1 files changed, 48 insertions, 1 deletions
diff --git a/src/2015/day25/aoc.cpp b/src/2015/day25/aoc.cpp
index dd622e9..9154c57 100644
--- a/src/2015/day25/aoc.cpp
+++ b/src/2015/day25/aoc.cpp
@@ -2,5 +2,52 @@
 
 namespace aoc2015 {
 
-std::pair<int64_t, int64_t> day25(line_view lv) { return {0, 0}; }
+/* So, to find the second code (which ends up in row 2, column 1),
+ * start with the previous value, 20151125.
+ * Multiply it by 252533 to get 5088824049625. Then, divide that by 33554393,
+ * which leaves a remainder of 31916031. That remainder is the second code.
+ */
+int64_t code_gen(int64_t code) {
+  code *= 252533;
+  code %= 33554393;
+  return code;
+}
+
+// n = 0 ..
+int64_t code_gen(int64_t code, size_t n) {
+  while (n-- > 0) {
+    code = code_gen(code);
+  }
+  return code;
+}
+
+// n = 1 ..
+int64_t total_number_by(size_t n) {
+  int64_t t{0};
+  for (size_t i = 1; i <= n; i++) {
+    t += i;
+  }
+  return t;
+}
+
+size_t row_number(int64_t r, int64_t c) { return r + c - 1; }
+
+int64_t code_by(size_t r, size_t c) {
+  size_t row = row_number(r, c);
+  int64_t total = total_number_by(row - 1);
+
+  for (size_t x = row; x >= 1; x--) {
+    if (x == r) {
+      return code_gen(20151125, total);
+    }
+    total += 1;
+  }
+  return 0;
+}
+
+// Enter the code at row 2947, column 3029.
+std::pair<int64_t, int64_t> day25(line_view lv) {
+  return {code_by(2947, 3029), 0};
+}
+
 } // namespace aoc2015