From 1984410ac998ac5561f4a3de87d3d125f2f61178 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bla=C5=BE=20Hrastnik?= <blaz@mxxn.io>
Date: Thu, 28 May 2020 14:59:50 +0900
Subject: [PATCH] Selection mapping over changesets.

---
 helix-core/src/lib.rs         |  2 +-
 helix-core/src/selection.rs   | 30 +++++++++++++++++++++++++++++-
 helix-core/src/transaction.rs |  7 +++++++
 3 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/helix-core/src/lib.rs b/helix-core/src/lib.rs
index e027f9434..421d8f3cb 100644
--- a/helix-core/src/lib.rs
+++ b/helix-core/src/lib.rs
@@ -13,4 +13,4 @@ pub use selection::Selection;
 
 pub use state::State;
 
-pub use transaction::{Change, ChangeSet, Transaction};
+pub use transaction::{Assoc, Change, ChangeSet, Transaction};
diff --git a/helix-core/src/selection.rs b/helix-core/src/selection.rs
index 0b2cd5a3b..98bbdb7fa 100644
--- a/helix-core/src/selection.rs
+++ b/helix-core/src/selection.rs
@@ -2,6 +2,7 @@
 //! single selection range.
 //!
 //! All positioning is done via `char` offsets into the buffer.
+use crate::{Assoc, ChangeSet};
 use smallvec::{smallvec, SmallVec};
 
 #[inline]
@@ -59,7 +60,18 @@ impl Range {
         }
     }
 
-    // TODO: map
+    /// Map a range through a set of changes. Returns a new range representing the same position
+    /// after the changes are applied.
+    pub fn map(self, changes: &ChangeSet) -> Self {
+        let anchor = changes.map_pos(self.anchor, Assoc::Before);
+        let head = changes.map_pos(self.head, Assoc::Before);
+
+        // TODO: possibly unnecessary
+        if self.anchor == anchor && self.head == head {
+            return self;
+        }
+        Self { anchor, head }
+    }
 
     /// Extend the range to cover at least `from` `to`.
     #[must_use]
@@ -115,6 +127,22 @@ impl Selection {
     // add_range // push
     // replace_range
 
+    /// Map selections over a set of changes. Useful for adjusting the selection position after
+    /// applying changes to a document.
+    pub fn map(self, changes: &ChangeSet) -> Self {
+        if changes.is_empty() {
+            return self;
+        }
+
+        Self::new(
+            self.ranges
+                .into_iter()
+                .map(|range| range.map(changes))
+                .collect(),
+            self.primary_index,
+        )
+    }
+
     #[must_use]
     /// Constructs a selection holding a single range.
     pub fn single(anchor: usize, head: usize) -> Self {
diff --git a/helix-core/src/transaction.rs b/helix-core/src/transaction.rs
index c6be89ac8..8da01c099 100644
--- a/helix-core/src/transaction.rs
+++ b/helix-core/src/transaction.rs
@@ -242,6 +242,13 @@ impl ChangeSet {
         }
     }
 
+    /// `true` when the set is empty.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        let len = self.changes.len();
+        len == 0 || (len == 1 && self.changes[0] == Change::Retain(self.len))
+    }
+
     /// Map a position through the changes.
     ///
     /// `assoc` indicates which size to associate the position with. `Before` will keep the