 artemkorsakov's solution

to Grains in the Go Track

Published at Mar 16 2019 · 3 comments
Instructions
Test suite
Solution

Note:

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.

Source

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.

cases_test.go

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,
},
}

grains_test.go

package grains

import (
"testing"
)

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 {
Square(test.input)
}

}
}

func BenchmarkTotal(b *testing.B) {
for i := 0; i < b.N; i++ {
Total()
}
}
package grains

import (
"errors"
)

// Square returns the count of grains in the given field.
func Square(input int) (uint64, error) {
if input < 1 || input > 64 {
return 0, errors.New("input must be between 1 and 64")
}
return 1 << uint(input-1), nil
}

// Total returns the sum of all grains.
func Total() uint64 {
return 1<<64 - 1
}

Find this solution interesting? Ask the author a question to learn more.  Solution Author
commented 213 days ago

Hi, Shahid4183!

Please, sorry for my English, I speak English a little.

The count of grains in the field with number N is Math.pow(2, N-1). The operator "<<" shifts the bit to the right. This is the same as multiplying by 2. It means that Math.pow(2, N-1) is the same as 1 << N-1. It is the first part.

The sum of all grains is the same as sum of Math.pow(2, 0), Math.pow(2, 1), Math.pow(2, 2) ... Math.pow(2, 63). The sum of the powers of 2 from 0 to N-1 is the same as the Nth power of 2 minus 1. It means that the sum of all grains is the same as Math.pow(2, 64) - 1. Great! Thanks a lot for your quick response!

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

• What compromises have been made?
• Are there new concepts here that you could read more about to improve your understanding?