download: Infer track and exercise slug from CWD

If I'm in the `exercism/c` directory and invoke:

    exercism download --exercise hello-world

I would like `--track c` to be inferred.

If I'm in the `exercism/c/hello-world` directory and invoke without
arguments:

    exercism download

I would like both `--track c` and `--exercise hello-world` to be
inferred.

cmd/download.go

@@ -162,6 +162,19 @@ func newDownload(flags *pflag.FlagSet, usrCfg *viper.Viper) (*download, error) {
 	d.apibaseurl = usrCfg.GetString("apibaseurl")
 	d.workspace = usrCfg.GetString("workspace")
 
+	if d.uuid == "" {
+		if d.slug == "" {
+			if _, slug, ok := trackAndSlugFromCwd(d.workspace); ok && slug != "" {
+				d.slug = slug
+			}
+		}
+		if d.track == "" && d.team == "" {
+			if track, _, ok := trackAndSlugFromCwd(d.workspace); ok {
+				d.track = track
+			}
+		}
+	}
+
 	if err = d.needsSlugXorUUID(); err != nil {
 		return nil, err
 	}
@@ -349,6 +362,43 @@ func (sf solutionFile) relativePath() string {
 	return filepath.FromSlash(file)
 }
 
+func trackAndSlugFromCwd(workspace string) (string, string, bool) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return "", "", false
+	}
+
+	if !strings.HasPrefix(cwd, workspace) {
+		return "", "", false
+	}
+
+	remaining := strings.TrimPrefix(cwd, workspace)
+	remaining = strings.TrimPrefix(remaining, os.PathSeparator)
+
+	components := strings.Split(remaining, os.PathSeparator)
+	if len(components) == 0 {
+		return "", "", false
+	}
+
+	// We have at least one component, possibly more.
+	// First is presumably the track, but check whether it actually is one.
+	track := components[0]
+	if !isTrack(track) {
+		return "", "", false
+	}
+
+	slug := ""
+	if len(components) >= 2 {
+		slug = components[1]
+	}
+
+	return track, slug, true
+}
+
+func isTrack(dir string) bool {
+	return dir != "users" && dir != "teams"
+}
+
 func setupDownloadFlags(flags *pflag.FlagSet) {
 	flags.StringP("uuid", "u", "", "the solution UUID")
 	flags.StringP("track", "t", "", "the track ID")

cmd/download_test.go

@@ -77,6 +77,79 @@ func TestDownloadWithoutFlags(t *testing.T) {
 	}
 }
 
+func TestDownloadInferringFlags(t *testing.T) {
+	co := newCapturedOutput()
+	co.override()
+	defer co.reset()
+
+	originalCwd, err := os.Getwd()
+	defer os.Chdir(originalCwd)
+	assert.NoError(t, err)
+
+	testCases := []struct {
+		cwd   string
+		flags map[string]string
+		ok    bool
+	}{
+		{
+			cwd:   "bogus-track",
+			flags: map[string]string{"exercise": "bogus-exercise"},
+			ok:    true,
+		},
+		{
+			cwd:   "bogus-track/bogus-exercise",
+			flags: nil,
+			ok:    true,
+		},
+		{
+			cwd:   "teams/bogus-team/bogus-track/bogus-exercise",
+			flags: nil,
+			ok:    false,
+		},
+		{
+			cwd:   "users/bogus-user/bogus-track/bogus-exercise",
+			flags: nil,
+			ok:    false,
+		},
+	}
+
+	for _, tc := range testCases {
+		tmpDir, err := ioutil.TempDir("", "download-infer")
+		defer os.RemoveAll(tmpDir)
+		assert.NoError(t, err)
+
+		subdir := filepath.Join(tmpDir, tc.cwd)
+		err = os.MkdirAll(subdir, 0755)
+		assert.NoError(t, err)
+		os.Chdir(subdir)
+
+		ts := fakeDownloadServer(strconv.FormatBool(true), "")
+		defer ts.Close()
+
+		v := viper.New()
+		v.Set("workspace", tmpDir)
+		v.Set("apibaseurl", ts.URL)
+		v.Set("token", "abc123")
+
+		cfg := config.Config{
+			UserViperConfig: v,
+		}
+		flags := pflag.NewFlagSet("fake", pflag.PanicOnError)
+		setupDownloadFlags(flags)
+		for name, value := range tc.flags {
+			flags.Set(name, value)
+		}
+
+		err = runDownload(cfg, flags, []string{})
+		if tc.ok {
+			assert.NoError(t, err)
+			assertDownloadedCorrectFiles(t, tmpDir)
+		} else {
+			assert.Error(t, err)
+		}
+	}
+}
+
 func TestSolutionFile(t *testing.T) {
 	testCases := []struct {
 		name, file, expectedPath, expectedURL string