feat(bookfile): add Contains and OpenReaderAt utilities

backend/internal/bookfile/reader_test.go

@@ -0,0 +1,51 @@
+package bookfile
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestContains(t *testing.T) {
+	dir := t.TempDir()
+	sub := filepath.Join(dir, "sub")
+	if err := os.MkdirAll(sub, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if !Contains(dir, sub) {
+		t.Fatal("sub should be inside dir")
+	}
+	if Contains(sub, dir) {
+		t.Fatal("dir should not be inside sub")
+	}
+	if Contains(dir, "/completely/different") {
+		t.Fatal("unrelated path should not be inside dir")
+	}
+	// Same path.
+	if !Contains(dir, dir) {
+		t.Fatal("dir should contain itself")
+	}
+}
+
+func TestOpenReaderAt(t *testing.T) {
+	dir := t.TempDir()
+	// Create a test file.
+	path := filepath.Join(dir, "test.txt")
+	if err := os.WriteFile(path, []byte("hello"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	f, size, err := OpenReaderAt(dir, "test.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	if size != 5 {
+		t.Fatalf("size = %d, want 5", size)
+	}
+
+	// Non-existent file.
+	_, _, err = OpenReaderAt(dir, "nope.txt")
+	if err == nil {
+		t.Fatal("expected error for non-existent file")
+	}
+}