docs: state unwritten contracts in doc comments

Author: Cameron Custer

library/graphs/dijkstra.hpp

@@ -4,6 +4,7 @@
 //!   auto d = dijkstra(g, source);
 //! @endcode
 //! d[v] = min dist from source->..->v
+//! requires weights >= 0
 //! @time O(n + (m log m))
 //! @space O(n + m)
 vector<ll> dijkstra(const auto& g, int s) {

library/graphs/uncommon/enumerate_triangles.hpp

@@ -5,6 +5,7 @@
 //!       //u, v, w form a triangle
 //!   });
 //! @endcode
+//! requires no self loops or multi edges
 //! @time O(n + m ^ (3/2))
 //! @space O(n + m)
 void enumerate_triangles(const vector<pii>& edges, int n,

library/loops/submasks.hpp

@@ -1,5 +1,6 @@
 #pragma once
 //! loops over submasks in decreasing order
+//! excludes the empty submask
 //! @param mask a submask of 2^n-1
 //! @time O(3^n) to iterate every submask
 //!   of every mask of size n

library/math/derangements.hpp

@@ -4,6 +4,7 @@
 //!   auto der = derangements(n, mod);
 //! @endcode
 //! der[i] = number of permutations p with p[i]!=i
+//! requires n >= 1
 //! @time O(n)
 //! @space O(n)
 vector<ll> derangements(int n, int mod) {

library/math/matrix_related/matrix_mult.hpp

@@ -1,5 +1,6 @@
 #pragma once
 //! https://codeforces.com/blog/entry/80195
+//! watch for overflow if T is an integer type
 //! @time O(n * m * inner)
 //! @space O(n * m)
 template<class T>

library/math/matrix_related/row_reduce.hpp

@@ -6,6 +6,7 @@
 //! columns [0,cols) of mat represent a matrix
 //! columns [cols,m) of mat are also
 //!   affected by row operations
+//! requires mod is prime
 //! @time O(n * m * min(cols, n))
 //! @space O(1)
 pii row_reduce(vector<vi>& mat, int cols) {

library/math/mod_division.hpp

@@ -3,6 +3,7 @@
 //! @code
 //!   int quotient = mod_div(x, y); // returns x * y^-1
 //! @endcode
+//! requires gcd(y, mod) == 1
 //! @time O(log(y))
 //! @space O(1)
 const int mod = 998244353;

library/strings/binary_trie.hpp

@@ -4,6 +4,8 @@
 //!   bt.update(num, 1); // insert
 //!   bt.update(num, -1); // erase
 //! @endcode
+//! requires 0 <= num < 2^61
+//! walk requires a non-empty trie
 //! @time O(q * mx_bit)
 //! @space O(q * mx_bit)
 const ll mx_bit = 1LL << 60;

library/strings/kmp.hpp

@@ -9,6 +9,7 @@
 //!   auto match2 = kmp1.find_str(s_vec);
 //! @endcode
 //! if match[i] == 1 then s[i,ssize(t)) == t
+//! requires t non-empty
 //! @time O(|s| + |t|)
 //! @space O(|s| + |t|)
 // NOLINTNEXTLINE(readability-identifier-naming)

library/trees/centroid_decomp.hpp

@@ -4,6 +4,7 @@
 //!   vector<basic_string<int>> g(n);
 //!   vector<int> p = cd(g, [&](int c) {});
 //! @endcode
+//! g is mutated (edges removed)
 //! @time O(n log n)
 //! @space O(n)
 vi cd(auto& g, auto f) {