error checking for Grid

lib/include/aocpp/Grid.hpp

@@ -5,6 +5,7 @@
 #include <vector>
 #include <utility>
 #include <type_traits>
+#include <stdexcept>
 
 #include <aocpp/Coord.hpp>
 
@@ -12,6 +13,21 @@ namespace aocpp {
 
 struct Grid {
   std::vector<std::string> rows;
+  std::size_t columns;
+  
+  Grid() : rows{}, columns{} {}
+  explicit Grid(std::vector<std::string> rows) : rows(std::move(rows))
+  {
+    columns = rows.empty() ? 0 : rows[0].size();
+    for (std::size_t i = 1; i < rows.size(); i++) {
+      if (rows[i].size() != columns) {
+        throw std::runtime_error{"grid not rectangular"};
+      }
+    }
+  }
+
+  auto Rows() const -> std::size_t { return rows.size(); }
+  auto Cols() const -> std::size_t { return columns; }
 
   auto contains(Coord c) const -> bool {
     return 
@@ -24,13 +40,26 @@ struct Grid {
   auto operator[](Coord c)       -> char& { return rows[c.y][c.x]; }
   auto operator[](Coord c) const -> char  { return rows[c.y][c.x]; }
 
+  /// @brief Parse a rectangular grid from a stream
+  /// @param in input stream read until eof
+  /// @return parsed grid
+  /// @throw std::runtime\_error when grid is not rectangular
   static auto Parse(std::istream & in) -> Grid {
     Grid result;
     std::string line;
-    while (std::getline(in, line)) {
+    if (std::getline(in, line)) {
+      result.columns = line.size();
       result.rows.emplace_back(std::move(line));
+      while (std::getline(in, line)) {
+        if (line.size() != result.columns) {
+          throw std::runtime_error{"bad grid"};
+        }
+        result.rows.emplace_back(std::move(line));
+      }
+    } else {
+      result.columns = 0;
     }
-    return {result};
+    return result;
   }
 
   template <typename F>