refactor: move things to packages (#39)

internal/updater/changelog.go

@@ -0,0 +1,32 @@
+package updater
+
+import (
+	"fmt"
+	"regexp"
+)
+
+const (
+	ChangelogHeader = "# Changelog"
+	ChangelogFile   = "CHANGELOG.md"
+)
+
+var (
+	ChangelogUpdaterHeaderRegex = regexp.MustCompile(`^# Changelog\n`)
+)
+
+func Changelog(info ReleaseInfo) Updater {
+	return func(content string) (string, error) {
+		headerIndex := ChangelogUpdaterHeaderRegex.FindStringIndex(content)
+		if headerIndex == nil && len(content) != 0 {
+			return "", fmt.Errorf("unexpected format of CHANGELOG.md, header does not match")
+		}
+		if headerIndex != nil {
+			// Remove the header from the content
+			content = content[headerIndex[1]:]
+		}
+
+		content = ChangelogHeader + "\n\n" + info.ChangelogEntry + content
+
+		return content, nil
+	}
+}