Initial commit

2020/puzzle-05-01.cc

@@ -0,0 +1,60 @@
+#include <cassert>
+#include <iostream>
+#include <string>
+
+/** \brief Return the position described by BSP in s.
+ *  \tparam L2 log2 of the max position.
+ *  \tparam LOWER character which means use lower half.
+ *  \tparam UPPER character which means use upper half.
+ *  \param  s     String to parse, must be L2 characters long
+ *  \return       Position described by s.
+ */
+template <unsigned L2, char LOWER, char UPPER>
+unsigned find_pos(std::string const &s) {
+  assert(s.length() == L2);
+  unsigned low = 0;
+  unsigned high = 1 << L2;
+  unsigned width = 1 << (L2 - 1);
+  for (auto c : s) {
+    assert(high != low);
+    assert(width != 0);
+    if (c == LOWER) {
+      high = low + width;
+    } else if (c == UPPER) {
+      low = low + width;
+    } else {
+      assert(false);
+    }
+    width >>= 1;
+  }
+
+  assert(low == high - 1);
+  return low;
+}
+
+template <unsigned L2R, unsigned L2C> struct Seat {
+  Seat(std::string const &s) {
+    assert(s.length() == L2R + L2C);
+    row_ = find_pos<L2R, 'F', 'B'>(s.substr(0, L2R));
+    col_ = find_pos<L2C, 'L', 'R'>(s.substr(L2R, L2C));
+    std::cout << s << ": row " << row_ << ", column " << col_ << ", seat ID "
+              << id() << ".\n";
+  }
+
+  unsigned id() const noexcept { return row_ * (1 << L2C) + col_; }
+
+  unsigned row_;
+  unsigned col_;
+};
+
+int main(int argc, char **argv) {
+  unsigned max_id = 0;
+
+  for (std::string line; std::getline(std::cin, line);) {
+    Seat<7, 3> seat(line);
+    max_id = std::max(max_id, seat.id());
+  }
+
+  std::cout << "Max ID: " << max_id << "\n";
+  return 0;
+}