chore: improve comments and factored out variables when moving iterator

doc/manual/source/command-ref/nix-collect-garbage.md

@@ -94,7 +94,7 @@ generations of each profile, do
 $ nix-collect-garbage -d
 ```
 
-## Keep most-recent by time (number fo days) and trim by amount
+## Keep most-recent by time (number of days) and trim by amount
 
 This command will delete generations older than a week if possible, while keeping an amount of generations between `10` and `20`.
 

src/libstore/include/nix/store/profiles.hh

@@ -135,17 +135,17 @@ void deleteGenerations(const Path & profile, const std::set<GenerationNumber> &
  *
  * Examples:
  * - All parameters are nullopt
- *   No generations are deleted
- * - keepMax is 5
- *   No generations are deleted
+ *   No generations are deleted.
+ * - keepMin is 5
+ *   No generations are deleted, only keepMax and olderThan delete generations.
  * - keepMax is 10
- *   10 generations after the current one are kept, the rest is deleted.
+ *   10 most recent generations after the current one are kept, the rest is deleted.
  * - olderThan is 2025-09-16
- *   Generations older than 2025-09-16 are deleted
+ *   Generations older than 2025-09-16 are deleted.
  * - olderThan is 2025-09-16, keepMin is 5, keepMax is 10 -
  *   Will try to delete generations older than 2025-09-16.
- *   If there are more than 10 generations to be kept, continues to delete old generations until there is 10.
- *   If there are less than 5 generations to be kept, preserves older generations until there is 5.
+ *   If there are more than 10 generations to be kept, continues to delete old generations until there are 10.
+ *   If there are less than 5 generations to be kept, preserves the most recent of generations to be deleted until there are 5.
  *
  * @param profile The profile, specified by its name and location combined into a path, whose generations we want to
  * delete.

src/libstore/profiles.cc

@@ -166,7 +166,7 @@ static inline void iterDropUntil(Generations & gens, auto && i, auto && cond)
 void deleteGenerationsFilter(const Path & profile, std::optional<time_t> olderThan, std::optional<GenerationNumber> keepMin, std::optional<GenerationNumber> keepMax, bool dryRun)
 {
     if (keepMin.has_value() && keepMax.has_value() && *keepMin > *keepMax)
-        throw Error("Keep min cannot be greater than keep max");
+        throw Error("--keep-min cannot be greater than --keep-max");
 
     PathLocks lock;
     lockProfile(lock, profile);
@@ -197,13 +197,18 @@ void deleteGenerationsFilter(const Path & profile, std::optional<time_t> olderTh
         iterDropUntil(gens, older, [&](auto & g) { return g.creationTime < *olderThan; });
     }
 
-    // Find first generation to delete by clamping
+    // Find first generation to delete by clamping between keepMin and keepMax
     auto toDelete = older;
-    for (int i = std::distance(gens.rbegin(), older) - std::distance(gens.rbegin(), end); i > 0; --i)
+
+    auto clampBackward = std::distance(gens.rbegin(), older) - std::distance(gens.rbegin(), end);
+    for (int i = clampBackward; i > 0; --i)
         --toDelete;
-    for (int i = std::distance(gens.rbegin(), start) - std::distance(gens.rbegin(), older); i > 0; --i)
+
+    auto clampForward = std::distance(gens.rbegin(), start) - std::distance(gens.rbegin(), older);
+    for (int i = clampForward; i > 0; --i)
         ++toDelete;
 
+    // Delete
     for (; toDelete != gens.rend(); ++toDelete)
         deleteGeneration2(profile, toDelete->number, dryRun);
 }

src/nix/nix-collect-garbage/nix-collect-garbage.cc

@@ -83,7 +83,7 @@ static int main_nix_collect_garbage(int argc, char ** argv)
                 deleteOlderThan = std::optional<time_t> { parseOlderThanTimeSpec(getArg(*arg, arg, end)) };
             } else if (*arg == "--keep-min")
                 keepMin = std::optional<GenerationNumber> { std::max(getIntArg<GenerationNumber>(*arg, arg, end, false), (GenerationNumber) 1) };
-            else if (*arg == "--keep-max"){
+            else if (*arg == "--keep-max") {
                 removeOld = true;
                 keepMax = std::optional<GenerationNumber> { std::max(getIntArg<GenerationNumber>(*arg, arg, end, false), (GenerationNumber) 1) };
             }