aoc 2015, day 25

1 files changed, 53 insertions, 0 deletions

diff --git a/2015/src/day25.cpp b/2015/src/day25.cpp
new file mode 100644
index 0000000..ed02c9b
--- /dev/null
+++ b/2015/src/day25.cpp
@@ -0,0 +1,53 @@
+#include <fstream>
+#include <iostream>
+#include <sstream>
+#include <vector>
+using namespace std;
+
+tuple<long, long>
+read_file(string_view filename)
+{
+        fstream input{ filename };
+
+        vector<long> values;
+        for ( string word; input >> word; ) {
+                long value{};
+                if ( stringstream(word) >> value ) {
+                        values.push_back(value);
+                }
+        }
+        return { values.at(0), values.at(1) };
+}
+
+long
+triangle_num(long row, long col)
+{
+        const auto side = row + col - 1;
+        return (side * (side + 1)) / 2 - row;
+}
+
+long
+powmod(long base, long pow, long mod)
+{
+        long result = 0;
+        for ( result = 1; pow != 0; pow /= 2 ) {
+                if ( pow % 2 == 1 ) {
+                        result = (result * base) % mod;
+                }
+                base = (base * base) % mod;
+        }
+        return result;
+}
+
+void
+part1(long row, long col)
+{
+        cout << (powmod(252533, triangle_num(row, col), 33554393) * 20151125) % 33554393 << endl;
+}
+
+int
+main()
+{
+        const auto [row, col] = read_file("data/day25.txt");
+        part1(row, col);
+}