1 files changed, 131 insertions, 0 deletions
diff --git a/year2021/day16/part2.cpp b/year2021/day16/part2.cpp
new file mode 100644
index 0000000..5a02df9
--- /dev/null
+++ b/year2021/day16/part2.cpp
@@ -0,0 +1,131 @@
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <list>
+#include <map>
+#include <string>
+#include <string_view>
+#include <tuple>
+
+static const std::map<char, const char *> hexToBin = {
+    {'0', "0000"}, {'1', "0001"}, {'2', "0010"}, {'3', "0011"},
+    {'4', "0100"}, {'5', "0101"}, {'6', "0110"}, {'7', "0111"},
+    {'8', "1000"}, {'9', "1001"}, {'A', "1010"}, {'B', "1011"},
+    {'C', "1100"}, {'D', "1101"}, {'E', "1110"}, {'F', "1111"}
+};
+
+static std::pair<uint64_t, std::string_view> solve(std::string_view packet);
+
+int main(int argc, const char *argv[])
+{
+    if (argc != 2)
+        return -1;
+
+    std::string packetBin;
+    std::string packetHex (argv[1]);
+    for (char c : packetHex)
+        packetBin += hexToBin.at(c);
+
+    std::cout << solve(packetBin).first << std::endl;
+
+    return 0;
+}
+
+uint64_t binStrToInt(std::string_view bin)
+{
+    uint64_t n = 0;
+
+    for (char c : bin) {
+        n = n * 2ull;
+        if (c == '1')
+            ++n;
+    }
+
+    return n;
+}
+
+bool isEmptyPacket(std::string_view packet)
+{
+    return packet.empty() ||
+           std::all_of(packet.cbegin(), packet.cend(),
+                       [](char c) { return c == '0'; });
+}
+
+std::pair<uint64_t, std::string_view> solve(std::string_view packet)
+{
+    // Remove version
+    packet = packet.substr(3);
+    // Pull type ID
+    const auto typeId = packet.substr(0, 3);
+    packet = packet.substr(3);
+    
+    if (typeId == "100") {
+        std::string numberStr;
+        while (1) {
+            const auto chunk = packet.substr(0, 5);
+            packet = packet.substr(5);
+            numberStr += chunk.substr(1);
+            if (chunk.front() == '0')
+                break;
+        }
+
+        return {binStrToInt(numberStr), isEmptyPacket(packet) ? "" : packet};
+    } else {
+        const bool isLengthBits = packet.front() == '0';
+        packet = packet.substr(1);
+        const auto lengthStr = packet.substr(0, isLengthBits ? 15 : 11);
+        const auto length = binStrToInt(lengthStr);
+        packet = packet.substr(isLengthBits ? 15 : 11);
+
+        std::list<uint64_t> args;
+        if (isLengthBits) {
+            auto rem = packet.substr(0, length);
+            while (1) {
+                const auto ret = solve(rem);
+                args.emplace_front(ret.first);
+                if (ret.second.empty() || isEmptyPacket(ret.second))
+                    break;
+                rem = ret.second;
+            }
+            packet = packet.substr(length);
+        } else {
+            for (uint64_t i = 0; i < length; ++i) {
+                const auto ret = solve(packet);
+                args.emplace_front(ret.first);
+                packet = ret.second;
+                if (isEmptyPacket(packet))
+                    break;
+            }
+        }
+
+        uint64_t result;
+        if (typeId == "000") {
+            result = 0;
+            for (auto a : args)
+                result += a;
+        } else if (typeId == "001") {
+            result = 1;
+            for (auto a : args)
+                result *= a;
+        } else if (typeId == "010") {
+            result = *std::min_element(args.cbegin(), args.cend());
+        } else if (typeId == "011") {
+            result = *std::max_element(args.cbegin(), args.cend());
+        } else if (typeId == "101") {
+            const auto a1 = args.back();
+            args.pop_back();
+            result = args.back() < a1;
+        } else if (typeId == "110") {
+            const auto a1 = args.back();
+            args.pop_back();
+            result = args.back() > a1;
+        } else if (typeId == "111") {
+            const auto a1 = args.back();
+            args.pop_back();
+            result = args.back() == a1;
+        }
+
+        return {result, isEmptyPacket(packet) ? "" : packet};
+    }
+}
+