Add 2021 day 16 puzzles

2021/puzzle-16-01.cc

@@ -0,0 +1,110 @@
+#include <iostream>
+#include <string>
+#include <vector>
+
+using UInt = unsigned long;
+
+namespace {
+
+auto bit(std::string const& stream, std::size_t idx) -> UInt
+{
+  std::string_view vals{"0123456789ABCDEF"};
+  auto val{vals.find(stream[idx / 4])};
+  assert(val != std::string::npos);
+  val = (val >> (3 - (idx % 4))) & 1;
+  return val;
+}
+
+auto read_number(std::string const& stream, std::size_t idx, std::size_t length) -> UInt
+{
+  UInt result{0};
+  while (length-- != 0) {
+    result <<= 1;
+    result |= bit(stream, idx++);
+  }
+  return result;
+}
+
+struct Operator
+{
+  static auto build(std::string const& stream) -> Operator
+  {
+    auto [result, idx] = build(stream, 0);
+    return result;
+  }
+
+  auto version_sum() const noexcept -> UInt
+  {
+    UInt version{version_};
+    for (auto const& child : children_) {
+      version += child.version_sum();
+    }
+    return version;
+  }
+
+private:
+  static auto build(std::string const& stream, std::size_t idx) -> std::pair<Operator, std::size_t>
+  {
+    UInt version{read_number(stream, idx, 3)};
+    idx += 3;
+    UInt type{read_number(stream, idx, 3)};
+    idx += 3;
+    Operator result{version, type};
+    if (type == 4) {
+      UInt value{0};
+      bool cont{false};
+      do {
+        cont = (bit(stream, idx++) == 1);
+        value <<= 4;
+        value |= read_number(stream, idx, 4);
+        idx += 4;
+      } while (cont);
+      result.value_ = value;
+    }
+    else {
+      bool count_subpackets{bit(stream, idx++) == 1};
+      if (count_subpackets) {
+        UInt count{read_number(stream, idx, 11)};
+        idx += 11;
+        while (count-- != 0) {
+          auto [subpacket, new_idx] = build(stream, idx);
+          result.children_.push_back(subpacket);
+          idx = new_idx;
+        }
+      }
+      else {
+        UInt size{read_number(stream, idx, 15)};
+        idx += 15;
+        auto target_idx{idx + size};
+        while (idx < target_idx) {
+          auto [subpacket, new_idx] = build(stream, idx);
+          result.children_.push_back(subpacket);
+          idx = new_idx;
+        }
+      }
+    }
+
+    return {result, idx};
+  }
+
+  Operator(UInt version, UInt type) : version_(version), type_(type) {}
+
+  UInt version_;
+  UInt type_;
+  UInt value_{0};
+  std::vector<Operator> children_;
+};
+}  // namespace
+
+auto main() -> int
+{
+  std::string line;
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read line\n";
+    return 1;
+  }
+
+  auto operators{Operator::build(line)};
+  std::cout << "Version sums: " << operators.version_sum() << '\n';
+  return 0;
+}