Implement checkout

commands/checkout.go

 package commands
 
 import (
+	"fmt"
 	"github.com/jingweno/gh/git"
+	"github.com/jingweno/gh/github"
 	"github.com/jingweno/gh/utils"
+	"regexp"
 )
 
 var cmdCheckout = &Command{
@@ -20,14 +23,54 @@ var cmdCheckout = &Command{
   $ gh checkout https://github.com/jingweno/gh/pull/73 custom-branch-name
 **/
 func checkout(command *Command, args []string) {
+	var err error
 	if len(args) > 0 {
-		args = transformCheckoutArgs(args)
+		args, err = transformCheckoutArgs(args)
+		utils.Check(err)
 	}
 
-  err := git.ExecCheckout(args)
+	err = git.ExecCheckout(args)
 	utils.Check(err)
 }
 
-func transformCheckoutArgs(args []string) []string {
-  return nil
+func transformCheckoutArgs(args []string) ([]string, error) {
+	id := parsePullRequestId(args[0])
+	if id != "" {
+		newArgs, url := removeItem(args, 0)
+		gh := github.New()
+		pullRequest, err := gh.PullRequest(id)
+		if err != nil {
+			return nil, err
+		}
+
+		user := pullRequest.User.Login
+		branch := pullRequest.Head.Ref
+		if pullRequest.Head.Repo.ID == 0 {
+			return nil, fmt.Errorf("%s's fork is not available anymore", user)
+		}
+		git.AddRemoteWithTrack(branch, user, url)
+
+		trackedBranch := fmt.Sprintf("%s/%s", user, branch)
+		var newBranchName string
+		if len(newArgs) > 0 {
+			newArgs, newBranchName = removeItem(newArgs, 0)
+		} else {
+			newBranchName = fmt.Sprintf("%s-%s", user, branch)
+		}
+
+		newArgs = append(newArgs, "--track", "-B", newBranchName, trackedBranch)
+
+		return newArgs, nil
+	}
+
+	return args, nil
+}
+
+func parsePullRequestId(url string) string {
+	pullURLRegex := regexp.MustCompile("https://github\\.com/.+/.+/pull/(\\d+)")
+	if pullURLRegex.MatchString(url) {
+		return pullURLRegex.FindStringSubmatch(url)[1]
+	}
+
+	return ""
 }

commands/checkout_test.go

+package commands
+
+import (
+	"github.com/bmizerany/assert"
+	"testing"
+)
+
+func TestParsePullRequestId(t *testing.T) {
+	url := "https://github.com/jingweno/gh/pull/73"
+	assert.Equal(t, "73", parsePullRequestId(url))
+
+	url = "https://github.com/jingweno/gh/pull/"
+	assert.Equal(t, "", parsePullRequestId(url))
+}

git/git.go

@@ -123,6 +123,12 @@ func AddRemote(name, url string) error {
 	return err
 }
 
+func AddRemoteWithTrack(trackedBranch, name, url string) error {
+	_, err := execGitCmd([]string{"remote", "add", "-f", "-t", trackedBranch, name, url})
+
+	return err
+}
+
 func ExecCheckout(args []string) error {
 	cmdArgs := make([]string, 0)
 	cmdArgs = append(cmdArgs, "checkout")

github/github.go

@@ -18,6 +18,12 @@ type GitHub struct {
 	config  *Config
 }
 
+func (gh *GitHub) PullRequest(id string) (*octokat.PullRequest, error) {
+	client := gh.client()
+
+	return client.PullRequest(gh.repo(), id)
+}
+
 func (gh *GitHub) CreatePullRequest(base, head, title, body string) (string, error) {
 	client := gh.client()
 	params := octokat.PullRequestParams{base, head, title, body}