Refactor Region struct (#4)

* Add some basic sanity tests for Region

This commit adds some basic sanity tests for `overlap` method
of `Region`.

* Refactor overlaps method of Region struct

This commit refactors `Region::overlaps` method.

* Add some docs

* Assert Region's len is nonzero

crates/memory/src/region.rs

@@ -1,3 +1,4 @@
+/// Represents a contiguous region in memory.
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct Region {
     pub start: u32,
@@ -5,6 +6,12 @@ pub struct Region {
 }
 
 impl Region {
+    pub fn new(start: u32, len: u32) -> Self {
+        assert!(len > 0, "Region cannot have 0 length");
+        Self { start, len }
+    }
+
+    /// Checks if this `Region` overlaps with `rhs` `Region`.
     pub fn overlaps(&self, rhs: Region) -> bool {
         let self_start = self.start as u64;
         let self_end = self_start + (self.len - 1) as u64;
@@ -12,26 +19,45 @@ impl Region {
         let rhs_start = rhs.start as u64;
         let rhs_end = rhs_start + (rhs.len - 1) as u64;
 
-        // start of rhs inside self:
+        if self_start <= rhs_start {
-        if rhs_start >= self_start && rhs_start < self_end {
+            self_end >= rhs_start
-            return true;
+        } else {
+            rhs_end >= self_start
+        }
+    }
 }
 
-        // end of rhs inside self:
+#[cfg(test)]
-        if rhs_end >= self_start && rhs_end < self_end {
+mod test {
-            return true;
+    use super::*;
+
+    #[test]
+    fn nonoverlapping() {
+        let r1 = Region::new(0, 10);
+        let r2 = Region::new(10, 10);
+        assert!(!r1.overlaps(r2));
+
+        let r1 = Region::new(10, 10);
+        let r2 = Region::new(0, 10);
+        assert!(!r1.overlaps(r2));
     }
 
-        // start of self inside rhs:
+    #[test]
-        if self_start >= rhs_start && self_start < rhs_end {
+    fn overlapping() {
-            return true;
+        let r1 = Region::new(0, 10);
-        }
+        let r2 = Region::new(9, 10);
+        assert!(r1.overlaps(r2));
 
-        // end of self inside rhs: XXX is this redundant? i suspect it is but im too tired
+        let r1 = Region::new(0, 10);
-        if self_end >= rhs_start && self_end < rhs_end {
+        let r2 = Region::new(2, 5);
-            return true;
+        assert!(r1.overlaps(r2));
-        }
 
-        false
+        let r1 = Region::new(9, 10);
+        let r2 = Region::new(0, 10);
+        assert!(r1.overlaps(r2));
+
+        let r1 = Region::new(2, 5);
+        let r2 = Region::new(0, 10);
+        assert!(r1.overlaps(r2));
     }
 }