Solution to https://github.com/JDUcraft/go-dependency-injection

compute-weeks-left.go

package cmd

import (
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
)

type TimeTraveler struct {
	*http.Client
	Source string
}

func NewDefaultTimeTraveler() *TimeTraveler {
	return &TimeTraveler{
		Client: http.DefaultClient,
		Source: "https://worldtimeapi.org/api/timezone/Europe/Paris",
	}
}

func (t *TimeTraveler) GetCurrentTime() (*TimeResponse, error) {
	res, err := t.Get(t.Source)
	if err != nil || res == nil {
		return nil, err
	}
	defer res.Body.Close()
	if res.StatusCode != http.StatusOK {
		return nil, errors.New(res.Status)
	}
	dec := json.NewDecoder(res.Body)
	var tr TimeResponse
	err = dec.Decode(&tr)
	if err != nil {
		return nil, err
	}
	return &tr, nil
}

func ComputeWeeksLeftBeforeNextYear(t *TimeTraveler) (weeksLeft string, err error) {
	tr, err := t.GetCurrentTime()
	if err != nil {
		return "Read failed", err
	}
	return fmt.Sprintf("~ %d weeks left this year", 52-tr.WeekNumber), err
}

type TimeResponse struct {
	WeekNumber int `json:"week_number"`
}

compute-weeks-left_test.go

package cmd

import (
	"encoding/json"
	"net/http"
	"net/http/httptest"
	"testing"
)

func TestComputeWeeksLeftBeforeNextYear(t *testing.T) {
	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		enc := json.NewEncoder(w)
		enc.Encode(&TimeResponse{
			WeekNumber: 52 - 46,
		})
	}))
	defer ts.Close()
	weeksLeft, _ := ComputeWeeksLeftBeforeNextYear(&TimeTraveler{
		Client: http.DefaultClient,
		Source: ts.URL,
	})
	if weeksLeft != "~ 46 weeks left this year" {
		t.Errorf("\nexpected: [~ 46 weeks left this year]\nreceived: [%s]", weeksLeft)
	}
}

main.go

package main

import (
	"fmt"
	"main/cmd"
	"os"
)

func main() {
	weeksLeft, err := cmd.ComputeWeeksLeftBeforeNextYear(cmd.NewDefaultTimeTraveler())
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%s", weeksLeft)
}