ProcessList: fix quadratic process removal when scanning

This commit changes ProcessList_scan to lazily remove Processes by
index, which is known, instead of performing a brute-force search by
pid and immediately reclaiming the lost vector space via compaction.

Searching by pid is potentially quadratic in ProcessList_scan because
the process we are searching for is always at the back of the vector
(the scan starts from the back of the vector). Additionally, removal
via Vector_remove immediately reclaims space (by sliding elements
down).

With these changes process removal in ProcessList_scan is now linear.

Changes:
  * ProcessList: add new ProcessList_removeIndex function to remove
    by index
  * Vector: add Vector_softRemove and Vector_compact functions to
    support lazy removal/deletion of entries Vector_softRemove
    Vector_compact
  * Vector: replace Vector_count with Vector_countEquals since it only
    used for consistency assertions.

1 files changed, 5 insertions, 1 deletions

diff --git a/Process.h b/Process.h
index e1408c21..ebaf230b 100644
--- a/Process.h
+++ b/Process.h
@@ -394,7 +394,11 @@ bool Process_changePriorityBy(Process* this, Arg delta);
 
 bool Process_sendSignal(Process* this, Arg sgn);
 
-int Process_pidCompare(const void* v1, const void* v2);
+static inline int Process_pidEqualCompare(const void* v1, const void* v2) {
+   const pid_t p1 = ((const Process*)v1)->pid;
+   const pid_t p2 = ((const Process*)v2)->pid;
+   return p1 != p2; /* return zero when equal */
+}
 
 int Process_compareByKey_Base(const Process* p1, const Process* p2, ProcessField key);