implement check command

check_command.go

@@ -0,0 +1,62 @@
+package resource
+
+import (
+	"errors"
+
+	"github.com/google/go-github/github"
+)
+
+type CheckCommand struct {
+	github GitHub
+}
+
+func NewCheckCommand(github GitHub) *CheckCommand {
+	return &CheckCommand{
+		github: github,
+	}
+}
+
+func (c *CheckCommand) Run(request CheckRequest) ([]Version, error) {
+	releases, err := c.github.ListReleases()
+	if err != nil {
+		return []Version{}, err
+	}
+
+	if len(releases) == 0 {
+		return []Version{}, errors.New("repository had no releases")
+	}
+
+	latestVersion := *releases[0].TagName
+
+	if request.Version.Tag == "" {
+		return []Version{
+			{Tag: latestVersion},
+		}, nil
+	}
+
+	if latestVersion == request.Version.Tag {
+		return []Version{}, nil
+	}
+
+	upToLatest := false
+	reversedVersions := []Version{}
+	for _, release := range reverse(releases) {
+		version := *release.TagName
+
+		if upToLatest {
+			reversedVersions = append(reversedVersions, Version{Tag: version})
+		} else {
+			upToLatest = request.Version.Tag == version
+		}
+	}
+
+	return reversedVersions, nil
+}
+
+func reverse(s []github.RepositoryRelease) []github.RepositoryRelease {
+	for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
+		s[i], s[j] = s[j], s[i]
+	}
+
+	return s
+}