From bfcd78c43e88d513fafdd7c4633d0857bdcf5fb1 Mon Sep 17 00:00:00 2001
From: Filip Tibell <filip.tibell@gmail.com>
Date: Mon, 25 Sep 2023 13:43:59 -0500
Subject: [PATCH] Round decimal place in Vector3 and CFrame when converting to
 dom type

---
 CHANGELOG.md                          |  1 +
 src/roblox/datatypes/mod.rs           |  2 ++
 src/roblox/datatypes/types/vector3.rs | 11 +++++++----
 src/roblox/datatypes/util.rs          | 16 ++++++++++++++++
 4 files changed, 26 insertions(+), 4 deletions(-)
 create mode 100644 src/roblox/datatypes/util.rs

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 25234ed..5cf906d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -58,6 +58,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Update to Luau version `0.594`
+- CFrame and Vector3 values are now rounded to the nearest 2 ^ 16 decimal place to reduce floating point errors and diff noise. Note that this does not affect intermediate calculations done in lua, and only happens when a property value is set on an Instance.
 
 ### Fixed
 
diff --git a/src/roblox/datatypes/mod.rs b/src/roblox/datatypes/mod.rs
index 6604d5f..5432c50 100644
--- a/src/roblox/datatypes/mod.rs
+++ b/src/roblox/datatypes/mod.rs
@@ -6,6 +6,8 @@ pub mod extension;
 pub mod result;
 pub mod types;
 
+mod util;
+
 use result::*;
 
 pub use crate::roblox::shared::userdata::*;
diff --git a/src/roblox/datatypes/types/vector3.rs b/src/roblox/datatypes/types/vector3.rs
index a7ba383..ef0307e 100644
--- a/src/roblox/datatypes/types/vector3.rs
+++ b/src/roblox/datatypes/types/vector3.rs
@@ -5,7 +5,10 @@ use glam::Vec3;
 use mlua::prelude::*;
 use rbx_dom_weak::types::Vector3 as DomVector3;
 
-use crate::{lune::util::TableBuilder, roblox::exports::LuaExportsTable};
+use crate::{
+    lune::util::TableBuilder,
+    roblox::{datatypes::util::round_float_decimal, exports::LuaExportsTable},
+};
 
 use super::{super::*, EnumItem};
 
@@ -212,9 +215,9 @@ impl From<DomVector3> for Vector3 {
 impl From<Vector3> for DomVector3 {
     fn from(v: Vector3) -> Self {
         DomVector3 {
-            x: v.0.x,
-            y: v.0.y,
-            z: v.0.z,
+            x: round_float_decimal(v.0.x),
+            y: round_float_decimal(v.0.y),
+            z: round_float_decimal(v.0.z),
         }
     }
 }
diff --git a/src/roblox/datatypes/util.rs b/src/roblox/datatypes/util.rs
new file mode 100644
index 0000000..7855507
--- /dev/null
+++ b/src/roblox/datatypes/util.rs
@@ -0,0 +1,16 @@
+// HACK: We round to the nearest Very Small Decimal
+// to reduce writing out floating point accumulation
+// errors to files (mostly relevant for xml formats)
+const ROUNDING: usize = 65_536; // 2 ^ 16
+
+pub fn round_float_decimal(value: f32) -> f32 {
+    let place = ROUNDING as f32;
+
+    // Round only the fractional part, we do not want to
+    // lose any float precision in case a user for some
+    // reason has very very large float numbers in files
+    let whole = value.trunc();
+    let fract = (value.fract() * place).round() / place;
+
+    whole + fract
+}