Merge pull request #1385 from NicholasPatrick/matrix_product_overflow_fix

Fix off-by-one error in a matrix product test.

Author: NachiaVivias

linear_algebra/matrix_product/gen/unsigned_overflow.cpp

@@ -1,5 +1,3 @@
-
-
 #include <iostream>
 #include <set>
 #include <limits>
@@ -9,17 +7,25 @@
 
 using namespace std;
 
-int main() {
+int main(int, char* argv[]) {
+    long long seed = atoll(argv[1]);
+
     int val = MOD - 2;
+    unsigned long long max_ull = numeric_limits<unsigned long long>::max();
 
-    // Find smallest size n where n * val * val > numeric_limits<unsigned long long>::max()
+    // Find smallest size n where n * val * val > max_ull
     int n = 0;
-    unsigned long long res = 0;
-    long long val2 = (long long) val * val;
-    while (res + val2 >= res) // Check if adding val2 to res will overflow
-    {
-        res += val2;
-        ++n;
+    unsigned long long val2 = (unsigned long long) val * val;
+    if (seed == 0) {
+        unsigned long long res = 0;
+        // Check if adding val2 to res will overflow
+        // this test mistakenly produces the largest n that does not overflow
+        while (res + val2 > res) {
+            res += val2;
+            ++n;
+        }
+    } else if (seed == 1) {
+        n = max_ull / val2 + 1;
     }
 
     cout << n << ' ' << n << ' ' << n << '\n';

linear_algebra/matrix_product/hash.json

@@ -40,5 +40,7 @@
   "small_09.in": "a5b051bd88e93503771c9681352029cb501aba6ff34136ae571ac148075d781d",
   "small_09.out": "2b456d5070b924b76d31b85945eb42362918491c0c7e2e8aedb4e3941563acfa",
   "unsigned_overflow_00.in": "b30d8bfb1e59cc60670096505b5eda866052fd8e839244263da5f75f108aeb9a",
-  "unsigned_overflow_00.out": "b776f9584be10a2f16b3fc22111c6123ddf89a4b6b6058e7a9cec320c2b6a6ab"
+  "unsigned_overflow_00.out": "b776f9584be10a2f16b3fc22111c6123ddf89a4b6b6058e7a9cec320c2b6a6ab",
+  "unsigned_overflow_01.in": "e772fad71a028698445b585feb8fcc95695841ef4d69181537e94f908700855d",
+  "unsigned_overflow_01.out": "b5273ee92bfc3cfb2cb42684db4622825ed605b31292a812f269cd6f8b58658d"
 }

linear_algebra/matrix_product/info.toml

@@ -19,7 +19,7 @@ forum = "https://github.com/yosupo06/library-checker-problems/issues/654"
     number = 1
 [[tests]]
     name = "unsigned_overflow.cpp"
-    number = 1
+    number = 2
 
 [params]
     N_MAX = 1_024