From c4270332e8046af6f40e99f0c9f3d2d1bc5cdc77 Mon Sep 17 00:00:00 2001
From: StepanTheGreat <117771383+StepanTheGreat@users.noreply.github.com>
Date: Sun, 21 Sep 2025 21:30:38 +0200
Subject: [PATCH] Allow updating a part of a buffer

---
 src/graphics.rs       | 10 +++++++++-
 src/graphics/gl.rs    | 14 ++++++++++----
 src/graphics/metal.rs | 11 ++++++++---
 3 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/src/graphics.rs b/src/graphics.rs
index dbdfffc6..62a80a7e 100644
--- a/src/graphics.rs
+++ b/src/graphics.rs
@@ -1280,7 +1280,15 @@ pub trait RenderingBackend {
     /// ```
     fn new_buffer(&mut self, type_: BufferType, usage: BufferUsage, data: BufferSource)
         -> BufferId;
-    fn buffer_update(&mut self, buffer: BufferId, data: BufferSource);
+
+    fn buffer_update(&mut self, buffer: BufferId, data: BufferSource) {
+        self.buffer_update_part(buffer, 0, data);
+    }
+
+    /// Update only a part of a buffer. 
+    /// 
+    /// The offset is given in bytes
+    fn buffer_update_part(&mut self, buffer: BufferId, offset: usize, data: BufferSource);
 
     /// Size of buffer in bytes.
     /// For 1 element, u16 buffer this will return 2.
diff --git a/src/graphics/gl.rs b/src/graphics/gl.rs
index 9c8c9844..04959847 100644
--- a/src/graphics/gl.rs
+++ b/src/graphics/gl.rs
@@ -1383,11 +1383,12 @@ impl RenderingBackend for GlContext {
         BufferId(self.buffers.add(buffer))
     }
 
-    fn buffer_update(&mut self, buffer: BufferId, data: BufferSource) {
+    fn buffer_update_part(&mut self, buffer: BufferId, offset: usize, data: BufferSource) {
         let data = match data {
             BufferSource::Slice(data) => data,
-            _ => panic!("buffer_update expects BufferSource::slice"),
+            _ => panic!("buffer_update_part expects BufferSource::slice"),
         };
+
         debug_assert!(data.is_slice);
         let buffer = &self.buffers[buffer.0];
 
@@ -1398,13 +1399,18 @@ impl RenderingBackend for GlContext {
 
         let size = data.size;
 
-        assert!(size <= buffer.size);
+        // Make sure that the offset isn't larger than the buffer's size itself
+        assert!(offset < buffer.size);
+        
+        // And that our data actually fits into the buffer with this offset
+        assert!(size <= buffer.size-offset);
 
         let gl_target = gl_buffer_target(&buffer.buffer_type);
         self.cache.store_buffer_binding(gl_target);
         self.cache
             .bind_buffer(gl_target, buffer.gl_buf, buffer.index_type);
-        unsafe { glBufferSubData(gl_target, 0, size as _, data.ptr as _) };
+
+        unsafe { glBufferSubData(gl_target, offset as _, size as _, data.ptr as _) };
         self.cache.restore_buffer_binding(gl_target);
     }
 
diff --git a/src/graphics/metal.rs b/src/graphics/metal.rs
index 39004579..9f58ed89 100644
--- a/src/graphics/metal.rs
+++ b/src/graphics/metal.rs
@@ -634,12 +634,17 @@ impl RenderingBackend for MetalContext {
         BufferId(self.buffers.len() - 1)
     }
 
-    fn buffer_update(&mut self, buffer: BufferId, data: BufferSource) {
+    fn buffer_update_part(&mut self, buffer: BufferId, offset: usize, data: BufferSource) {
         let data = match data {
             BufferSource::Slice(data) => data,
-            _ => panic!("buffer_update expects BufferSource::slice"),
+            _ => panic!("buffer_update_part expects BufferSource::slice"),
         };
         let buffer = &mut self.buffers[buffer.0];
+
+        // Make sure that the offset isn't larger than the buffer's size itself
+        assert!(offset < buffer.size);
+
+        // And that our data actually fits into the buffer with this offset
         assert!(data.size <= buffer.size);
 
         unsafe {
@@ -647,7 +652,7 @@ impl RenderingBackend for MetalContext {
             std::ptr::copy(data.ptr, dest, data.size);
 
             #[cfg(target_os = "macos")]
-            msg_send_![buffer.raw[buffer.next_value], didModifyRange:NSRange::new(0, data.size as u64)];
+            msg_send_![buffer.raw[buffer.next_value], didModifyRange:NSRange::new(offset as u64, data.size as u64)];
         }
         buffer.value = buffer.next_value;
     }