feat: rewrite random.cpp

src/8-misc/random.cpp

@@ -1,28 +1,8 @@
 #include "../common/common.hpp"
 
-// what: tiny RNG wrapper with fixed default seed for reproducible tests/shuffles.
-// time: O(1) per call; memory: O(1)
-// constraint: uses mt19937_64.
-// usage: rng rd(2); ll x = rd.ll_range(l, r); rd.shuf(v);
-struct rng {
-    static constexpr ull DEFAULT_SEED = 712367;
-    mt19937_64 eng;
-
-    rng(ull seed = DEFAULT_SEED) : eng(seed) {}
-
-    ll ll_range(ll l, ll r) {
-        // edge: l <= r
-        uniform_int_distribution<ll> dis(l, r);
-        return dis(eng);
-    }
-
-    int int_range(int l, int r) {
-        uniform_int_distribution<int> dis(l, r);
-        return dis(eng);
-    }
-
-    template <class T>
-    void shuf(vector<T> &v) {
-        shuffle(all(v), eng);
-    }
-};
+// what: mt19937_64 quick usage snippet (time-seeded + rand range + shuffle).
+// time: O(1) per draw; memory: O(1)
+// constraint: uniform_int_distribution is inclusive [l, r].
+// constraint: chrono seed isn't cryptographically secure.
+// usage: mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
+// usage: ll x = uniform_int_distribution<ll>(l, r)(rng); shuffle(all(v), rng);