diff --git a/package-lock.json b/package-lock.json
index 83821ba..3bd60f7 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,6 +1,6 @@
 {
   "name": "pixelmanipulator",
-  "version": "5.5.1",
+  "version": "5.5.2",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
@@ -1615,6 +1615,12 @@
         }
       }
     },
+    "ava-fast-check": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/ava-fast-check/-/ava-fast-check-5.0.0.tgz",
+      "integrity": "sha512-ZXXVVJ74KntRkG1Iolgiia+UUNU/uN8PZpvae3BiuZVkGW9oyhuy3FUHEEzjYvpkwK7gzNw0rUzsuyLidXPNSQ==",
+      "dev": true
+    },
     "available-typed-arrays": {
       "version": "1.0.5",
       "resolved": "https://registry.npmjs.org/available-typed-arrays/-/available-typed-arrays-1.0.5.tgz",
@@ -3335,6 +3341,15 @@
       "integrity": "sha1-lpGEQOMEGnpBT4xS48V06zw+HgU=",
       "dev": true
     },
+    "fast-check": {
+      "version": "2.25.0",
+      "resolved": "https://registry.npmjs.org/fast-check/-/fast-check-2.25.0.tgz",
+      "integrity": "sha512-wRUT2KD2lAmT75WNIJIHECawoUUMHM0I5jrlLXGtGeqmPL8jl/EldUDjY1VCp6fDY8yflyfUeIOsOBrIbIiArg==",
+      "dev": true,
+      "requires": {
+        "pure-rand": "^5.0.1"
+      }
+    },
     "fast-deep-equal": {
       "version": "3.1.3",
       "resolved": "https://registry.npmjs.org/fast-deep-equal/-/fast-deep-equal-3.1.3.tgz",
@@ -5306,6 +5321,12 @@
       "integrity": "sha1-wNWmOycYgArY4esPpSachN1BhF4=",
       "dev": true
     },
+    "pure-rand": {
+      "version": "5.0.1",
+      "resolved": "https://registry.npmjs.org/pure-rand/-/pure-rand-5.0.1.tgz",
+      "integrity": "sha512-ksWccjmXOHU2gJBnH0cK1lSYdvSZ0zLoCMSz/nTGh6hDvCSgcRxDyIcOBD6KNxFz3xhMPm/T267Tbe2JRymKEQ==",
+      "dev": true
+    },
     "qs": {
       "version": "6.5.3",
       "resolved": "https://registry.npmjs.org/qs/-/qs-6.5.3.tgz",
diff --git a/package.json b/package.json
index c9313fb..4d03039 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "pixelmanipulator",
-  "version": "5.5.1",
+  "version": "5.5.2",
   "description": "Run any cellular automata on an html5 canvas.",
   "main": "dist/main.js",
   "browser": "dist/browser.js",
@@ -68,6 +68,7 @@
     "@typescript-eslint/eslint-plugin": "^4.0.1",
     "@typescript-eslint/parser": "^4.0.0",
     "ava": "^4.2.0",
+    "ava-fast-check": "^5.0.0",
     "bootstrap-dark-5": "^1.1.3",
     "browserify": "^17.0.0",
     "c8": "^7.11.2",
@@ -78,6 +79,7 @@
     "eslint-plugin-node": "^11.1.0",
     "eslint-plugin-promise": "^4.2.1 || ^5.0.0",
     "eslint-plugin-tsdoc": "^0.2.16",
+    "fast-check": "^2.25.0",
     "fps-control": "^1.0.0",
     "gh-pages": "^3.2.3",
     "parcel": "^2.5.0",
diff --git a/src/lib/neighborhoods.ts b/src/lib/neighborhoods.ts
index 266a958..4da60c4 100644
--- a/src/lib/neighborhoods.ts
+++ b/src/lib/neighborhoods.ts
@@ -26,7 +26,7 @@ export type Hitbox=Location[]
 */
 export function rect (topLeft: Location, bottomRight: Location): Location[] {
   const output: Hitbox = []
-  for (let x = topLeft.x; x <= bottomRight.y; x++) {
+  for (let x = topLeft.x; x <= bottomRight.x; x++) {
     for (let y = topLeft.y; y <= bottomRight.y; y++) {
       output.push({ x, y })
     }
diff --git a/tests/lib/test-neighborhoods.ts b/tests/lib/test-neighborhoods.ts
new file mode 100644
index 0000000..dcbf791
--- /dev/null
+++ b/tests/lib/test-neighborhoods.ts
@@ -0,0 +1,59 @@
+import { testProp, fc } from 'ava-fast-check'
+import { neighborhoods } from '../../src/lib/pixelmanipulator'
+const { rect } = neighborhoods
+
+testProp(
+  'rect\'s size output is the area',
+  // Maximums provided to ensure reasonable time and memory is spent
+  [fc.integer(), fc.integer(), fc.nat({ max: 25 }), fc.nat({ max: 25 })],
+  (t, x, y, w, h) => {
+    const topLeft = {
+      x,
+      y
+    }
+    const bottomRight = {
+      x: x + w,
+      y: y + h
+    }
+    const expectedArea = (w + 1) * (h + 1) // (0,0) to (1,1) is area of 4
+    const list = rect(topLeft, bottomRight)
+    t.is(list.length, expectedArea)
+  }
+)
+testProp(
+  'return from rect has no duplicates',
+  [fc.integer(), fc.integer(), fc.nat({ max: 25 }), fc.nat({ max: 25 })],
+  (t, x, y, w, h) => {
+    const topLeft = {
+      x,
+      y
+    }
+    const bottomRight = {
+      x: x + w,
+      y: y + h
+    }
+    const list = rect(topLeft, bottomRight).map(pos => JSON.stringify(pos))
+    const set = new Set(list)
+    t.is(list.length, set.size)
+  }
+)
+testProp(
+  'return from rect keeps all values within bounds',
+  [fc.integer(), fc.integer(), fc.nat({ max: 25 }), fc.nat({ max: 25 })],
+  (t, x, y, w, h) => {
+    const topLeft = {
+      x,
+      y
+    }
+    const bottomRight = {
+      x: x + w,
+      y: y + h
+    }
+    const list = rect(topLeft, bottomRight)
+    t.plan(list.length * 2)
+    list.forEach(pos => {
+      t.true(topLeft.x <= pos.x && pos.x <= bottomRight.x)
+      t.true(topLeft.y <= pos.y && pos.y <= bottomRight.y)
+    })
+  }
+)