diff --git a/2019/17.cpp b/2019/17.cpp
index d29e0e9..5a45333 100644
--- a/2019/17.cpp
+++ b/2019/17.cpp
@@ -17,7 +17,7 @@ namespace {
 
 auto GatherOutput(Machine m) -> Grid
 {
-    Grid grid;
+    std::vector<std::string> rows;
     bool eol = true; // next output starts a new line
 
     Run(m, []() -> ValueType {
@@ -25,16 +25,16 @@ auto GatherOutput(Machine m) -> Grid
     },
     [&](ValueType c) -> void {
       if (eol) {
-        grid.rows.emplace_back();
+        rows.emplace_back();
         eol = false;
       }
       if ('\n' == c) {
         eol = true;
       } else {
-        grid.rows.back().push_back(static_cast<char>(c));}
+        rows.back().push_back(static_cast<char>(c));}
       }
     );
-    return grid;
+    return Grid{std::move(rows)};
 }
 
 /// Find all the 4-way intersections.
diff --git a/2019/20.cpp b/2019/20.cpp
index fa7f882..a1d33d3 100644
--- a/2019/20.cpp
+++ b/2019/20.cpp
@@ -35,8 +35,8 @@ using Distances = std::map<Name,std::vector<std::pair<Name, std::int64_t>>>;
 auto FindPortals(Grid const& grid) -> Portals {
   Portals portals;
 
-  std::int64_t w = grid.rows[0].size();
-  std::int64_t h = grid.rows.size();
+  std::int64_t w = grid.Cols();
+  std::int64_t h = grid.Rows();
 
   for (std::int64_t x = 1; x < w-1; x++) {
     for (std::int64_t y = 1; y < h-1; y++) {
diff --git a/2019/24.cpp b/2019/24.cpp
index 72e6620..70156cb 100644
--- a/2019/24.cpp
+++ b/2019/24.cpp
@@ -24,8 +24,8 @@ namespace {
 auto FindBugs(Grid const& grid) -> std::vector<Coord> {
   std::vector<Coord> result;
 
-  std::int64_t w = grid.rows[0].size();
-  std::int64_t h = grid.rows.size();
+  std::int64_t w = grid.Cols();
+  std::int64_t h = grid.Rows();
 
   grid.each([&](Coord c, char v) {
     if (v == '#') result.push_back(c);
diff --git a/2020/03.cpp b/2020/03.cpp
index a06e007..fd4e005 100644
--- a/2020/03.cpp
+++ b/2020/03.cpp
@@ -22,11 +22,12 @@ namespace {
 
 auto Ski(Grid const& grid, Coord step) -> std::size_t
 {
-  std::size_t const w = grid.rows[0].size();
+  std::size_t const w = grid.Cols();
+  std::size_t const h = grid.Rows();
   std::size_t trees {0};
   Coord here {0,0};
 
-  while(here.y < grid.rows.size()) {
+  while(here.y < h) {
     if (grid[here] == '#') trees++;
     here += step;
     while (here.x >= w) { here.x -= w; }
diff --git a/lib/include/aocpp/Grid.hpp b/lib/include/aocpp/Grid.hpp
index 990ad77..5f17b77 100644
--- a/lib/include/aocpp/Grid.hpp
+++ b/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>