Advent of Code 2020 - Day 3 (Toboggan Trajectory)

day03.cpp

#include <fstream>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
const char TREE = '#';

int slope(vector<string> &forest, int right, int down){
    int row=0, col=0, cols_max=forest[0].size();
    int row_target=forest.size()-1, trees_count=0;

    while(row < row_target){
        col += right;
        row += down;
        if(col >= cols_max) col %= cols_max; // also col -= cols_max
        if(forest[row][col] == TREE) trees_count++;
    }
    return trees_count;
}

int main() {
    ios::sync_with_stdio(0);cin.tie(0);

    // read inputs
    string line;
    vector<string> forest;
    while (getline(cin, line)) {
        forest.push_back(line);
    }

    // part 1
    int slope31 = slope(forest, 3, 1);
    cout << "Part 1: " << slope31 << "\n";

    // part 2
    ll answer=slope31;
    vector<pii> moves = {{1,1}, {5,1}, {7,1}, {1,2}};
    for(auto &[right, left]: moves){
        answer *= slope(forest, right, left);
    }
    cout << "Part 2: " << answer << "\n";

    return 0;
}