Forecast from the right end of the window, and from the level shown

Four defects stacked into one nonsense report: cartridges at 20% claiming four
days, cartridges at 1% claiming weeks.

The root cause is a Zabbix API detail. `limit` caps the whole result set rather
than each item, and the query sorted ascending, so the cap kept the OLDEST rows
in the window. A four-cartridge printer polled every five minutes writes over
100k readings in 90 days; the forecast was fitted to the first few days of that
and nothing since. Every rate was real and every rate described a cartridge
thrown away three months ago. Nothing in the output looks wrong, which is why
it needed pinning in a test rather than a comment.

A 90-day burn rate does not need every individual poll, so a long window now
reads hourly trends - the table meant for this, a tenth of the rows, and kept
longer. Raw history serves short windows and any item a site keeps no trends
for. Both are fetched newest-first with the budget scaled per item.

Second, the countdown was computed from the last stored reading while the level
displayed was the live one, so the two could disagree by a whole cartridge. The
live level is now what the countdown divides. A live level far above the stored
run means it was swapped since the last reading, and that is reported as a
replacement rather than as a collapse in the burn rate.

Third, at or below 5% a cartridge reads as empty rather than as a slow drain.
At 1% losing a tenth of a point a day the arithmetic says ten days. The printer
is out of toner, and it is the first thing to order.

Fourth, the days-left column spanned the printer's rows, so the printer's
soonest figure was printed beside every supply it had. That alone accounts for
the shape of both complaints: a healthy cartridge wearing its neighbour's
deadline, and an empty one wearing a number that belonged to nothing on its row.

Also fixes float-typed supplies vanishing from any printer that also had an
integer-typed one - they live in different history tables and the fetch stopped
at whichever answered first.

Not verified against live data: Zabbix is not reachable from the dev box.

plugins/printers/services/supply_history.py

@@ -29,6 +29,13 @@ MIN_POINTS_FOR_ESTIMATE = 4
 # says "never", which is worse than saying nothing.
 MIN_DROP_FOR_ESTIMATE = 2
 
+# At or below this, the cartridge is done and the arithmetic stops being the
+# useful answer. A supply sitting at 1% that drains a tenth of a point a day
+# computes to ten days; a printer at 1% is out of toner as far as anyone
+# standing at it is concerned, and it is what should be ordered first. Rate is
+# still reported - only the days-left figure is floored.
+EMPTY_LEVEL = 5
+
 
 def _asfloat(value):
     try:
@@ -101,11 +108,17 @@ def burn_rate(points):
     return drop / days
 
 
-def analyse(points, rise=REPLACEMENT_RISE):
+def analyse(points, rise=REPLACEMENT_RISE, currentlevel=None):
     """Everything the report needs about one supply.
 
+    `currentlevel` is the live reading from the printer, when the caller has
+    one. It is what the days-left figure is computed against, because it is
+    what the report displays: taking the level from the display and the
+    countdown from the last stored history point lets the two disagree, and a
+    row reading "20% - 4 days left" is read as a broken report, correctly.
+
     Returns:
-      currentlevel     latest reading, or None
+      currentlevel     live reading if given, else the latest stored one
       daysleft         at the current rate, or None when there is no estimate
       burnrateperday   percent per day, or None
       reason           why there is no estimate - shown rather than hidden, so
@@ -117,12 +130,16 @@ def analyse(points, rise=REPLACEMENT_RISE):
     """
     points = normalise(points)
     result = {
-        'currentlevel': None, 'daysleft': None, 'burnrateperday': None,
+        'currentlevel': currentlevel, 'daysleft': None, 'burnrateperday': None,
         'reason': None, 'replacements': 0, 'lastreplaced': None,
         'basisdays': 0, 'points': [],
     }
     if not points:
-        result['reason'] = 'no history'
+        # A live level with no history is still worth acting on when it is low.
+        if currentlevel is not None and currentlevel <= EMPTY_LEVEL:
+            result['daysleft'] = 0
+        else:
+            result['reason'] = 'no history'
         return result
 
     replacements = find_replacements(points, rise=rise)
@@ -130,13 +147,30 @@ def analyse(points, rise=REPLACEMENT_RISE):
     result['lastreplaced'] = replacements[-1].isoformat() if replacements else None
 
     run = current_run(points, rise=rise)
-    result['currentlevel'] = run[-1][1]
+    stored = run[-1][1]
+    level = stored if currentlevel is None else currentlevel
+    result['currentlevel'] = level
     result['points'] = [(when.isoformat(), level) for when, level in run]
     if len(run) >= 2:
         result['basisdays'] = round(
             (run[-1][0] - run[0][0]).total_seconds() / 86400, 1)
 
+    # A live level well above the stored run means it was swapped since the
+    # last stored reading. The run describes the cartridge that came out.
+    if currentlevel is not None and currentlevel - stored >= rise:
+        result['replacements'] += 1
+        result['reason'] = 'replaced recently'
+        return result
+
     rate = burn_rate(run)
+
+    # Empty is empty. Ordering by a rate below this level ranks a dead
+    # cartridge behind a healthy one that happens to be draining faster.
+    if level <= EMPTY_LEVEL:
+        result['daysleft'] = 0
+        result['burnrateperday'] = round(rate, 2) if rate is not None else None
+        return result
+
     if rate is None:
         # Say which of the three it is; "no estimate" alone invites a bug report.
         if len(run) < MIN_POINTS_FOR_ESTIMATE:
@@ -147,7 +181,7 @@ def analyse(points, rise=REPLACEMENT_RISE):
         return result
 
     result['burnrateperday'] = round(rate, 2)
-    result['daysleft'] = max(0, int(run[-1][1] / rate))
+    result['daysleft'] = max(0, int(level / rate))
     return result