2 files changed, 91 insertions, 2 deletions

diff --git a/helix-core/src/object.rs b/helix-core/src/object.rs
index 717c5994..21fa24fb 100644
--- a/helix-core/src/object.rs
+++ b/helix-core/src/object.rs
@@ -1,7 +1,5 @@
 use crate::{Range, RopeSlice, Selection, Syntax};
 
-// TODO: to contract_selection we'd need to store the previous ranges before expand.
-// Maybe just contract to the first child node?
 pub fn expand_selection(syntax: &Syntax, text: RopeSlice, selection: &Selection) -> Selection {
     let tree = syntax.tree();
 
@@ -34,3 +32,30 @@ pub fn expand_selection(syntax: &Syntax, text: RopeSlice, selection: &Selection)
         }
     })
 }
+
+pub fn shrink_selection(syntax: &Syntax, text: RopeSlice, selection: &Selection) -> Selection {
+    let tree = syntax.tree();
+
+    selection.clone().transform(|range| {
+        let from = text.char_to_byte(range.from());
+        let to = text.char_to_byte(range.to());
+
+        let descendant = match tree.root_node().descendant_for_byte_range(from, to) {
+            // find first child, if not possible, fallback to the node that contains selection
+            Some(descendant) => match descendant.child(0) {
+                Some(child) => child,
+                None => descendant,
+            },
+            None => return range,
+        };
+
+        let from = text.byte_to_char(descendant.start_byte());
+        let to = text.byte_to_char(descendant.end_byte());
+
+        if range.head < range.anchor {
+            Range::new(to, from)
+        } else {
+            Range::new(from, to)
+        }
+    })
+}
diff --git a/helix-core/src/selection.rs b/helix-core/src/selection.rs
index 06ea9d67..1515c4fc 100644
--- a/helix-core/src/selection.rs
+++ b/helix-core/src/selection.rs
@@ -140,6 +140,11 @@ impl Range {
         self.from() == other.from() || (self.to() > other.from() && other.to() > self.from())
     }
 
+    #[inline]
+    pub fn contains_range(&self, other: &Self) -> bool {
+        self.from() <= other.from() && self.to() >= other.to()
+    }
+
     pub fn contains(&self, pos: usize) -> bool {
         self.from() <= pos && pos < self.to()
     }
@@ -544,6 +549,39 @@ impl Selection {
     pub fn len(&self) -> usize {
         self.ranges.len()
     }
+
+    // returns true if self ⊇ other
+    pub fn contains(&self, other: &Selection) -> bool {
+        // can't contain other if it is larger
+        if other.len() > self.len() {
+            return false;
+        }
+
+        let (mut iter_self, mut iter_other) = (self.iter(), other.iter());
+        let (mut ele_self, mut ele_other) = (iter_self.next(), iter_other.next());
+
+        loop {
+            match (ele_self, ele_other) {
+                (Some(ra), Some(rb)) => {
+                    if !ra.contains_range(rb) {
+                        // `self` doesn't contain next element from `other`, advance `self`, we need to match all from `other`
+                        ele_self = iter_self.next();
+                    } else {
+                        // matched element from `other`, advance `other`
+                        ele_other = iter_other.next();
+                    };
+                }
+                (None, Some(_)) => {
+                    // exhausted `self`, we can't match the reminder of `other`
+                    return false;
+                }
+                (_, None) => {
+                    // no elements from `other` left to match, `self` contains `other`
+                    return true;
+                }
+            }
+        }
+    }
 }
 
 impl<'a> IntoIterator for &'a Selection {
@@ -982,4 +1020,30 @@ mod test {
             &["", "abcd", "efg", "rs", "xyz"]
         );
     }
+    #[test]
+    fn test_selection_contains() {
+        fn contains(a: Vec<(usize, usize)>, b: Vec<(usize, usize)>) -> bool {
+            let sela = Selection::new(a.iter().map(|a| Range::new(a.0, a.1)).collect(), 0);
+            let selb = Selection::new(b.iter().map(|b| Range::new(b.0, b.1)).collect(), 0);
+            sela.contains(&selb)
+        }
+
+        // exact match
+        assert!(contains(vec!((1, 1)), vec!((1, 1))));
+
+        // larger set contains smaller
+        assert!(contains(vec!((1, 1), (2, 2), (3, 3)), vec!((2, 2))));
+
+        // multiple matches
+        assert!(contains(vec!((1, 1), (2, 2)), vec!((1, 1), (2, 2))));
+
+        // smaller set can't contain bigger
+        assert!(!contains(vec!((1, 1)), vec!((1, 1), (2, 2))));
+
+        assert!(contains(
+            vec!((1, 1), (2, 4), (5, 6), (7, 9), (10, 13)),
+            vec!((3, 4), (7, 9))
+        ));
+        assert!(!contains(vec!((1, 1), (5, 6)), vec!((1, 6))));
+    }
 }