diff --git a/src/constraints.rs b/src/constraints.rs
new file mode 100644
index 0000000..02ac2c3
--- /dev/null
+++ b/src/constraints.rs
@@ -0,0 +1,23 @@
+use std::os::raw::*;
+
+const MAX_BORDER_WIDTH: c_int = 10000;
+
+pub fn border_width(border_width: c_int) -> c_int {
+	if border_width > MAX_BORDER_WIDTH { return MAX_BORDER_WIDTH }
+	border_width
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn test_border_width() {
+		assert_eq!(border_width(0), 0);
+		assert_eq!(border_width(10), 10);
+		assert_eq!(border_width(10_000), 10_000);
+
+		assert_eq!(border_width(10_001), 10_000);
+		assert_eq!(border_width(20_000), 10_000);
+	}
+}
diff --git a/src/lib.rs b/src/lib.rs
index eba48aa..a80bb72 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,24 +1,8 @@
+mod constraints;
+
 use std::os::raw::*;
 
-const MAX_BORDER_WIDTH: c_int = 10000;
-
 #[no_mangle]
-pub extern "C" fn constraints_border_width(border_width: c_int) -> c_int {
-	if border_width > MAX_BORDER_WIDTH { return MAX_BORDER_WIDTH }
-	border_width
-}
-
-#[cfg(test)]
-mod tests {
-	use super::*;
-
-	#[test]
-	fn test_constraints_border_width() {
-		assert_eq!(constraints_border_width(0), 0);
-		assert_eq!(constraints_border_width(10), 10);
-		assert_eq!(constraints_border_width(10_000), 10_000);
-
-		assert_eq!(constraints_border_width(10_001), 10_000);
-		assert_eq!(constraints_border_width(20_000), 10_000);
-	}
+extern "C" fn constraints_border_width(border_width: c_int) -> c_int {
+	constraints::border_width(border_width)
 }