From e5b2bc8abbe041d56beb57b95cc444c20e526367 Mon Sep 17 00:00:00 2001
From: HampusM <hampus@hampusmat.com>
Date: Sat, 12 Sep 2026 12:50:44 +0200
Subject: feat(engine): add naive matrix multiplication fn

---
 engine/src/data_types/matrix.rs | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/engine/src/data_types/matrix.rs b/engine/src/data_types/matrix.rs
index 3c54bb1..e28c73f 100644
--- a/engine/src/data_types/matrix.rs
+++ b/engine/src/data_types/matrix.rs
@@ -1,4 +1,4 @@
-use std::ops::{Index, IndexMut, Mul};
+use std::ops::{Add, Index, IndexMut, Mul};
 
 use crate::data_types::dimens::Dimens3;
 use crate::vector::{Vec3, Vec4};
@@ -284,6 +284,36 @@ impl Mul<f32> for Matrix<f32, 4, 4>
     }
 }
 
+impl<Value, const ROWS: usize, const COLUMNS: usize, const RHS_COLUMNS: usize>
+    Mul<Matrix<Value, ROWS, RHS_COLUMNS>> for Matrix<Value, ROWS, COLUMNS>
+where
+    Value: Mul<Output = Value> + Add<Output = Value> + Default + Copy,
+{
+    type Output = Matrix<Value, ROWS, RHS_COLUMNS>;
+
+    fn mul(self, rhs: Matrix<Value, ROWS, RHS_COLUMNS>) -> Self::Output
+    {
+        // https://en.wikipedia.org/wiki/Matrix_multiplication
+
+        let mut out = Self::Output::new();
+
+        for (row_index, row) in self.items.iter().enumerate() {
+            for (column, item) in row.iter().enumerate() {
+                let rhs_items = &rhs.items[column];
+
+                for (rhs_item_index, rhs_item) in rhs_items.iter().enumerate() {
+                    let prev = out[CellPos { row: row_index, col: rhs_item_index }];
+
+                    out[CellPos { row: row_index, col: rhs_item_index }] =
+                        prev + (*item * *rhs_item);
+                }
+            }
+        }
+
+        out
+    }
+}
+
 impl<Value, const ROWS: usize, const COLUMNS: usize> Index<CellPos>
     for Matrix<Value, ROWS, COLUMNS>
 {
-- 
cgit v1.2.3-18-g5258