]int64, n)
+	l := New(a...)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < n; j++ {
+			_ = l.Get(j)
+		}
+	}
+}
+
+func BenchmarkSet(b *testing.B) {
+	b.StopTimer()
+	a := make([]int64, n)
+	l := New(a...)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < n; j++ {
+			l.Set(j, int64(j))
+		}
+	}
+}
+
+func BenchmarkAdd(b *testing.B) {
 	b.StopTimer()
 	a := make([]int64, n)
+	l := New(a...)
 	b.StartTimer()
 	for i := 0; i < b.N; i++ {
 		for j := 0; j < n; j++ {
-			var sum int64
-			for k := 0; k < j; k++ {
-				sum += a[k]
-			}
-			_ = sum
+			l.Add(j, int64(j))
 		}
 	}
 }

list.go

@@ -13,7 +13,8 @@
 //
 package fenwick
 
-// List represents a Fenwick tree.
+// List represents a list of numbers with support for efficient
+// prefix sum computation. The zero value is an empty list.
 type List struct {
 	// The tree slice stores range sums of an underlying array t.
 	// To compute the prefix sum t[0] + t[1] + t[k-1], add elements
@@ -50,12 +51,22 @@ func (l *List) Len() int {
 
 // Get returns the element at index i.
 func (l *List) Get(i int) int64 {
-	return l.SumRange(i, i+1)
+	sum := l.tree[i]
+	j := i + 1
+	j -= j & -j
+	for i > j {
+		sum -= l.tree[i-1]
+		i -= i & -i
+	}
+	return sum
 }
 
 // Set sets the element at index i to n.
 func (l *List) Set(i int, n int64) {
-	l.Add(i, n-l.Get(i))
+	n -= l.Get(i)
+	for len := len(l.tree); i < len; i |= i + 1 {
+		l.tree[i] += n
+	}
 }
 
 // Add adds n to the element at index i.

list_test.go

@@ -5,13 +5,12 @@ import (
 )
 
 func TestGet(t *testing.T) {
-	n := 50
+	n := 100
 	a := make([]int64, n)
-	l := New()
 	for i := range a {
 		a[i] = int64(i)
 	}
-	l = New(a...)
+	l := New(a...)
 	for i := range a {
 		if a[i] != l.Get(i) {
 			t.Errorf("Get(%d) = %d; want %d", i, l.Get(i), a[i])
@@ -20,13 +19,12 @@ func TestGet(t *testing.T) {
 }
 
 func TestSet(t *testing.T) {
-	n := 50
+	n := 100
 	a := make([]int64, n)
-	l := New()
 	for i := range a {
 		a[i] = int64(i)
 	}
-	l = New(a...)
+	l := New(a...)
 	for i := range a {
 		l.Set(i, 100)
 		if l.Get(i) != 100 {
@@ -36,13 +34,12 @@ func TestSet(t *testing.T) {
 }
 
 func TestAdd(t *testing.T) {
-	n := 50
+	n := 100
 	a := make([]int64, n)
-	l := New()
 	for i := range a {
 		a[i] = int64(i)
 	}
-	l = New(a...)
+	l := New(a...)
 	for i := range a {
 		l.Add(i, 100)
 		if l.Get(i) != a[i]+100 {
@@ -52,13 +49,12 @@ func TestAdd(t *testing.T) {
 }
 
 func TestSum(t *testing.T) {
-	n := 50
+	n := 100
 	a := make([]int64, n)
-	l := New()
 	for i := range a {
 		a[i] = int64(i)
 	}
-	l = New(a...)
+	l := New(a...)
 	for i := range a {
 		var res int64
 		for j := 0; j < i; j++ {
@@ -71,13 +67,12 @@ func TestSum(t *testing.T) {
 }
 
 func TestSumRange(t *testing.T) {
-	n := 50
+	n := 100
 	a := make([]int64, n)
-	l := New()
 	for i := range a {
 		a[i] = int64(i)
 	}
-	l = New(a...)
+	l := New(a...)
 	for i := range a {
 		for j := i; j < n; j++ {
 			var res int64
@@ -92,9 +87,12 @@ func TestSumRange(t *testing.T) {
 }
 
 func TestAppend(t *testing.T) {
-	n := 50
+	n := 100
 	a := make([]int64, n)
-	l := New()
+	l := new(List)
+	if l.Len() != 0 {
+		t.Errorf("Len() = %d; want %d", l.Len(), 0)
+	}
 	for i := range a {
 		a[i] = int64(i)
 		l.Append(int64(i))