AorB HackerRank - C++ study code #4

AorB_studyCplusplus_ex4.cpp

#include "bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
static const int INF = 0x3f3f3f3f; static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> static void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> static void amax(T &x, U y) { if(x < y) x = y; }

const char *digits = "0123456789ABCDEF";
int hexmap[128];

bool getbit(const string &s, int i) {
	if(i >= (int)s.size() * 4)
		return false;
	else
		return hexmap[s[i / 4]] >> (i % 4) & 1;
}

string toHex(const vector<bool> &v) {
	assert(v.size() % 4 == 0);
	string s;
	rep(i, v.size() / 4) {
		int x = 0;
		rep(j, 4)
			x |= (v[i * 4 + j] ? 1 : 0) << j;
		s += digits[x];
	}
	while(s.size() > 1 && s.back() == '0')
		s.pop_back();
	reverse(s.begin(), s.end());
	return s;
}

int main() {
	rep(i, 16)
		hexmap[digits[i]] = i;
	int T;
	scanf("%d", &T);
	for(int ii = 0; ii < T; ++ ii) {
		int K;
		scanf("%d", &K);
		string A, B, C;
		cin >> A >> B >> C;
		reverse(A.begin(), A.end());
		reverse(B.begin(), B.end());
		reverse(C.begin(), C.end());
		int n = (int)max({ A.size(), B.size(), C.size() }) * 4;
		vector<bool> newA(n), newB(n);
		rep(i, n) {
			newA[i] = getbit(A, i);
			newB[i] = getbit(B, i);
		}
		int diff = 0;
		vector<bool> change(n);
		rep(i, n) {
			bool a = newA[i], b = newB[i], c = getbit(C, i);
			if((a || b) != c) {
				if(!c) {
					diff += a + b;
					newA[i] = newB[i] = false;
				} else {
					diff += 1;
					newB[i] = true;
				}
				change[i] = true;
			}
		}
		if(diff > K) {
			puts("-1");
			continue;
		}
		K -= diff;
		for(int i = n - 1; i >= 0; -- i) {
			if(getbit(C, i)) {
				assert(newA[i] || newB[i]);
				if(newA[i] && !newB[i]) {
					if(K >= 2) {
						newA[i] = false;
						newB[i] = true;
						K -= 2;
					}
				} else if(newA[i] && newB[i]) {
					if(K >= 1) {
						newA[i] = false;
						K -= 1;
					}
				}
			}
		}
		string ansA = toHex(newA), ansB = toHex(newB);
		puts(ansA.c_str());
		puts(ansB.c_str());
	}
	return 0;
}

can you please explain static const int INF = 0x3f3f3f3f why this is used in line 6? Also I've seen people using this in many coding contests. what is this and why is this preferred?