calculate sum use all cpu core

sum.go

package main

import (
	"fmt"
	"runtime"
)

func Sum(start, end int) chan int {
	c := make(chan int)
	s := 0
	go func() {
		for i := start; i <= end; i ++ {
			s += i
		}
		c <- s
		close(c)
	}()
	return c
}

func SumAll(n int) int {
	cpuNum := runtime.GOMAXPROCS(0)

	fmt.Printf("cpuNum : %+v\n", cpuNum)

	step := n / cpuNum
	if step == 0 {
		cpuNum = n
		step = 1
	}

	resChan := make(chan int)
	for i := 0; i < cpuNum; i ++ {
		go func(i int) {
			start := i * step + 1
			var end int
			if i == cpuNum - 1 {
				end = n
			} else {
				end = (i+1) * step
			}
			resChan <- <- Sum(start, end)
		}(i)
	}

	result := 0
	for i := 0; i < cpuNum; i ++ {
		result += <-resChan
	}
	close(resChan)

	return result
}

sum_test.go

package main

import "testing"

func TestSum(t *testing.T) {
	res := <- Sum(0, 1)

	need := 1
	if res != need {
		t.Errorf("res should be %+v", need)
	}


	res = <- Sum(1, 2)
	need = 3
	if res != need {
		t.Errorf("res should be %+v", need)
	}
}


func TestSumAll(t *testing.T) {
	res := SumAll(1)
	need := 1
	if res != need {
		t.Errorf("res should be %+v, got %+v", need, res)
	}


	res = SumAll(2)
	need = 3
	if res != need {
		t.Errorf("res should be %+v, got %+v", need, res)
	}

	res = SumAll(3)
	need = 6
	if res != need {
		t.Errorf("res should be %+v, got %+v", need, res)
	}

	res = SumAll(10)
	need = 55
	if res != need {
		t.Errorf("res should be %+v, got %+v", need, res)
	}

	res = SumAll(100)
	need = 5050
	if res != need {
		t.Errorf("res should be %+v, got %+v", need, res)
	}
}