fix: always evict at least one cache entry when over max

wttrin--cleanup-cache-if-needed removed floor(count * 0.20) entries, which is 0
for a small cache (e.g. two entries at max one), so the cache could sit over its
advertised maximum indefinitely. It now removes at least one whenever it is over.

2 files changed, 9 insertions, 7 deletions

diff --git a/tests/test-wttrin--cleanup-cache-if-needed.el b/tests/test-wttrin--cleanup-cache-if-needed.el
index 1850d17..7533bfa 100644
--- a/tests/test-wttrin--cleanup-cache-if-needed.el
+++ b/tests/test-wttrin--cleanup-cache-if-needed.el
@@ -108,16 +108,17 @@
         (should (= 1 (testutil-wttrin-cache-size))))
     (test-wttrin--cleanup-cache-if-needed-teardown)))
 
-(ert-deftest test-wttrin--cleanup-cache-if-needed-boundary-two-entries-at-max-one-removes-none ()
-  "Test that two entries at max=1 removes no entries due to integer division."
+(ert-deftest test-wttrin--cleanup-cache-if-needed-boundary-two-entries-at-max-one-reduces-to-max ()
+  "Boundary: two entries at max=1 reduces to the max.
+floor(2 * 0.20) is 0, but cleanup must still remove at least one entry when the
+cache is over max, or it would exceed its advertised maximum indefinitely."
   (test-wttrin--cleanup-cache-if-needed-setup)
   (unwind-protect
       (testutil-wttrin-with-cache-max 1
         (testutil-wttrin-add-to-cache "old" "data1" 100)
         (testutil-wttrin-add-to-cache "new" "data2" 50)
         (wttrin--cleanup-cache-if-needed)
-        ;; 2 entries / 5 = 0 in integer division, so no entries removed
-        (should (= 2 (testutil-wttrin-cache-size))))
+        (should (= 1 (testutil-wttrin-cache-size))))
     (test-wttrin--cleanup-cache-if-needed-teardown)))
 
 (ert-deftest test-wttrin--cleanup-cache-if-needed-boundary-one-over-max-removes-oldest ()
diff --git a/wttrin.el b/wttrin.el
index cdef10a..4bc1b3a 100644
--- a/wttrin.el
+++ b/wttrin.el
@@ -1283,12 +1283,13 @@ Returns a list where each element is a cons cell (key . timestamp)."
 (defun wttrin--cleanup-cache-if-needed ()
   "Remove oldest entries if cache exceeds max size.
 Removes oldest entries based on `wttrin--cache-cleanup-percentage'
-when cache count exceeds `wttrin-cache-max-entries'.
+when cache count exceeds `wttrin-cache-max-entries', and always at least one
+so the cache can't sit over its maximum when the percentage floors to zero.
 This creates headroom to avoid frequent cleanups."
   (when (> (hash-table-count wttrin--cache) wttrin-cache-max-entries)
     (let* ((entries-by-age (wttrin--get-cache-entries-by-age))
-           (num-to-remove (floor (* (length entries-by-age)
-                                    wttrin--cache-cleanup-percentage))))
+           (num-to-remove (max 1 (floor (* (length entries-by-age)
+                                           wttrin--cache-cleanup-percentage)))))
       (dotimes (i num-to-remove)
         (remhash (car (nth i entries-by-age)) wttrin--cache)))))