finish extra/queue-to-do

Submitting solution...
Submission: SUCCESSFUL. Completed in: 9 hrs, 13 mins, 42 secs.

README.md

@@ -76,3 +76,6 @@ CFcSBwgCCBoHRhkKU1cGAA4AGU5YQR5THBwNFwoGGAxTQQMQVBUSBg4EAAwQRhUQVBUHFAQTGRpT QQM
 
 ### fuel-injection-perfection
 * Completed in: 1 hr, 2 secs.
+
+### queue-to-do
+* Completed in: 9 hrs, 13 mins, 42 secs.

extra/queue-to-do/solution.py

@@ -2,8 +2,23 @@ def solution(start, length):
     checksum = 0
     for i in range(length):
         base = start + length * i
-        for j in range(length - i):
+        checksum ^= calc_checksum_range(base, base + length - i)
-            checksum ^= base + j
+    return checksum
+
+# P = [01]*
+# P^P = 0
+# P00^P01^P10^P11 = 0
+def calc_checksum_range(_from, _to): # _from included, _to is NOT included
+    checksum = 0
+    remain = (-_from) % 4
+    for i in range(_from, min(_from + remain, _to)):
+        checksum ^= i
+    checkpoint = _from + remain
+
+    remain = _to % 4
+    for i in range(max(_from, checkpoint, _to - remain), _to):
+        checksum ^= i
+
     return checksum
 
 tests = [