Fix get latest panic on single entry changelog

internal/get/get.go

@@ -28,8 +28,10 @@ func parseChangelog(fileName string) (changelog.Changelog, error) {
 func changelogWithSingleEntry(entry entry.Entry, repoName, repoOwner string) changelog.Changelog {
 	// Isolate the entry
 	entry.Next = nil
-	entry.PrevTag = entry.Previous.Tag
-	entry.Previous = nil
+	if entry.Previous != nil {
+		entry.PrevTag = entry.Previous.Tag
+		entry.Previous = nil
+	}
 
 	cl := changelog.NewChangelog(repoOwner, repoName)
 	cl.Insert(entry)

internal/get/get_test.go

@@ -8,6 +8,7 @@ import (
 )
 
 var fileName string = "CHANGELOG.md"
+var singleEntryFileName string = "single_CHANGELOG.md"
 
 func TestGetAll(t *testing.T) {
 	cl, err := get.GetAll(fileName)
@@ -32,6 +33,18 @@ func TestGetLatest(t *testing.T) {
 	assert.Equal(t, "v0.13.0", cl.GetEntries()[0].PrevTag)
 }
 
+func TestGetLatestWithNoPrevious(t *testing.T) {
+	cl, err := get.GetLatest(singleEntryFileName)
+
+	// Should not error
+	assert.Nil(t, err)
+
+	// Should have 1 entry
+	count := len(cl.GetEntries())
+	assert.Equal(t, 1, count)
+	assert.Equal(t, "", cl.GetEntries()[0].PrevTag)
+}
+
 func TestGetVersionWithAValidVersion(t *testing.T) {
 	// Should not error when version is found
 	cl, err := get.GetVersion(fileName, "v0.9.0")

internal/get/single_CHANGELOG.md

@@ -0,0 +1,10 @@
+<!-- markdownlint-disable MD024 -->
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/) and this project adheres to [Semantic Versioning](http://semver.org).
+
+## [v0.1.0](https://github.com/chelnak/gh-changelog/tree/v0.1.0) - 2022-04-15
+
+[Full Changelog](https://github.com/chelnak/gh-changelog/compare/42d4c93b23eaf307c5f9712f4c62014fe38332bd...v0.1.0)