diff --git a/catalog.go b/catalog.go
index 90c2c95..301eb42 100644
--- a/catalog.go
+++ b/catalog.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"hash/fnv"
 	"os"
+	"path/filepath"
 	"sort"
 	"strings"
 	"time"
@@ -29,12 +30,13 @@ func hashZoneName(zone string) string {
 	h := fnv.New32a()
 	h.Write([]byte(zone))
 	sum := h.Sum32()
-	b := make([]byte, 4)
-	b[0] = byte(sum >> 24)
-	b[1] = byte(sum >> 16)
-	b[2] = byte(sum >> 8)
-	b[3] = byte(sum)
-	return b32.EncodeToString(b)
+	b := [4]byte{
+		byte(sum >> 24),
+		byte(sum >> 16),
+		byte(sum >> 8),
+		byte(sum),
+	}
+	return b32.EncodeToString(b[:])
 }
 
 // generateCatalogZone builds the zone file content for a single catalog.
@@ -76,20 +78,22 @@ func generateCatalogZone(catName string, cfg *Config, members []ZoneEntry, seria
 		return sorted[i].Zone < sorted[j].Zone
 	})
 
-	// Check for hash collisions
-	hashToZone := make(map[string]string)
+	// Check for hash collisions and cache results
+	hashToZone := make(map[string]string, len(sorted))
+	zoneHash := make(map[string]string, len(sorted))
 	for _, entry := range sorted {
 		h := hashZoneName(entry.Zone)
 		if existing, ok := hashToZone[h]; ok && existing != entry.Zone {
-			return "", fmt.Errorf("error: %s:%d: hash collision between %s and %s in catalog %q",
+			return "", fmt.Errorf("%s:%d: hash collision between %s and %s in catalog %q",
 				entry.File, entry.Line, existing, entry.Zone, catName)
 		}
 		hashToZone[h] = entry.Zone
+		zoneHash[entry.Zone] = h
 	}
 
 	// Member records
 	for _, entry := range sorted {
-		h := hashZoneName(entry.Zone)
+		h := zoneHash[entry.Zone]
 
 		// PTR record
 		ptrOwner := fmt.Sprintf("%s.zones.%s", h, origin)
@@ -126,58 +130,48 @@ func generateCatalogZone(catName string, cfg *Config, members []ZoneEntry, seria
 	return strings.Join(records, "\n") + "\n", nil
 }
 
-// readExistingSerial reads an existing zone file and extracts the SOA serial.
-// Returns 0, nil if the file doesn't exist.
-func readExistingSerial(path string) (uint32, error) {
-	f, err := os.Open(path)
+// readExisting reads an existing zone file and returns its content and SOA serial.
+// Returns ("", 0, nil) if the file doesn't exist.
+func readExisting(path string) (string, uint32, error) {
+	data, err := os.ReadFile(path)
 	if os.IsNotExist(err) {
-		return 0, nil
+		return "", 0, nil
 	}
 	if err != nil {
-		return 0, fmt.Errorf("reading existing zone %s: %w", path, err)
+		return "", 0, fmt.Errorf("reading existing zone %s: %w", path, err)
 	}
-	defer f.Close()
+	content := string(data)
 
-	zp := dns.NewZoneParser(f, "", path)
+	zp := dns.NewZoneParser(strings.NewReader(content), "", path)
 	for rr, ok := zp.Next(); ok; rr, ok = zp.Next() {
 		if soa, ok := rr.(*dns.SOA); ok {
-			return soa.Serial, nil
+			return content, soa.Serial, nil
 		}
 	}
 	if err := zp.Err(); err != nil {
-		return 0, fmt.Errorf("parsing existing zone %s: %w", path, err)
+		return "", 0, fmt.Errorf("parsing existing zone %s: %w", path, err)
 	}
 
-	return 0, nil
+	return content, 0, nil
 }
 
-// readExistingContent reads the full content of an existing zone file.
-// Returns empty string if file doesn't exist.
-func readExistingContent(path string) (string, error) {
-	data, err := os.ReadFile(path)
-	if os.IsNotExist(err) {
-		return "", nil
-	}
-	if err != nil {
-		return "", err
-	}
-	return string(data), nil
+// dateBase returns the YYYYMMDD00 serial base for the given time.
+func dateBase(now time.Time) uint32 {
+	return uint32(now.Year())*1000000 +
+		uint32(now.Month())*10000 +
+		uint32(now.Day())*100
 }
 
 // defaultSerial returns a serial for today with sequence 01: YYYYMMDD01.
 func defaultSerial(now time.Time) uint32 {
-	return uint32(now.Year())*1000000 +
-		uint32(now.Month())*10000 +
-		uint32(now.Day())*100 + 1
+	return dateBase(now) + 1
 }
 
 // bumpSerial increments a serial. If same date, bumps the sequence number.
 // If different date, starts at YYYYMMDD01.
 // Returns error if sequence reaches 99 and needs another bump.
 func bumpSerial(old uint32, now time.Time) (uint32, error) {
-	todayBase := uint32(now.Year())*1000000 +
-		uint32(now.Month())*10000 +
-		uint32(now.Day())*100
+	todayBase := dateBase(now)
 
 	if old >= todayBase && old < todayBase+100 {
 		// Same date, bump sequence
@@ -196,10 +190,10 @@ func bumpSerial(old uint32, now time.Time) (uint32, error) {
 // Returns true if the file was written (changed), false if unchanged.
 func processCatalog(catName string, cfg *Config, members []ZoneEntry, outputDir string, now time.Time) (bool, error) {
 	catCfg := cfg.Catalogs[catName]
-	outputPath := outputDir + "/" + catCfg.Zone + "zone"
+	outputPath := filepath.Join(outputDir, catCfg.Zone+"zone")
 
-	// Read existing serial
-	oldSerial, err := readExistingSerial(outputPath)
+	// Read existing file (content + serial in one pass)
+	existing, oldSerial, err := readExisting(outputPath)
 	if err != nil {
 		return false, err
 	}
@@ -217,11 +211,6 @@ func processCatalog(catName string, cfg *Config, members []ZoneEntry, outputDir
 	}
 
 	// Compare with existing file
-	existing, err := readExistingContent(outputPath)
-	if err != nil {
-		return false, err
-	}
-
 	if content == existing {
 		return false, nil
 	}
diff --git a/catalog_test.go b/catalog_test.go
index 72414e9..5daf8a1 100644
--- a/catalog_test.go
+++ b/catalog_test.go
@@ -1,6 +1,8 @@
 package main
 
 import (
+	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -159,12 +161,12 @@ func TestGenerateCatalogZone(t *testing.T) {
 	aIdx := -1
 	bIdx := -1
 	for i, line := range lines {
-		if containsStr(line, "a.example.com.") {
+		if strings.Contains(line, "a.example.com.") {
 			if aIdx == -1 {
 				aIdx = i
 			}
 		}
-		if containsStr(line, "b.example.com.") {
+		if strings.Contains(line, "b.example.com.") {
 			if bIdx == -1 {
 				bIdx = i
 			}
@@ -180,7 +182,7 @@ func TestGenerateCatalogZone(t *testing.T) {
 	// a.example.com should have a group TXT
 	foundGroup := false
 	for _, line := range lines {
-		if containsStr(line, "group.") && containsStr(line, "\"mygroup\"") {
+		if strings.Contains(line, "group.") && strings.Contains(line, "\"mygroup\"") {
 			foundGroup = true
 		}
 	}
@@ -211,7 +213,7 @@ func TestGenerateCatalogZoneCOO(t *testing.T) {
 
 	foundCOO := false
 	for _, line := range splitLines(content) {
-		if containsStr(line, "coo.") && containsStr(line, "old.example.com.") {
+		if strings.Contains(line, "coo.") && strings.Contains(line, "old.example.com.") {
 			foundCOO = true
 		}
 	}
@@ -220,69 +222,51 @@ func TestGenerateCatalogZoneCOO(t *testing.T) {
 	}
 }
 
-func TestReadExistingSerial(t *testing.T) {
+func TestReadExisting(t *testing.T) {
 	t.Run("file does not exist", func(t *testing.T) {
-		serial, err := readExistingSerial("/nonexistent/path.zone")
+		content, serial, err := readExisting("/nonexistent/path.zone")
 		if err != nil {
 			t.Fatalf("unexpected error: %v", err)
 		}
 		if serial != 0 {
 			t.Errorf("serial = %d, want 0", serial)
 		}
+		if content != "" {
+			t.Errorf("content = %q, want empty", content)
+		}
 	})
 
 	t.Run("valid zone file", func(t *testing.T) {
 		dir := t.TempDir()
-		path := dir + "/test.zone"
-		content := "example.com. 0 IN SOA ns1.example.com. hostmaster.example.com. 2026030205 900 600 2147483646 0\nexample.com. 0 IN NS invalid.\n"
-		writeTestFile(t, dir, "test.zone", content)
+		zoneContent := "example.com. 0 IN SOA ns1.example.com. hostmaster.example.com. 2026030205 900 600 2147483646 0\nexample.com. 0 IN NS invalid.\n"
+		writeTestFile(t, dir, "test.zone", zoneContent)
 
-		serial, err := readExistingSerial(path)
+		content, serial, err := readExisting(filepath.Join(dir, "test.zone"))
 		if err != nil {
 			t.Fatalf("unexpected error: %v", err)
 		}
 		if serial != 2026030205 {
 			t.Errorf("serial = %d, want 2026030205", serial)
 		}
+		if content != zoneContent {
+			t.Errorf("content mismatch: got %q", content)
+		}
 	})
 
 	t.Run("zone file with no SOA", func(t *testing.T) {
 		dir := t.TempDir()
-		path := dir + "/test.zone"
-		content := "example.com. 0 IN NS invalid.\n"
-		writeTestFile(t, dir, "test.zone", content)
+		zoneContent := "example.com. 0 IN NS invalid.\n"
+		writeTestFile(t, dir, "test.zone", zoneContent)
 
-		serial, err := readExistingSerial(path)
+		content, serial, err := readExisting(filepath.Join(dir, "test.zone"))
 		if err != nil {
 			t.Fatalf("unexpected error: %v", err)
 		}
 		if serial != 0 {
 			t.Errorf("serial = %d, want 0 (no SOA found)", serial)
 		}
-	})
-}
-
-func TestReadExistingContent(t *testing.T) {
-	t.Run("file does not exist", func(t *testing.T) {
-		content, err := readExistingContent("/nonexistent/path.zone")
-		if err != nil {
-			t.Fatalf("unexpected error: %v", err)
-		}
-		if content != "" {
-			t.Errorf("content = %q, want empty", content)
-		}
-	})
-
-	t.Run("file exists", func(t *testing.T) {
-		dir := t.TempDir()
-		writeTestFile(t, dir, "test.zone", "hello\n")
-
-		content, err := readExistingContent(dir + "/test.zone")
-		if err != nil {
-			t.Fatalf("unexpected error: %v", err)
-		}
-		if content != "hello\n" {
-			t.Errorf("content = %q, want %q", content, "hello\n")
+		if content != zoneContent {
+			t.Errorf("content mismatch: got %q", content)
 		}
 	})
 }
diff --git a/helpers_test.go b/helpers_test.go
index a5bcc6a..d9ff4e6 100644
--- a/helpers_test.go
+++ b/helpers_test.go
@@ -24,7 +24,3 @@ func assertContains(t *testing.T, s, substr string) {
 		t.Errorf("expected %q to contain %q", s, substr)
 	}
 }
-
-func containsStr(s, substr string) bool {
-	return strings.Contains(s, substr)
-}
diff --git a/input.go b/input.go
index 8e5d7a4..650e5ec 100644
--- a/input.go
+++ b/input.go
@@ -54,7 +54,7 @@ func parseLine(line, file string, lineNum int) (ZoneEntry, error) {
 	// Split on whitespace first, then handle comma separation within tokens
 	tokens := tokenize(line)
 	if len(tokens) < 2 {
-		return ZoneEntry{}, fmt.Errorf("error: %s:%d: expected zone name followed by at least one catalog name", file, lineNum)
+		return ZoneEntry{}, fmt.Errorf("%s:%d: expected zone name followed by at least one catalog name", file, lineNum)
 	}
 
 	entry := ZoneEntry{
@@ -69,16 +69,16 @@ func parseLine(line, file string, lineNum int) (ZoneEntry, error) {
 			switch strings.ToLower(key) {
 			case "group":
 				if value == "" {
-					return ZoneEntry{}, fmt.Errorf("error: %s:%d: empty group value", file, lineNum)
+					return ZoneEntry{}, fmt.Errorf("%s:%d: empty group value", file, lineNum)
 				}
 				entry.Group = value
 			case "coo":
 				if value == "" {
-					return ZoneEntry{}, fmt.Errorf("error: %s:%d: empty coo value", file, lineNum)
+					return ZoneEntry{}, fmt.Errorf("%s:%d: empty coo value", file, lineNum)
 				}
 				entry.COO = normalizeFQDN(value)
 			default:
-				return ZoneEntry{}, fmt.Errorf("error: %s:%d: unknown property %q", file, lineNum, key)
+				return ZoneEntry{}, fmt.Errorf("%s:%d: unknown property %q", file, lineNum, key)
 			}
 		} else {
 			// Bare name = catalog assignment
@@ -87,7 +87,7 @@ func parseLine(line, file string, lineNum int) (ZoneEntry, error) {
 	}
 
 	if len(entry.Catalogs) == 0 {
-		return ZoneEntry{}, fmt.Errorf("error: %s:%d: no catalog assignment for zone %s", file, lineNum, entry.Zone)
+		return ZoneEntry{}, fmt.Errorf("%s:%d: no catalog assignment for zone %s", file, lineNum, entry.Zone)
 	}
 
 	return entry, nil
@@ -95,10 +95,7 @@ func parseLine(line, file string, lineNum int) (ZoneEntry, error) {
 
 // tokenize splits a line on whitespace and commas, stripping empty tokens.
 func tokenize(line string) []string {
-	// Replace commas with spaces, then split on whitespace
-	line = strings.ReplaceAll(line, ",", " ")
-	fields := strings.Fields(line)
-	return fields
+	return strings.Fields(strings.ReplaceAll(line, ",", " "))
 }
 
 func buildCatalogMembers(entries []ZoneEntry, cfg *Config) (CatalogMembers, error) {
@@ -110,14 +107,14 @@ func buildCatalogMembers(entries []ZoneEntry, cfg *Config) (CatalogMembers, erro
 	for _, entry := range entries {
 		for _, catName := range entry.Catalogs {
 			if _, ok := cfg.Catalogs[catName]; !ok {
-				return nil, fmt.Errorf("error: %s:%d: unknown catalog %q", entry.File, entry.Line, catName)
+				return nil, fmt.Errorf("%s:%d: unknown catalog %q", entry.File, entry.Line, catName)
 			}
 
 			if seen[catName] == nil {
 				seen[catName] = make(map[string]int)
 			}
 			if prevLine, dup := seen[catName][entry.Zone]; dup {
-				return nil, fmt.Errorf("error: %s:%d: zone %s already assigned to catalog %q at line %d",
+				return nil, fmt.Errorf("%s:%d: zone %s already assigned to catalog %q at line %d",
 					entry.File, entry.Line, entry.Zone, catName, prevLine)
 			}
 			seen[catName][entry.Zone] = entry.Line
diff --git a/input_test.go b/input_test.go
index 5a46e3e..a0c3ca7 100644
--- a/input_test.go
+++ b/input_test.go
@@ -1,7 +1,6 @@
 package main
 
 import (
-	"os"
 	"path/filepath"
 	"testing"
 )
@@ -206,9 +205,7 @@ zone.example.org    catalog1, catalog2
 
 test.example.net    catalog1, group=internal
 `
-	if err := os.WriteFile(inputPath, []byte(content), 0o644); err != nil {
-		t.Fatal(err)
-	}
+	writeTestFile(t, dir, "zones.txt", content)
 
 	members, err := parseInput(inputPath, cfg)
 	if err != nil {
@@ -240,9 +237,7 @@ func TestParseInputErrors(t *testing.T) {
 	t.Run("invalid line in input", func(t *testing.T) {
 		dir := t.TempDir()
 		path := filepath.Join(dir, "zones.txt")
-		if err := os.WriteFile(path, []byte("zone-with-no-catalog\n"), 0o644); err != nil {
-			t.Fatal(err)
-		}
+		writeTestFile(t, dir, "zones.txt", "zone-with-no-catalog\n")
 		_, err := parseInput(path, cfg)
 		if err == nil {
 			t.Fatal("expected error for invalid line")
diff --git a/main.go b/main.go
index 44376be..f279a92 100644
--- a/main.go
+++ b/main.go
@@ -41,7 +41,7 @@ func main() {
 
 	members, err := parseInput(inputFile, cfg)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "%s\n", err)
+		fmt.Fprintf(os.Stderr, "error: %s\n", err)
 		os.Exit(1)
 	}
 
@@ -58,15 +58,15 @@ func main() {
 	for _, catName := range catNames {
 		changed, err := processCatalog(catName, cfg, members[catName], *outputDir, now)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "%s\n", err)
+			fmt.Fprintf(os.Stderr, "error: %s\n", err)
 			hasErrors = true
 			continue
 		}
-		catZone := cfg.Catalogs[catName].Zone
+		zoneFile := cfg.Catalogs[catName].Zone + "zone"
 		if changed {
-			fmt.Fprintf(os.Stderr, "%s%s: updated\n", catZone, "zone")
+			fmt.Fprintf(os.Stderr, "%s: updated\n", zoneFile)
 		} else {
-			fmt.Fprintf(os.Stderr, "%s%s: unchanged\n", catZone, "zone")
+			fmt.Fprintf(os.Stderr, "%s: unchanged\n", zoneFile)
 		}
 	}
 
diff --git a/main_test.go b/main_test.go
index 074741c..82ff143 100644
--- a/main_test.go
+++ b/main_test.go
@@ -6,7 +6,6 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
-	"time"
 )
 
 func TestIntegrationEndToEnd(t *testing.T) {
@@ -213,7 +212,7 @@ soa:
 	// Find PTR lines and verify order
 	var ptrZones []string
 	for _, line := range lines {
-		if containsStr(line, "\tPTR\t") && !containsStr(line, "coo.") {
+		if strings.Contains(line, "\tPTR\t") && !strings.Contains(line, "coo.") {
 			// Extract the PTR target
 			parts := strings.Split(line, "\t")
 			ptrZones = append(ptrZones, parts[len(parts)-1])
@@ -313,9 +312,7 @@ soa:
 		if err == nil {
 			t.Error("expected error for unknown catalog")
 		}
-		if !containsStr(string(out), "unknown catalog") {
-			t.Errorf("expected 'unknown catalog' in error output, got: %s", out)
-		}
+		assertContains(t, string(out), "unknown catalog")
 	})
 }
 
@@ -405,7 +402,7 @@ soa:
 	assertContains(t, content2, "2026011601")
 
 	// Simulate not using the tool for a while, running on a much later date
-	day3 := time.Date(2026, 6, 15, 0, 0, 0, 0, time.UTC)
+	day3 := fixedTime(2026, 6, 15)
 	members3 := []ZoneEntry{
 		{Zone: "a.example.com.", Catalogs: []string{"cat1"}, File: "test", Line: 1},
 		{Zone: "b.example.com.", Catalogs: []string{"cat1"}, File: "test", Line: 2},