logic.go

func gravatarHash(email string) []byte {
	return md5.Sum([]byte(email))[:]
}

func gravatarURL(hash [16]byte, size uint32) string {
	return fmt.Sprintf("https://www.gravatar.com/avatar/%x?s=%d", hash, size)
}

func gravatar(email string, size uint32) string {
	hash := gravatarHash(email)

	return gravatarURL(hash, size)
}

logic_test.go

import (
	"fmt"
	"github.com/stretchr/testify/assert"
	"testing"
)

func TestGravatar(t *testing.T) {
	var size uint32 = 10
	endpoint := "https://www.gravatar.com/avatar/cf38500a2cd3b6a2c8c1d4d8259e83f8?s=%v"
	email := "[email protected]"
	url := gravatar(email, size)
	expected := fmt.Sprintf(endpoint, size)

	assert.Equal(t, url, expected, "URLs are not the same.")
}