Implement factory offset saving

rc_core/src/cubes/offsetter.rs

@@ -0,0 +1,26 @@
+pub struct OffsetParser;
+
+impl OffsetParser {
+    pub fn with_cubes<'a, I: std::iter::Iterator<Item=&'a crate::persist::Cube>>(_iter: I) -> Self {
+        Self
+    }
+
+    /// offset is (x, y, z)
+    pub fn offset_inplace_by(&self, cubes: &mut [u8], colours: &mut [u8], offset: (i16, i16, i16)) {
+        // this assumes cubes and colours are valid and length-prefixed
+        let cubes = &mut cubes[4..];
+        let colours = &mut colours[4..];
+        for cube in cubes.chunks_mut(8) {
+            // bytes 4, 5, 6 are x, y, z (respectively)
+            cube[4] = (cube[4] as i16 + offset.0) as _;
+            cube[5] = (cube[5] as i16 + offset.1) as _;
+            cube[6] = (cube[6] as i16 + offset.2) as _;
+        }
+        for colour in colours.chunks_mut(4) {
+            // last 3 bytes are x, y, z (respectively)
+            colour[1] = (colour[1] as i16 + offset.0) as _;
+            colour[2] = (colour[2] as i16 + offset.1) as _;
+            colour[3] = (colour[3] as i16 + offset.2) as _;
+        }
+    }
+}