From ea1d06e02b7e5c92c94871267a4db2dc95843a90 Mon Sep 17 00:00:00 2001
From: MoBit <mo8it@protonmail.com>
Date: Fri, 3 Dec 2021 14:41:01 +0100
Subject: [PATCH] gyration_tensor

---
 src/shape.jl | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/src/shape.jl b/src/shape.jl
index fbfc09b..084a88b 100644
--- a/src/shape.jl
+++ b/src/shape.jl
@@ -1,4 +1,4 @@
-using StaticArrays: SVector
+using StaticArrays: SVector, SMatrix
 
 function project_to_unit_circle(x, l)
     φ = (x + l) * π / l
@@ -42,4 +42,24 @@ function center_of_mass(args)
         project_back_from_unit_circle(x_θ, args.l),
         project_back_from_unit_circle(y_θ, args.l),
     )
+end
+
+function gyration_tensor(args)
+    COM = center_of_mass(args)
+
+    S11 = 0.0
+    S12 = 0.0
+    S22 = 0.0
+
+    for p in args.particles
+        c = p.c
+        x = c[1] - COM[1]
+        y = c[2] - COM[2]
+
+        S11 += x^2
+        S12 += x * y
+        S22 += y^2
+    end
+
+    return SMatrix{2,2}(S11, S12, S12, S22)
 end
\ No newline at end of file