fix: seed rand source for --theme random to avoid deterministic output

2 files changed, 44 insertions, 9 deletions

diff --git a/cmd/snonux/main.go b/cmd/snonux/main.go
index 42a377f..a494854 100644
--- a/cmd/snonux/main.go
+++ b/cmd/snonux/main.go
@@ -17,6 +17,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"time"
 
 	"codeberg.org/snonux/snonux/internal/config"
 	"codeberg.org/snonux/snonux/internal/generator"
@@ -34,7 +35,7 @@ const (
 )
 
 func main() {
-	cfg, mode, err := parseFlags(os.Args[1:])
+	cfg, mode, err := parseFlags(os.Args[1:], rand.New(rand.NewSource(time.Now().UnixNano())))
 	if err != nil {
 		log.Fatalf("error: %v", err)
 	}
@@ -64,7 +65,8 @@ var errParseFlags = errors.New("flag parse error")
 
 // parseFlags reads CLI flags and returns a validated Config.
 // Special theme value "random" picks a theme at random from the registry.
-func parseFlags(args []string) (*config.Config, cliMode, error) {
+// The rng parameter must be non-nil; it is used for theme selection.
+func parseFlags(args []string, rng *rand.Rand) (*config.Config, cliMode, error) {
 	cfg := &config.Config{}
 	fs := flag.NewFlagSet("snonux", flag.ContinueOnError)
 	fs.SetOutput(io.Discard)
@@ -94,8 +96,11 @@ func parseFlags(args []string) (*config.Config, cliMode, error) {
 
 	// Resolve the special "random" value before any further validation.
 	if cfg.Theme == "random" {
+		if rng == nil {
+			return nil, modeRun, fmt.Errorf("theme %q requires a seeded rng", cfg.Theme)
+		}
 		themes := generator.ListThemes()
-		cfg.Theme = themes[rand.Intn(len(themes))]
+		cfg.Theme = themes[rng.Intn(len(themes))]
 		log.Printf("random theme selected: %s", cfg.Theme)
 	}
 
diff --git a/cmd/snonux/main_test.go b/cmd/snonux/main_test.go
index 7f05c8b..a3ff292 100644
--- a/cmd/snonux/main_test.go
+++ b/cmd/snonux/main_test.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"errors"
+	"math/rand"
 	"os"
 	"path/filepath"
 	"strings"
@@ -61,7 +62,7 @@ func TestEnsureDir_createsAndRejectsFile(t *testing.T) {
 func TestParseFlags_version(t *testing.T) {
 	t.Parallel()
 
-	_, mode, err := parseFlags([]string{"-version"})
+	_, mode, err := parseFlags([]string{"-version"}, rand.New(rand.NewSource(1)))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -73,7 +74,7 @@ func TestParseFlags_version(t *testing.T) {
 func TestParseFlags_listThemes(t *testing.T) {
 	t.Parallel()
 
-	_, mode, err := parseFlags([]string{"-list-themes"})
+	_, mode, err := parseFlags([]string{"-list-themes"}, rand.New(rand.NewSource(1)))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -92,7 +93,7 @@ func TestParseFlags_run(t *testing.T) {
 		"-output", out,
 		"-theme", "neon",
 		"-base-url", "https://t.test",
-	})
+	}, rand.New(rand.NewSource(1)))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -114,7 +115,7 @@ func TestParseFlags_sync(t *testing.T) {
 		"-output", out,
 		"-theme", "neon",
 		"-sync",
-	})
+	}, rand.New(rand.NewSource(1)))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -131,7 +132,7 @@ func TestParseFlags_randomTheme(t *testing.T) {
 
 	in := t.TempDir()
 	out := t.TempDir()
-	cfg, _, err := parseFlags([]string{"-input", in, "-output", out, "-theme", "random"})
+	cfg, _, err := parseFlags([]string{"-input", in, "-output", out, "-theme", "random"}, rand.New(rand.NewSource(42)))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -144,10 +145,39 @@ func TestParseFlags_randomTheme(t *testing.T) {
 	}
 }
 
+// TestParseFlags_randomTheme_deterministic verifies that when a seeded
+// *rand.Rand is passed in, the "random" theme resolve predictably.
+// This ensures the rng parameter is actually used.
+func TestParseFlags_randomTheme_deterministic(t *testing.T) {
+	t.Parallel()
+
+	in := t.TempDir()
+	out := t.TempDir()
+
+	// NewSource(1) results in a deterministic first draw across multiple calls.
+	src := rand.NewSource(1)
+	rng := rand.New(src)
+
+	cfg, _, err := parseFlags([]string{"-input", in, "-output", out, "-theme", "random"}, rng)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Reset the source and re-invoke parseFlags to confirm the same theme
+	// is selected again (determinism).
+	cfg2, _, err := parseFlags([]string{"-input", in, "-output", out, "-theme", "random"}, rand.New(rand.NewSource(1)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.Theme != cfg2.Theme {
+		t.Fatalf("expected deterministic theme %q, got %q", cfg.Theme, cfg2.Theme)
+	}
+}
+
 func TestParseFlags_unknownFlag(t *testing.T) {
 	t.Parallel()
 
-	_, _, err := parseFlags([]string{"-not-a-real-flag"})
+	_, _, err := parseFlags([]string{"-not-a-real-flag"}, rand.New(rand.NewSource(1)))
 	if err == nil {
 		t.Fatal("expected error")
 	}