service.go

package main

import (
	"errors"
	"time"

	"github.com/go-kit/kit/log"
	"golang.org/x/net/context"
)

type Service interface {
	Sum(ctx context.Context, a, b int) (int, error)
}

var (
	ErrTwoZeroes   = errors.New("can't sum two zeroes")
	ErrIntOverflow = errors.New("integer overflow")
)

func str2err(s string) error {
	if s == "" {
		return nil
	}
	return errors.New(s)
}

func err2str(err error) string {
	if err == nil {
		return ""
	}
	return err.Error()
}

func NewBasicService() Service {
	return basicService{}
}

type basicService struct{}

const (
	intMax = 1<<31 - 1
	intMin = -(intMax + 1)
)

func (s basicService) Sum(_ context.Context, a, b int) (int, error) {
	if a == 0 && b == 0 {
		return 0, ErrTwoZeroes
	}
	if (b > 0 && a > (intMax-b)) || (b < 0 && a < (intMin-b)) {
		return 0, ErrIntOverflow
	}
	return a + b, nil
}

type Middleware func(Service) Service

func ServiceLoggingMiddleware(logger log.Logger) Middleware {
	return func(next Service) Service {
		return serviceLoggingMiddleware{
			logger: logger,
			next:   next,
		}
	}
}

type serviceLoggingMiddleware struct {
	logger log.Logger
	next   Service
}

func (mw serviceLoggingMiddleware) Sum(ctx context.Context, a, b int) (v int, err error) {
	defer func(begin time.Time) {
		mw.logger.Log(
			"method", "Sum",
			"a", a, "b", b, "result", v, "error", err,
			"took", time.Since(begin),
		)
	}(time.Now())
	return mw.next.Sum(ctx, a, b)
}