diff --git a/2018/loc.svg b/2018/loc.svg
index 3f034c5..6f60c7e 100644
--- a/2018/loc.svg
+++ b/2018/loc.svg
@@ -32,10 +32,10 @@ z
       <defs>
        <path d="M 0 0 
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-" id="m73b8dde3be" style="stroke:#000000;stroke-width:0.8;"/>
+" id="m7029ccf672" style="stroke:#000000;stroke-width:0.8;"/>
       </defs>
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="60.305455" xlink:href="#m73b8dde3be" y="307.584"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="60.305455" xlink:href="#m7029ccf672" y="307.584"/>
       </g>
      </g>
      <g id="text_1">
@@ -71,7 +71,7 @@ z
     <g id="xtick_2">
      <g id="line2d_2">
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="127.941818" xlink:href="#m73b8dde3be" y="307.584"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="127.941818" xlink:href="#m7029ccf672" y="307.584"/>
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      </g>
      <g id="text_2">
@@ -110,7 +110,7 @@ z
     <g id="xtick_3">
      <g id="line2d_3">
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="195.578182" xlink:href="#m73b8dde3be" y="307.584"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="195.578182" xlink:href="#m7029ccf672" y="307.584"/>
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      <g id="text_3">
@@ -139,7 +139,7 @@ z
     <g id="xtick_4">
      <g id="line2d_4">
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="263.214545" xlink:href="#m73b8dde3be" y="307.584"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="263.214545" xlink:href="#m7029ccf672" y="307.584"/>
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      </g>
      <g id="text_4">
@@ -153,7 +153,7 @@ z
     <g id="xtick_5">
      <g id="line2d_5">
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="330.850909" xlink:href="#m73b8dde3be" y="307.584"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="330.850909" xlink:href="#m7029ccf672" y="307.584"/>
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      <g id="text_5">
@@ -193,7 +193,7 @@ z
     <g id="xtick_6">
      <g id="line2d_6">
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="398.487273" xlink:href="#m73b8dde3be" y="307.584"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="398.487273" xlink:href="#m7029ccf672" y="307.584"/>
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      <g id="text_6">
@@ -286,10 +286,10 @@ z
       <defs>
        <path d="M 0 0 
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-" id="mc9d31d9449" style="stroke:#000000;stroke-width:0.8;"/>
+" id="m9772f4f0de" style="stroke:#000000;stroke-width:0.8;"/>
       </defs>
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#mc9d31d9449" y="295.488"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#m9772f4f0de" y="295.488"/>
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      </g>
      <g id="text_8">
@@ -302,7 +302,7 @@ L -3.5 0
     <g id="ytick_2">
      <g id="line2d_8">
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#mc9d31d9449" y="247.488"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#m9772f4f0de" y="247.488"/>
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      <g id="text_9">
@@ -316,7 +316,7 @@ L -3.5 0
     <g id="ytick_3">
      <g id="line2d_9">
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#mc9d31d9449" y="199.488"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#m9772f4f0de" y="199.488"/>
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      </g>
      <g id="text_10">
@@ -331,7 +331,7 @@ L -3.5 0
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       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#mc9d31d9449" y="151.488"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#m9772f4f0de" y="151.488"/>
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@@ -346,7 +346,7 @@ L -3.5 0
     <g id="ytick_5">
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       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#mc9d31d9449" y="103.488"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#m9772f4f0de" y="103.488"/>
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      </g>
      <g id="text_12">
@@ -361,7 +361,7 @@ L -3.5 0
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      <g id="line2d_12">
       <g>
-       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#mc9d31d9449" y="55.488"/>
+       <use style="stroke:#000000;stroke-width:0.8;" x="57.6" xlink:href="#m9772f4f0de" y="55.488"/>
       </g>
      </g>
      <g id="text_13">
@@ -577,7 +577,7 @@ z
     </g>
    </g>
    <g id="line2d_13">
-    <path clip-path="url(#p01ec4ed421)" d="M 73.832727 282.048 
+    <path clip-path="url(#p6b7ab71ead)" d="M 73.832727 282.048 
 L 87.36 277.248 
 L 100.887273 270.528 
 L 114.414545 269.568 
@@ -605,7 +605,7 @@ L 398.487273 289.728
 " style="fill:none;stroke:#1f77b4;stroke-linecap:square;stroke-width:1.5;"/>
    </g>
    <g id="line2d_14">
-    <path clip-path="url(#p01ec4ed421)" d="M 73.832727 295.488 
+    <path clip-path="url(#p6b7ab71ead)" d="M 73.832727 295.488 
 L 87.36 293.568 
 L 100.887273 295.488 
 L 114.414545 295.488 
@@ -633,7 +633,7 @@ L 398.487273 295.488
 " style="fill:none;stroke:#ff7f0e;stroke-linecap:square;stroke-width:1.5;"/>
    </g>
    <g id="line2d_15">
-    <path clip-path="url(#p01ec4ed421)" d="M 73.832727 232.128 
+    <path clip-path="url(#p6b7ab71ead)" d="M 73.832727 232.128 
 L 87.36 218.688 
 L 100.887273 208.128 
 L 114.414545 191.808 
@@ -650,7 +650,7 @@ L 249.687273 205.248
 L 263.214545 53.568 
 L 276.741818 81.408 
 L 290.269091 176.448 
-L 303.796364 146.688 
+L 303.796364 143.808 
 L 317.323636 277.248 
 L 330.850909 277.248 
 L 344.378182 277.248 
@@ -870,7 +870,7 @@ L 348.72625 83.926688
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  </g>
  <defs>
-  <clipPath id="p01ec4ed421">
+  <clipPath id="p6b7ab71ead">
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diff --git a/2018/src/day18.rs b/2018/src/day18.rs
index dc6fbd2..9690824 100644
--- a/2018/src/day18.rs
+++ b/2018/src/day18.rs
@@ -12,10 +12,11 @@ enum Tile {
     Tree,
     Lumber,
     Open,
+    Invalid,
 }
 
 impl Tile {
-    pub fn next(self, counts: &[usize; 3]) -> Self {
+    pub fn next(self, counts: &[usize; 4]) -> Self {
         match self {
             Tile::Open => {
                 if counts[Tile::Tree as usize] >= 3 {
@@ -38,6 +39,7 @@ impl Tile {
                     Tile::Open
                 }
             }
+            Tile::Invalid => Tile::Invalid,
         }
     }
 }
@@ -48,7 +50,7 @@ impl From<char> for Tile {
             '|' => Tile::Tree,
             '#' => Tile::Lumber,
             '.' => Tile::Open,
-            _ => panic!("Invalid tile '{}'", c)
+            _ => Tile::Invalid
         }
     }
 }
@@ -59,6 +61,7 @@ impl From<Tile> for char {
             Tile::Tree => '|',
             Tile::Lumber => '#',
             Tile::Open => '.',
+            Tile::Invalid => 'X',
         }
     }
 }
@@ -98,7 +101,7 @@ impl Day18 {
             let ymax = if y < height - 1 { y + 1 } else { y };
 
             for x in 0..self.width {
-                let mut counts = [0; 3];
+                let mut counts = [0; 4];
                 let xmin = if x > 0 { x - 1 } else { x };
                 let xmax = if x < width - 1 { x + 1 } else { x };