Teaching and learning resources • Re: Advent of Code 2023

The result is a worst case time complexity of O((N+M)²) versus O((N*M)²). Unless I missed something, it would appear the universe is so sparsely populated that either method results in reasonably fast solutions.

Your approach is twice as fast even in a sparse universe.

Code:

--- Day 11: Cosmic Expansion ---Part 1  - 9509330Part 2  - 635832237682Elapsed - 0.125 ms.

Using your algorithm, I think?

Code:

constexpr auto title = "--- Day 11: Cosmic Expansion ---";#include <chrono>#include <fstream>#include <iostream>#include <string>#include <vector>using namespace std;using namespace chrono;static vector<string> lines;static int count_row(int row) {    int count = 0;    for (char c : lines[row])        if (c == '#')            count++;    return count;}static int count_col(int col) {    int count = 0;    for (string& row : lines)        if (row[col] == '#')            count++;    return count;}int64_t part(uint32_t delta) {    vector<int> row_pos, row_count, col_pos, col_count;    int y_inc = 0;    for (int y = 0; y < lines.size(); y++) {        int row_n = count_row(y);        if (row_n == 0)            y_inc += delta;        else {            row_pos.push_back(y + y_inc);            row_count.push_back(row_n);        }    }    int x_inc = 0;    for (int x = 0; x < lines[0].size(); x++) {        int col_n = count_col(x);        if (col_n == 0)            x_inc += delta;        else {            col_pos.push_back(x + x_inc);            col_count.push_back(col_n);        }    }    int64_t dist = 0;    for (int y = 0; y < row_pos.size() - 1; y++)        for (int y2 = y + 1; y2 < row_pos.size(); y2++)            dist += int64_t(row_pos[y2] - row_pos[y]) * row_count[y] * row_count[y2];    for (int x = 0; x < col_pos.size() - 1; x++)        for (int x2 = x + 1; x2 < col_pos.size(); x2++)            dist += int64_t(col_pos[x2] - col_pos[x]) * col_count[x] * col_count[x2];    return dist;}int main() {    auto start = high_resolution_clock::now();    ifstream f("day11.txt");    string line;    while (getline(f, line))        lines.push_back(line);    cout << title << endl         << "Part 1  - " << part(1) << endl         << "Part 2  - " << part(999999) << endl         << "Elapsed - "         << duration_cast<microseconds>(high_resolution_clock::now() - start).count() / 1000.0         << " ms." << endl;}

Statistics: Posted by ejolson — Sat Jan 06, 2024 3:04 am