fix: a dry run asks no install, counts systems

install.py

@@ -5608,7 +5608,13 @@ def _print_plan(
             print(line)
     else:
         largest = sorted(by_system, key=lambda s: (-_bytes(by_system[s]), s))
-        print(f"  {_plural(len(largest), 'system')}, the largest first:")
+        # A file several systems share is a group of its own: "88 systems"
+        # were 66 systems, as the full screen counts them, and 22 groups.
+        systems = {name for entry in to_download for name in entry_systems(entry)}
+        groups = (f"{_plural(len(systems), 'system')} in {len(largest)} groups"
+                  if systems and len(systems) != len(largest)
+                  else _plural(len(largest), 'system'))
+        print(f"  {groups}, the largest first:")
         for system in largest[:PLAN_LARGEST_SYSTEMS]:
             print(f"  {term.bold(system)}  {_sized(by_system[system])}")
         if len(largest) > PLAN_LARGEST_SYSTEMS:
@@ -6486,7 +6492,7 @@ INSTALLED_ONLY = "installed"
 def _prompt_custom_selection(
     plat: str, files: list[dict], omitted: list[dict],
     installed: tuple[list[dict], list[dict]] | None = None,
-    name: str = "",
+    name: str = "", dry_run: bool = False,
 ) -> tuple[list[str], list[str], list[str]] | str | None:
     """Offer to narrow the install by system, core and region.
 
@@ -6496,7 +6502,9 @@ def _prompt_custom_selection(
     everything on its axis.
     """
     print(f"\n{name or display_name(plat)}: {_sized(files)}.")
-    print("  Enter) install everything")
+    # A dry run installs nothing: "install everything" was asked before it.
+    print("  Enter) list everything, nothing is written" if dry_run
+          else "  Enter) install everything")
     if installed is not None:
         print(f"  i) only what the installed cores read: {_sized(installed[0])}")
     print("  c) choose systems, cores and regions")
@@ -9610,7 +9618,7 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
             )
             selection = _prompt_custom_selection(
                 plat_name, files, omitted_files, installed,
-                display_name(plat_name, manifest),
+                display_name(plat_name, manifest), args.dry_run,
             )
             if selection is None:
                 smaller = ("  Nothing downloaded. To install less, run the same command "

tests/test_install_experience.py

@@ -1191,6 +1191,33 @@ class OfflineWithNothingFound(unittest.TestCase):
         self.assertIn("Use --platform and --dest", said)
 
 
+class TheDryRunWords(unittest.TestCase):
+    """The FAQ's dry run asked "Enter) install everything" before a run
+    that installs nothing, then counted 88 systems where the full screen
+    lists 66: a shared file made a group of its own."""
+
+    def test_the_question_says_nothing_is_written(self):
+        out = io.StringIO()
+        files = [{"dest": "a.bin", "size": 1, "systems": ["x"]}]
+        with mock.patch.object(install, "_ask", return_value=""), \
+                contextlib.redirect_stdout(out):
+            install._prompt_custom_selection("retroarch", files, [], None, "RetroArch",
+                                             dry_run=True)
+        self.assertNotIn("install everything", out.getvalue())
+        self.assertIn("nothing is written", out.getvalue())
+
+    def test_the_largest_systems_are_counted_as_systems(self):
+        files = [{"dest": f"{n}.bin", "size": n + 1, "systems": [f"s{n}"]}
+                 for n in range(12)]
+        files.append({"dest": "shared.bin", "size": 1, "systems": ["s0", "s1"]})
+        out = io.StringIO()
+        with mock.patch.object(install.shutil, "get_terminal_size",
+                               return_value=os.terminal_size((80, 10))), \
+                contextlib.redirect_stdout(out):
+            install._print_plan(files, Path("dest"), install.Terminal(io.StringIO()))
+        self.assertIn("12 systems in 13 groups, the largest first", out.getvalue())
+
+
 def _source_of(function) -> str:
     import inspect