From b9ec10d7aef4ae145e1200899376bdb4d22e8491 Mon Sep 17 00:00:00 2001
From: Simmo Saan <simmo.saan@gmail.com>
Date: Sun, 10 Dec 2023 12:35:51 +0200
Subject: [PATCH] Extract Geometry.polygonArea to library

---
 .../eu/sim642/adventofcode2023/Day10.scala     | 12 ++----------
 .../eu/sim642/adventofcodelib/Geometry.scala   | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 10 deletions(-)
 create mode 100644 src/main/scala/eu/sim642/adventofcodelib/Geometry.scala

diff --git a/src/main/scala/eu/sim642/adventofcode2023/Day10.scala b/src/main/scala/eu/sim642/adventofcode2023/Day10.scala
index 3f277278..8a69f9ed 100644
--- a/src/main/scala/eu/sim642/adventofcode2023/Day10.scala
+++ b/src/main/scala/eu/sim642/adventofcode2023/Day10.scala
@@ -1,6 +1,6 @@
 package eu.sim642.adventofcode2023
 
-import eu.sim642.adventofcodelib.Grid
+import eu.sim642.adventofcodelib.{Geometry, Grid}
 import eu.sim642.adventofcodelib.GridImplicits.*
 import eu.sim642.adventofcodelib.IteratorImplicits.*
 import eu.sim642.adventofcodelib.graph.{BFS, Distances, GraphTraversal, UnitNeighbors}
@@ -97,7 +97,6 @@ object Day10 {
   /**
    * Solution, which calculates inside points using Pick's theorem.
    * Area of the polygon is calculated using the shoelace formula.
-   * @see [[https://en.wikipedia.org/wiki/Shoelace_formula]]
    * @see [[https://en.wikipedia.org/wiki/Pick%27s_theorem]]
    */
   object PicksTheoremPart2Solution extends Part2Solution {
@@ -128,14 +127,7 @@ object Day10 {
 
       val loop = dfs(grid.posOf('S'), Set.empty, Nil)
 
-      val area = {
-        ((loop.last :: loop).iterator
-          .zipWithTail
-          .map(_ cross _)
-          .sum / 2).abs
-      }
-
-      area - loop.size / 2 + 1
+      Geometry.polygonArea(loop) - loop.size / 2 + 1
     }
   }
 
diff --git a/src/main/scala/eu/sim642/adventofcodelib/Geometry.scala b/src/main/scala/eu/sim642/adventofcodelib/Geometry.scala
new file mode 100644
index 00000000..a3f82be9
--- /dev/null
+++ b/src/main/scala/eu/sim642/adventofcodelib/Geometry.scala
@@ -0,0 +1,18 @@
+package eu.sim642.adventofcodelib
+
+import eu.sim642.adventofcodelib.IteratorImplicits.*
+import eu.sim642.adventofcodelib.pos.Pos
+
+object Geometry {
+
+  /**
+   * Calculates the area of a simple polygon using the shoelace formula.
+   * @see [[https://en.wikipedia.org/wiki/Shoelace_formula]]
+   */
+  def polygonArea(poss: Seq[Pos]): Int = {
+    ((poss.last +: poss).iterator
+      .zipWithTail
+      .map(_ cross _)
+      .sum / 2).abs
+  }
+}