Add and pervasively use grid type (Bitmap)

lib/algorithms/geometry.dart

@@ -0,0 +1,47 @@
+// Dart has a habit of using double-inclusive bounds,
+// and I strongly prefer half-open bounds
+//
+// So: Here's a reimplementation of the geometry I need
+class Size {
+  final int dx;
+  final int dy;
+
+  Size(this.dx, this.dy) {
+    assert(dx >= 0);
+    assert(dy >= 0);
+  }
+}
+
+class Offset {
+  final int x;
+  final int y;
+
+  const Offset(this.x, this.y);
+}
+
+class Rect {
+  final int x0;
+  final int y0;
+  final int dx;
+  final int dy;
+
+  int get x1 => x0 + dx;
+  int get y1 => y0 + dy;
+
+  Rect(this.x0, this.y0, this.dx, this.dy) {
+    assert(dx >= 0);
+    assert(dy >= 0);
+  }
+
+  bool contains(int x, int y) {
+    return x0 <= x && x < x1 && y0 <= y && y < y1;
+  }
+
+  bool containsPoint(Offset xy) {
+    return contains(xy.x, xy.y);
+  }
+
+  bool containsRect(Rect rect) {
+    return x0 <= rect.x0 && y0 <= rect.y0 && rect.x1 <= x1 && rect.y1 <= y1;
+  }
+}

lib/algorithms/regionalize.dart

@@ -1,5 +1,7 @@
 import 'dart:math' as math;
 
+import 'package:dartterm/algorithms/geometry.dart' as geo;
+
 class Region {
   final math.Rectangle<int> rect;
   final Set<(int, int)> points;
@@ -25,14 +27,13 @@ class Region {
   }
 }
 
-List<Region> regionalize(
-    math.Rectangle<int> rect, bool Function(int, int) isAccessible) {
+List<Region> regionalize(geo.Rect rect, bool Function(int, int) isAccessible) {
   int nextRegion = 0;
   Map<(int, int), int> regions = {};
 
   int floodfill(int x, int y) {
     int workDone = 0;
-    if (!rect.containsPoint(math.Point(x, y))) {
+    if (!rect.containsPoint(geo.Offset(x, y))) {
       return workDone;
     }
     if (regions[(x, y)] != null) {
@@ -53,8 +54,8 @@ List<Region> regionalize(
 
   // TODO: This can be done more efficiently with a union/find data structure
   // But this is an easy implementation to understand
-  for (var y = rect.top; y < rect.bottom; y++) {
-    for (var x = rect.left; x < rect.right; x++) {
+  for (var y = rect.y0; y < rect.y1; y++) {
+    for (var x = rect.x0; x < rect.x1; x++) {
       if (regions[(x, y)] == null) {
         if (floodfill(x, y) > 0) {
           nextRegion += 1;