Day 4 2024

5 files changed, 236 insertions, 0 deletions

diff --git a/2024/04/Makefile b/2024/04/Makefile
new file mode 100644
index 0000000..855c831
--- /dev/null
+++ b/2024/04/Makefile
@@ -0,0 +1,24 @@
+CC=g++ -std=c++20 -g -Wall
+
+a:
+        ${CC} -o a.out day04a.cpp
+
+b:
+        ${CC} -o b.out day04b.cpp
+
+clean:
+        rm -f a b
+
+atest: a
+        ./a.out
+
+btest: b
+        ./b.out
+
+arun: a
+        ./a.out < input
+
+brun: b
+        ./b.out < input
+
+.PHONY: a b clean atest btest arun brun
diff --git a/2024/04/b.out b/2024/04/b.out
new file mode 100755
index 0000000..d0afbc2
--- /dev/null
+++ b/2024/04/b.out
diff --git a/2024/04/day04a.cpp b/2024/04/day04a.cpp
new file mode 100644
index 0000000..887ebef
--- /dev/null
+++ b/2024/04/day04a.cpp
@@ -0,0 +1,96 @@
+#include <algorithm>
+#include <iostream>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <vector>
+using namespace std;
+
+enum class Direction { U, D, R, L, UR, UL, DR, DL };
+Direction all_directions[] = {
+        Direction::U, Direction::D, Direction::R, Direction::L,
+        Direction::UR, Direction::UL, Direction::DR, Direction::DL
+};
+
+pair<int, int> step(pair<int, int> p, Direction d) {
+        auto [i, j] = p;
+
+        switch (d) {
+        case Direction::U:
+                return make_pair(i-1, j);
+        case Direction::D:
+                return make_pair(i+1, j);
+        case Direction::R:
+                return make_pair(i, j+1);
+        case Direction::L:
+                return make_pair(i, j-1);
+        case Direction::UR:
+                return make_pair(i-1, j+1);
+        case Direction::UL:
+                return make_pair(i-1, j-1);
+        case Direction::DR:
+                return make_pair(i+1, j+1);
+        case Direction::DL:
+                return make_pair(i+1, j-1);
+        }
+
+        return make_pair(-999,-999);
+}
+
+class Board {
+public:
+        int M, N;
+
+        Board(vector<string> &lines) {
+                N = lines.size();
+                M = 0;
+                for (string l : lines)
+                        M = max(M, (int)l.size());
+                cells = new char[M * N];
+                for (int i = 0; i < N; i++)
+                        for (int j = 0; j < M; j++)
+                                cells[N*i + j] = j < (int)lines[i].size() ?
+                                    lines[i][j] : out_of_bound;
+        }
+
+        string word_at(int i, int j, Direction d, int l) {
+                string ret = "";
+                auto p = make_pair(i, j);
+                for (int k = 0; k < l; k++, p = step(p, d))
+                        ret += (*this)[p];
+                return ret;
+        }
+
+        ~Board() {
+                delete []cells;
+        }
+
+private:
+        const char out_of_bound = '.';
+        char *cells;
+
+        char operator[](pair<int, int> p) {
+                auto [i, j] = p;
+                if (i >= N || i < 0 || j >= M || j < 0)
+                        return out_of_bound;
+                return cells[N*i + j];
+        }
+};
+
+int main() {
+        string line;
+        vector<string> lines;
+        while (getline(cin, line))
+                lines.push_back(line);
+
+        Board board(lines);
+        int tot = 0;
+        for (int i = 0; i < board.N; i++)
+                for (int j = 0; j < board.M; j++)
+                        for (Direction d : all_directions)
+                                if (board.word_at(i, j, d, 4) == "XMAS")
+                                        tot++;
+        cout << tot << endl;
+
+        return 0;
+}
diff --git a/2024/04/day04b.cpp b/2024/04/day04b.cpp
new file mode 100644
index 0000000..3d338d8
--- /dev/null
+++ b/2024/04/day04b.cpp
@@ -0,0 +1,114 @@
+#include <algorithm>
+#include <iostream>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <vector>
+using namespace std;
+
+enum class Direction { U, D, R, L, UR, UL, DR, DL };
+Direction all_directions[] = {
+        Direction::U, Direction::D, Direction::R, Direction::L,
+        Direction::UR, Direction::UL, Direction::DR, Direction::DL
+};
+
+pair<int, int> step(pair<int, int> p, Direction d) {
+        auto [i, j] = p;
+
+        switch (d) {
+        case Direction::U:
+                return make_pair(i-1, j);
+        case Direction::D:
+                return make_pair(i+1, j);
+        case Direction::R:
+                return make_pair(i, j+1);
+        case Direction::L:
+                return make_pair(i, j-1);
+        case Direction::UR:
+                return make_pair(i-1, j+1);
+        case Direction::UL:
+                return make_pair(i-1, j-1);
+        case Direction::DR:
+                return make_pair(i+1, j+1);
+        case Direction::DL:
+                return make_pair(i+1, j-1);
+        }
+
+        return make_pair(-999,-999);
+}
+
+class Board {
+public:
+        int M, N;
+
+        Board(vector<string> &lines) {
+                N = lines.size();
+                M = 0;
+                for (string l : lines)
+                        M = max(M, (int)l.size());
+                cells = new char[M * N];
+                for (int i = 0; i < N; i++)
+                        for (int j = 0; j < M; j++)
+                                cells[N*i + j] = j < (int)lines[i].size() ?
+                                    lines[i][j] : out_of_bound;
+        }
+
+        string word_at(int i, int j, Direction d, int l) {
+                string ret = "";
+                auto p = make_pair(i, j);
+                for (int k = 0; k < l; k++, p = step(p, d))
+                        ret += (*this)[p];
+                return ret;
+        }
+
+        ~Board() {
+                delete []cells;
+        }
+
+private:
+        const char out_of_bound = '.';
+        char *cells;
+
+        char operator[](pair<int, int> p) {
+                auto [i, j] = p;
+                if (i >= N || i < 0 || j >= M || j < 0)
+                        return out_of_bound;
+                return cells[N*i + j];
+        }
+};
+
+int main() {
+        string line;
+        vector<string> lines;
+        while (getline(cin, line))
+                lines.push_back(line);
+
+        Board board(lines);
+        int tot = 0;
+        for (int i = 0; i < board.N; i++) {
+                for (int j = 0; j < board.M; j++) {
+                        int tmp = 0;
+                        auto p = make_pair(i, j);
+                        auto [x, y] = step(p, Direction::UL);
+                        string wUL = board.word_at(x, y, Direction::DR, 3);
+                        tmp += wUL == "MAS";
+
+                        pair<int&, int&>(x, y) = step(p, Direction::UR);
+                        string wUR = board.word_at(x, y, Direction::DL, 3);
+                        tmp += wUR == "MAS";
+
+                        pair<int&, int&>(x, y) = step(p, Direction::DL);
+                        string wDL = board.word_at(x, y, Direction::UR, 3);
+                        tmp += wDL == "MAS";
+
+                        pair<int&, int&>(x, y) = step(p, Direction::DR);
+                        string wDR = board.word_at(x, y, Direction::UL, 3);
+                        tmp += wDR == "MAS";
+
+                        if (tmp == 2) tot++;
+                }
+        }
+        cout << tot << endl;
+
+        return 0;
+}
diff --git a/2024/learned.txt b/2024/learned.txt
index 63a0a71..4ab29b1 100644
--- a/2024/learned.txt
+++ b/2024/learned.txt
@@ -3,3 +3,5 @@ List of things I learned (or refreshed) with this year's AoC.
 * Day 1: std::count()
 * Day 2: std::sstream
 * Day 3: std::string_view
+* Day 4: nothing special, some practice with classic and a weird trick
+  to assign two variables at once with a pair of references pair<int&, int&>