CLI: -z and -x run without -o, writing nothing

The output path is now optional in both directions: without -o the tool
performs the full operation (compression with round-trip verification, or
decompression of a container) and prints the usual report without writing
a file. Verified: no-output runs exit 0 with the reference CR, and runs
with -o still produce byte-identical containers.

Author: JiajunHuang1999

README.md

@@ -114,7 +114,7 @@ fsz -t f64 -i data.d64 -eb rel 1e-3                   any mode on float64 data
 | Option | Meaning |
 |---|---|
 | `-z` / `-x` | force compress / decompress. Without either, files starting with the FSZ magic are decompressed and raw inputs get the report mode. |
-| `-i`, `-o` | input / output path (`-o` required for `-z` / `-x`). |
+| `-i`, `-o` | input / output path. Without `-o`, `-z` and `-x` run fully but write nothing. |
 | `-eb abs V` | absolute error bound `V`. |
 | `-eb rel V` | relative error bound: `V * (max - min)` of the input data. |
 | `-t f32\|f64` | element type of a raw input file (default `f32`). Decompression of `.fsz` files reads the type from the header instead. |

tools/fsz.cu

@@ -110,7 +110,8 @@ void usage(std::FILE* out) {
         "  -z                  force compression\n"
         "  -x                  force decompression\n"
         "  -i <path>           input file\n"
-        "  -o <path>           output file, required with -z and -x\n"
+        "  -o <path>           output file; without it -z and -x run fully\n"
+        "                      but write nothing\n"
         "  -t f32|f64          element type of the raw array, float32 or\n"
         "                      float64 (default f32)\n"
         "  -eb abs <value>     absolute error bound\n"
@@ -447,9 +448,8 @@ int run_compress(const Options& o, bool write_output) {
     if (o.eb_mode.empty()) {
         die_usage("no error bound; pass -eb abs <value> or -eb rel <value>");
     }
-    if (write_output && o.out_path.empty()) {
-        die_usage("-z needs an output path; pass -o <path>");
-    }
+    // -z without -o runs the full compression and report but writes nothing.
+    const bool write_file = write_output && !o.out_path.empty();
 
     std::vector<T> h_in = read_values<T>(o.in_path);
     const uint64_t n_file = (uint64_t)h_in.size();
@@ -529,7 +529,7 @@ int run_compress(const Options& o, bool write_output) {
     const bool ok = (nonfinite == 0) && (max_err <= 1.01 * (double)eb_abs);
 
     uint64_t total_bytes = 0;
-    if (write_output) {
+    if (write_file) {
         if (o.bare) {
             write_all(o.out_path, h_cmp.data(), h_cmp.size());
             total_bytes = (uint64_t)h_cmp.size();
@@ -564,7 +564,7 @@ int run_compress(const Options& o, bool write_output) {
                 o.eb_mode.c_str(), o.eb_value, (double)eb_abs);
     std::printf("  compressed = %zu bytes\n", (size_t)r.cmp_size);
     std::printf("  CR         = %.4f\n", cr);
-    if (write_output) {
+    if (write_file) {
         if (o.bare) {
             std::printf("  output     = %s (%llu bytes, bare bitstream)\n",
                         o.out_path.c_str(), (unsigned long long)total_bytes);
@@ -663,7 +663,7 @@ int run_decompress_typed(const Options& o, const DecompressPlan& p) {
     cudaFree(d_out);
     cudaFree(d_cmp);
 
-    write_all(o.out_path, h_out.data(), n * sizeof(T));
+    if (!o.out_path.empty()) write_all(o.out_path, h_out.data(), n * sizeof(T));
 
     const size_t out_bytes = n * sizeof(T);
     const double cr = (double)n * (double)sizeof(T) / (double)p.payload_size;
@@ -678,7 +678,8 @@ int run_decompress_typed(const Options& o, const DecompressPlan& p) {
                 p.from_header ? "  (from the file header)" : "");
     std::printf("  compressed = %zu bytes\n", p.payload_size);
     std::printf("  CR         = %.4f\n", cr);
-    std::printf("  output     = %s (%zu bytes)\n", o.out_path.c_str(), out_bytes);
+    if (!o.out_path.empty())
+        std::printf("  output     = %s (%zu bytes)\n", o.out_path.c_str(), out_bytes);
     std::printf("  decompress = %.2f ms (%.2f GB/s)\n", d_ms, gbps(out_bytes, d_ms));
 
     if (o.csv) {
@@ -692,8 +693,6 @@ int run_decompress_typed(const Options& o, const DecompressPlan& p) {
 }
 
 int run_decompress(const Options& o) {
-    if (o.out_path.empty()) die_usage("-x needs an output path; pass -o <path>");
-
     std::vector<unsigned char> raw = read_all(o.in_path);
 
     DecompressPlan p;