solution to CHANDF.cpp

1. https://www.youtube.com/watch?v=-F7cHQ-gWS4
2. Greediest Competitive Programming problem ever
3. Awesome Logical Reasoning / Problem Solving skills are needed to solve this.

Author: rachitiitr

MyOnlineSubmissions/Codechef/MAY20/CHANDF.cpp

@@ -0,0 +1,122 @@
+// Video Explanation at https://www.youtube.com/watch?v=-F7cHQ-gWS4
+#include<bits/stdc++.h>
+using namespace std;
+
+#define ll long long
+
+ll f(ll &x, ll &y, ll &z) {
+    return (x&z) * (y&z);
+}
+
+class Bit{
+    ll x;
+    int _bits; // we need only these many bits, in this case 40
+    
+    #define mask(b) (1LL<<(_bits-b)) // remember 0th bit is actually 39th bit
+public:
+    Bit() {}
+    
+    Bit(ll &y, int bits=40) { 
+        init(y, bits);
+    }
+
+    void init(ll &y, int bits=40) {
+        x = y;
+        _bits = bits-1;
+    }
+    
+    bool get(int &b) {
+        return (x & mask(b)) != 0;
+    }
+
+    void set(int &b) {
+        x |= mask(b);
+    }
+
+    void reset(int &b) {
+        if(get(b))
+            x ^= mask(b);
+    }
+
+    void set(int &b,int k) {
+        if(k == 1) {
+            set(b);
+        }
+        else {
+            reset(b);
+        }
+    }
+
+    ll toInt() {
+        return x;
+    }
+} bitX, bitY;
+
+vector<ll> getValidZValues(ll &L, ll &R) {
+    vector<ll> validZ = {L, R}; // L and R are always valid Z values we should handle
+    Bit bitL(L);
+    Bit bitR(R);
+    int k = 0;
+    while(k<40 and bitL.get(k) == bitR.get(k)) k++;
+
+    for(int l = k+1; l < 40; l++) {
+        if(bitL.get(l) != 0) continue;
+        Bit z(L);
+        z.set(l);
+        for(int i=l+1; i<40; i++) {
+            if(bitX.get(i) == 0 and bitY.get(i) == 0) {
+                z.set(i, 0); // minimize Z
+            }
+            else {
+                z.set(i, 1); // maximize F(X,Y,Z)
+            }
+        }
+        validZ.push_back(z.toInt());
+    }
+    for(int r = k+1; r < 40; r++) {
+        if(bitR.get(r) != 1) continue;
+        Bit z(R);
+        z.reset(r);
+        for(int i=r+1; i<40; i++) {
+            if(bitX.get(i) == 0 and bitY.get(i) == 0) {
+                z.set(i, 0);
+            }
+            else {
+                z.set(i, 1);
+            }
+        }
+        validZ.push_back(z.toInt());
+    }
+
+    sort(validZ.begin(), validZ.end());
+    return validZ;
+}
+
+int main() {
+    ios_base::sync_with_stdio(0);
+    int t;
+    cin >> t;
+
+    while(t--) {
+        ll x, y, l, r;
+        cin >> x >> y >> l >> r;
+
+        bitX.init(x);
+        bitY.init(y);
+
+        vector<ll> Zcandidates = getValidZValues(l, r);
+
+        ll mx = -1, ans = r;
+        for(ll z: Zcandidates) {
+            ll currentF = f(x, y, z);
+            if(currentF > mx) {
+                mx = currentF;
+                ans = z;
+            }
+        }
+
+        cout << ans << endl;
+    }
+    
+    return 0;
+}