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to Grains in the Go Track

Published at Aug 05 2018 · 0 comments
Test suite


This exercise has changed since this solution was written.

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board. Two grains on the next. Four on the third, and so on.

There are 64 squares on a chessboard.

Write code that shows:

  • how many grains were on each square, and
  • the total number of grains

For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

  • Optimize for speed.
  • Optimize for readability.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

Running the tests

To run the tests run the command go test from within the exercise directory.

If the test suite contains benchmarks, you can run these with the --bench and --benchmem flags:

go test -v --bench . --benchmem

Keep in mind that each reviewer will run benchmarks on a different machine, with different specs, so the results from these benchmark tests may vary.

Further information

For more detailed information about the Go track, including how to get help if you're having trouble, please visit the exercism.io Go language page.


JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


package grains

// Source: exercism/problem-specifications
// Commit: f079c2d grains: Move input (square) to input object (#1191)
// Problem Specifications Version: 1.1.0

// returns the number of grains on the square
var squareTests = []struct {
	description string
	input       int
	expectedVal uint64
	expectError bool
		description: "1",
		input:       1,
		expectedVal: 1,
		description: "2",
		input:       2,
		expectedVal: 2,
		description: "3",
		input:       3,
		expectedVal: 4,
		description: "4",
		input:       4,
		expectedVal: 8,
		description: "16",
		input:       16,
		expectedVal: 32768,
		description: "32",
		input:       32,
		expectedVal: 2147483648,
		description: "64",
		input:       64,
		expectedVal: 9223372036854775808,
		description: "square 0 returns an error",
		input:       0,
		expectError: true,
		description: "negative square returns an error",
		input:       -1,
		expectError: true,
		description: "square greater than 64 returns an error",
		input:       65,
		expectError: true,


package grains

import (

func TestSquare(t *testing.T) {
	for _, test := range squareTests {
		actualVal, actualErr := Square(test.input)

		// check actualVal only if no error expected
		if !test.expectError && actualVal != test.expectedVal {
			t.Fatalf("FAIL: %s\nSquare(%d) expected %d, Actual %d", test.description, test.input, test.expectedVal, actualVal)

		// if we expect an error and there isn't one
		if test.expectError && actualErr == nil {
			t.Fatalf("FAIL: %s\nSquare(%d) expected an error, but error is nil", test.description, test.input)
		// if we don't expect an error and there is one
		if !test.expectError && actualErr != nil {
			var _ error = actualErr
			t.Fatalf("FAIL: %s\nSquare(%d) expected no error, but error is: %s", test.description, test.input, actualErr)
		t.Logf("PASS: %s", test.description)

func TestTotal(t *testing.T) {
	var expected uint64 = 18446744073709551615
	if actual := Total(); actual != expected {
		t.Errorf("Total() expected %d, Actual %d", expected, actual)

func BenchmarkSquare(b *testing.B) {

	for i := 0; i < b.N; i++ {

		for _, test := range squareTests {


func BenchmarkTotal(b *testing.B) {
	for i := 0; i < b.N; i++ {
package grains help compute the number of grains on a chessboard (64 squares).
The 1st square has 1 grains, each subsequent squares has twice the amount of the previous.

package grains

import "errors"

const (
	noOfCells = 64

// Square returns the numer of grains in the n-th square.
func Square(n int) (uint64, error) {
	if !(1 <= n && n <= 64) {
		return 0, errors.New("invalid args: must be in [1, 64]")

	return 1 << uint64(n-1), nil

// Total returns the total number of grains on the board.
func Total() uint64 {
	var total uint64 = 0
	for i := 1; i <= noOfCells; i++ {
		total |= 1 << uint64(i-1)

	return total

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