learngo/16-slices/exercises/24-fix-the-memory-leak/main.go

// Copyright © 2018 Inanc Gumus
// Learn Go Programming Course
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
//
// For more tutorials  : https://learngoprogramming.com
// In-person training  : https://www.linkedin.com/in/inancgumus/
// Follow me on twitter: https://twitter.com/inancgumus

package main

import (
	"fmt"
	"io/ioutil"

	"github.com/inancgumus/learngo/16-slices/exercises/24-fix-the-memory-leak/api"
)

// ---------------------------------------------------------
// EXERCISE: Fix the memory leak
//
//  WARNING
//
//    This is a very difficult exercise. You need to
//    do some research on your own to solve it. Please don't
//    get discouraged if you can't solve it yet.
//
//
//  GOAL
//
//    In this exercise, your goal is to reduce the memory
//    usage. To do that, you need to find and fix the memory
//    leak within `main()`.
//
//
//  PROBLEM
//
//    `main()` calls `api.Report()` that reports the current
//    memory usage.
//
//    After that, `main()` calls `api.Read()` that returns
//    a slice with 10 millions of elements. But you only need
//    the last 10 elements of the returned slice.
//
//
//  WHAT YOU NEED TO DO
//
//    You only need to change the code in `main()`. Please
//    do not touch the code in `api/api.go`.
//
//
//  CURRENT OUTPUT
//
//    > Memory Usage: 113 KB
//
//    Last 10 elements: [...]
//
//    > Memory Usage: 65651 KB
//
//      + Before `api.Read()` call: It uses 113 KB of memory.
//
//      + After `api.Read()` call : It uses  65 MB of memory.
//
//      + This means that, `main()` never releases the memory.
//        This is the leak.
//
//      + Your goal is to release the unused memory. Remember,
//        you only need 10 elements but in the current code
//        below you have a slice with 10 millions of elements.
//
//
//  EXPECTED OUTPUT
//
//    > Memory Usage: 116 KB
//
//    Last 10 elements: [...]
//
//    > Memory Usage: 118 KB
//
//      + In the expected output, `main()` releases the memory.
//
//        It no longer uses 65 MB of memory. Instead, it only
//        uses 118 KB of memory. That's why the second
//        `api.Report()` call reports 118 KB.
//
//
//  ADDITIONAL NOTE
//
//    Memory leak means: Your program is using unnecessary
//    computer memory. It doesn't release memory that is
//    no longer needed.
//
//    See this for more information:
//    https://en.wikipedia.org/wiki/Memory_leak
//
//
//  HINTS
//
//    Check out `hints.md` file if you get stuck.
//
// ---------------------------------------------------------

func main() {
	// reports the initial memory usage
	api.Report()

	// returns a slice with 10 million elements.
	// it allocates 65 MB of memory space.
	millions := api.Read()

	// -----------------------------------------------------
	// ✪ ONLY CHANGE THE CODE IN THIS AREA ✪

	last10 := millions[len(millions)-10:]

	fmt.Printf("\nLast 10 elements: %d\n\n", last10)

	// ✪ ONLY CHANGE THE CODE IN THIS AREA ✪
	// -----------------------------------------------------

	api.Report()

	// don't worry about this code.
	fmt.Fprintln(ioutil.Discard, millions[0])
}