1 files changed, 84 insertions, 0 deletions

diff --git a/2016/src/day16.cpp b/2016/src/day16.cpp
new file mode 100644
index 0000000..7d24dd3
--- /dev/null
+++ b/2016/src/day16.cpp
@@ -0,0 +1,84 @@
+#include <iostream>
+#include <string_view>
+
+using namespace std;
+
+namespace {
+
+size_t
+smallest_dragon_length(string_view start_bits, size_t disc_size)
+{
+        auto length = start_bits.size();
+        while ( length < disc_size ) {
+                length = 2 * length + 1;
+        }
+        return length;
+}
+
+char
+bit_at(size_t pos, size_t total_length, string_view start_bits)
+{
+        size_t inversions = 0;
+
+        auto invert_if_needed = [&](char chr) {
+                if ( (inversions % 2) == 1 ) {
+                        return (chr == '0') ? '1' : '0';
+                }
+                else {
+                        return chr;
+                }
+        };
+
+        while ( total_length > start_bits.size() ) {
+                auto prev_length = (total_length - 1) / 2;
+
+                if ( pos == prev_length ) {
+                        return invert_if_needed('0');
+                }
+                else if ( pos > prev_length ) {
+                        pos = prev_length - 1 - (pos - prev_length - 1);
+                        inversions++;
+                        total_length = prev_length;
+                }
+                else {
+                        total_length = prev_length;
+                }
+        }
+
+        auto bit = start_bits[pos];
+
+        return invert_if_needed(bit);
+}
+
+void
+checksum(string_view start_bits, size_t disc_size)
+{
+        auto data_length = smallest_dragon_length(start_bits, disc_size);
+
+        string bits;
+        for ( size_t i = 0; i != disc_size; ++i ) {
+                bits += bit_at(i, data_length, start_bits);
+        }
+
+        while ( bits.size() % 2 == 0 ) {
+                string new_bits;
+
+                for ( size_t i = 0; i != bits.size(); i += 2 ) {
+                        new_bits += (bits[i] == bits[i + 1]) ? '1' : '0';
+                }
+
+                bits = std::move(new_bits);
+        }
+
+        cout << bits << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+        // checksum("10000", 20);
+        checksum("00111101111101000", 272);
+        checksum("00111101111101000", 35651584);
+}