Implement day 16 part 1.

2019/src/day16.cpp

@@ -1,10 +1,57 @@
 #include <iostream>
+#include <array>
+#include <vector>
+#include <cassert>
+#include <iterator>
 #include "days.hpp"
 
+namespace {
+    std::array<int, 4> base_pattern{0, 1, 0, -1};
+
+    int get_modifier(int rank, int pos) {
+        pos += 1;
+        pos /= rank + 1;
+
+        return base_pattern[pos % 4];
+    }
+
+    std::vector<int> read_input(std::istream &input) {
+        std::vector<int> result;
+
+        for (char c; input >> c;) {
+            assert(std::isdigit(c));
+            result.push_back(c - '0');
+        }
+
+        return result;
+    }
+}
+
 void aoc2019::day16_part1(std::istream &input, std::ostream &output) {
-	output << "Not implemented\n";
+    auto numbers = read_input(input);
+
+    for (int i = 0; i < 100; ++i) {
+        std::vector<int> new_numbers;
+        new_numbers.reserve(numbers.size());
+
+        for (int rank = 0; rank < numbers.size(); ++rank) {
+            int n = 0;
+            for (int pos = 0; pos < numbers.size(); ++pos) {
+                n += get_modifier(rank, pos) * numbers[pos];
+            }
+
+            n = std::abs(n % 10);
+
+            new_numbers.push_back(n);
+        }
+
+        numbers = new_numbers;
+    }
+
+    std::copy(numbers.begin(), numbers.begin() + 8, std::ostream_iterator<int>(output));
+    output << std::endl;
 }
 
 void aoc2019::day16_part2(std::istream &input, std::ostream &output) {
-	output << "Not implemented\n";
+    output << "Not implemented\n";
 }