Fix bigint sub, use smaller precision for inv in divmod

Author: adamant-pwn

cp-algo/math/bigint.hpp

@@ -134,10 +134,16 @@ namespace cp_algo::math {
                 if (i < N) {
                     d_ptr[i]--;
                 } else {
-                    d_ptr[0]--;
+                    // Two's complement: flip all digits then add 1
                     for (i = 0; i < N; i++) {
                         d_ptr[i] = base - d_ptr[i] - 1;
                     }
+                    bool carry = true;
+                    for (i = 0; i < N && carry; i++) {
+                        d_ptr[i]++;
+                        carry = d_ptr[i] >= base;
+                        d_ptr[i] -= carry * base;
+                    }
                     negate();
                 }
             }

cp-algo/math/decimal.hpp

@@ -155,12 +155,16 @@ namespace cp_algo::math {
         // General case using decimal arithmetic
         auto A = decimal<base>(a);
         auto B = decimal<base>(b);
-        auto d = (A * B.inv(A.magnitude())).trunc();
+        auto d = (A * B.inv(A.magnitude() - B.magnitude() + 1)).round();
         auto r = a - d * b;
         if (r >= b) {
             d += 1;
             r -= b;
         }
+        if (r < bigint<base>(0)) {
+            d -= 1;
+            r += b;
+        }
         return std::pair{d, r};
     }
 }