3 files changed, 48 insertions, 20 deletions

diff --git a/src/eval/array.rs b/src/eval/array.rs
index 6fb278e3..9abab8cf 100644
--- a/src/eval/array.rs
+++ b/src/eval/array.rs
@@ -48,8 +48,7 @@ impl Array {
 
     /// Borrow the value at the given index.
     pub fn get(&self, index: i64) -> StrResult<&Value> {
-        usize::try_from(index)
-            .ok()
+        self.locate(index)
             .and_then(|i| self.0.get(i))
             .ok_or_else(|| out_of_bounds(index, self.len()))
     }
@@ -57,8 +56,7 @@ impl Array {
     /// Mutably borrow the value at the given index.
     pub fn get_mut(&mut self, index: i64) -> StrResult<&mut Value> {
         let len = self.len();
-        usize::try_from(index)
-            .ok()
+        self.locate(index)
             .and_then(move |i| Arc::make_mut(&mut self.0).get_mut(i))
             .ok_or_else(|| out_of_bounds(index, len))
     }
@@ -77,8 +75,8 @@ impl Array {
     /// Insert a value at the specified index.
     pub fn insert(&mut self, index: i64, value: Value) -> StrResult<()> {
         let len = self.len();
-        let i = usize::try_from(index)
-            .ok()
+        let i = self
+            .locate(index)
             .filter(|&i| i <= self.0.len())
             .ok_or_else(|| out_of_bounds(index, len))?;
 
@@ -89,8 +87,8 @@ impl Array {
     /// Remove and return the value at the specified index.
     pub fn remove(&mut self, index: i64) -> StrResult<()> {
         let len = self.len();
-        let i = usize::try_from(index)
-            .ok()
+        let i = self
+            .locate(index)
             .filter(|&i| i < self.0.len())
             .ok_or_else(|| out_of_bounds(index, len))?;
 
@@ -106,16 +104,17 @@ impl Array {
     /// Extract a contigous subregion of the array.
     pub fn slice(&self, start: i64, end: Option<i64>) -> StrResult<Self> {
         let len = self.len();
-        let start = usize::try_from(start)
-            .ok()
+        let start = self
+            .locate(start)
             .filter(|&start| start <= self.0.len())
             .ok_or_else(|| out_of_bounds(start, len))?;
 
         let end = end.unwrap_or(self.len());
-        let end = usize::try_from(end)
-            .ok()
+        let end = self
+            .locate(end)
             .filter(|&end| end <= self.0.len())
-            .ok_or_else(|| out_of_bounds(end, len))?;
+            .ok_or_else(|| out_of_bounds(end, len))?
+            .max(start);
 
         Ok(Self::from_vec(self.0[start .. end].to_vec()))
     }
@@ -225,11 +224,20 @@ impl Array {
         self.0.as_slice()
     }
 
-
     /// Iterate over references to the contained values.
     pub fn iter(&self) -> std::slice::Iter<Value> {
         self.0.iter()
     }
+
+    /// Resolve an index.
+    fn locate(&self, index: i64) -> Option<usize> {
+        usize::try_from(if index >= 0 {
+            index
+        } else {
+            self.len().checked_add(index)?
+        })
+        .ok()
+    }
 }
 
 /// The out of bounds access error message.
diff --git a/tests/typ/code/array.typ b/tests/typ/code/array.typ
index df37dd45..cd163175 100644
--- a/tests/typ/code/array.typ
+++ b/tests/typ/code/array.typ
@@ -31,21 +31,33 @@
 
 ---
 // Test rvalue out of bounds.
+// Error: 2-14 array index out of bounds (index: 5, len: 3)
+{(1, 2, 3)(5)}
+
+---
+// Test lvalue out of bounds.
 {
   let array = (1, 2, 3)
-  // Error: 3-11 array index out of bounds (index: 5, len: 3)
-  array(5)
+  // Error: 3-11 array index out of bounds (index: 3, len: 3)
+  array(3) = 5
 }
 
 ---
-// Test lvalue out of bounds.
+// Test negative indices.
 {
-  let array = (1, 2, 3)
-  // Error: 3-12 array index out of bounds (index: -1, len: 3)
-  array(-1) = 5
+  let array = (1, 2, 3, 4)
+  test(array(0), 1)
+  test(array(-1), 4)
+  test(array(-2), 3)
+  test(array(-3), 2)
+  test(array(-4), 1)
 }
 
 ---
+// Error: 2-15 array index out of bounds (index: -4, len: 3)
+{(1, 2, 3)(-4)}
+
+---
 // Test non-collection indexing.
 
 {
diff --git a/tests/typ/utility/collection.typ b/tests/typ/utility/collection.typ
index 924200cb..3414f0a9 100644
--- a/tests/typ/utility/collection.typ
+++ b/tests/typ/utility/collection.typ
@@ -38,12 +38,20 @@
 #test((1, 2, 3, 4).slice(2), (3, 4))
 #test(range(10).slice(2, 6), (2, 3, 4, 5))
 #test(range(10).slice(4, count: 3), (4, 5, 6))
+#test((1, 2, 3).slice(2, -2), ())
+#test((1, 2, 3).slice(-2, 2), (2,))
+#test((1, 2, 3).slice(-3, 2), (1, 2))
+#test("ABCD".split("").slice(1, -1).join("-"), "A-B-C-D")
 
 ---
 // Error: 3-31 array index out of bounds (index: 12, len: 10)
 { range(10).slice(9, count: 3) }
 
 ---
+// Error: 3-25 array index out of bounds (index: -4, len: 3)
+{ (1, 2, 3).slice(0, -4) }
+
+---
 // Error: 2:17-2:19 missing argument: index
 #let numbers = ()
 { numbers.insert() }