feat(commands): shrink_selection (#1340)

* feat(commands): shrink_selection

Add `shrink_selection` command that can be used to shrink
previously expanded selection.

To make `shrink_selection` work it was necessary to add
selection history to the Document since we want to shrink
the selection towards the syntax tree node that was initially
selected.

Selection history is cleared any time the user changes
selection other way than by `expand_selection`. This ensures
that we don't get some funky edge cases when user calls
`shrink_selection`.

Related: https://github.com/helix-editor/helix/discussions/1328

* Refactor shrink_selection, move history to view

* Remove useless comment

* Add default key mapping for extend&shrink selection

* Rework contains_selection method

* Shrink selection without expand selects first child

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))));
+    }
 }