feat: generate compare links for changelog references

internal/vociferate/vociferate.go

@@ -9,6 +9,7 @@ package vociferate
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strconv"
@@ -246,7 +247,7 @@ func updateChangelog(rootDir, version, releaseDate, changelogPath string) error
 	updated := text[:afterHeader] + "\n" + defaultUnreleasedTemplate + "\n" + newSection + text[nextSectionStart:]
 	repoURL, ok := deriveRepositoryURL(rootDir)
 	if ok {
-		updated = addChangelogLinks(updated, repoURL)
+		updated = addChangelogLinks(updated, repoURL, rootDir)
 	}
 	if err := os.WriteFile(path, []byte(updated), 0o644); err != nil {
 		return fmt.Errorf("write changelog: %w", err)
@@ -426,7 +427,7 @@ func normalizeRepoURL(remoteURL string) (string, bool) {
 	return "", false
 }
 
-func addChangelogLinks(text, repoURL string) string {
+func addChangelogLinks(text, repoURL, rootDir string) string {
 	repoURL = strings.TrimSuffix(strings.TrimSpace(repoURL), "/")
 	if repoURL == "" {
 		return text
@@ -456,16 +457,60 @@ func addChangelogLinks(text, repoURL string) string {
 	text = strings.Join(lines[:cutAt], "\n") + "\n"
 
 	// Build and append reference link definitions.
-	linkDefs := []string{fmt.Sprintf("[Unreleased]: %s/src/branch/main", repoURL)}
+	releasedMatches := releasedSectionRe.FindAllStringSubmatch(text, -1)
-	for _, match := range releasedSectionRe.FindAllStringSubmatch(text, -1) {
+	releasedVersions := make([]string, 0, len(releasedMatches))
+	for _, match := range releasedMatches {
 		if len(match) >= 2 {
-			linkDefs = append(linkDefs, fmt.Sprintf("[%s]: %s/releases/tag/v%s", match[1], repoURL, match[1]))
+			releasedVersions = append(releasedVersions, match[1])
 		}
 	}
 
+	linkDefs := make([]string, 0, len(releasedVersions)+1)
+	if len(releasedVersions) > 0 {
+		latest := releasedVersions[0]
+		linkDefs = append(linkDefs, fmt.Sprintf("[Unreleased]: %s/compare/v%s...main", repoURL, latest))
+	} else {
+		linkDefs = append(linkDefs, fmt.Sprintf("[Unreleased]: %s/src/branch/main", repoURL))
+	}
+
+	firstCommitShort, hasFirstCommit := firstCommitShortHash(rootDir)
+	for i, version := range releasedVersions {
+		if i+1 < len(releasedVersions) {
+			previousVersion := releasedVersions[i+1]
+			linkDefs = append(linkDefs, fmt.Sprintf("[%s]: %s/compare/v%s...v%s", version, repoURL, previousVersion, version))
+			continue
+		}
+
+		if hasFirstCommit {
+			linkDefs = append(linkDefs, fmt.Sprintf("[%s]: %s/compare/%s...v%s", version, repoURL, firstCommitShort, version))
+			continue
+		}
+
+		linkDefs = append(linkDefs, fmt.Sprintf("[%s]: %s/compare/v%s...main", version, repoURL, version))
+	}
+
 	return strings.TrimRight(text, "\n") + "\n\n" + strings.Join(linkDefs, "\n") + "\n"
 }
 
+func firstCommitShortHash(rootDir string) (string, bool) {
+	command := exec.Command("git", "-C", rootDir, "rev-list", "--max-parents=0", "--abbrev-commit", "HEAD")
+	output, err := command.Output()
+	if err != nil {
+		return "", false
+	}
+
+	commit := strings.TrimSpace(string(output))
+	if commit == "" {
+		return "", false
+	}
+
+	if strings.Contains(commit, "\n") {
+		commit = strings.SplitN(commit, "\n", 2)[0]
+	}
+
+	return commit, true
+}
+
 func parseSemver(version string) (semver, error) {
 	parts := strings.Split(strings.TrimSpace(version), ".")
 	if len(parts) != 3 {