From 721635d115510004803d258097e6523a6ea1d03d Mon Sep 17 00:00:00 2001
From: kirawi <67773714+kirawi@users.noreply.github.com>
Date: Thu, 4 Jan 2024 18:56:50 -0500
Subject: [PATCH] fix merge logic

---
 helix-core/src/history.rs     | 17 +++++++++--------
 helix-core/src/transaction.rs |  4 +---
 2 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/helix-core/src/history.rs b/helix-core/src/history.rs
index 1db4eb22902f6..0c56f8f9c4b7b 100644
--- a/helix-core/src/history.rs
+++ b/helix-core/src/history.rs
@@ -266,7 +266,7 @@ impl History {
     ///        E -> F
     /// ```
     pub fn merge(&mut self, mut other: History) -> anyhow::Result<()> {
-        let after_n = self
+        let n = self
             .revisions
             .iter()
             .zip(other.revisions.iter())
@@ -275,14 +275,15 @@ impl History {
             })
             .count();
 
-        let revisions = self.revisions.split_off(after_n);
-        other.revisions.reserve_exact(revisions.len());
+        let new_revs = self.revisions.split_off(n);
+        if new_revs.is_empty() {
+            return Ok(());
+        }
 
-        // Converts the number of new elements to an index offset
-        let offset = (other.revisions.len() - after_n) - 1;
-        for mut r in revisions {
-            // Update parents of new revisions
-            if r.parent >= after_n {
+        // Only unique revisions in `self` matter, so saturating_sub(1) is sound as it going to 0 means there are no new revisions in the other history that aren't in `self`
+        let offset = (other.revisions.len() - n).saturating_sub(1);
+        for mut r in new_revs {
+            if r.parent >= n {
                 r.parent += offset;
             }
             debug_assert!(r.parent < other.revisions.len());
diff --git a/helix-core/src/transaction.rs b/helix-core/src/transaction.rs
index c34aedc0aa5d3..4ac1e05105203 100644
--- a/helix-core/src/transaction.rs
+++ b/helix-core/src/transaction.rs
@@ -379,9 +379,7 @@ impl ChangeSet {
             macro_rules! map {
                 ($map: expr, $i: expr) => {
                     loop {
-                        let Some((pos, assoc)) = positions.peek_mut() else {
-                            return;
-                        };
+                        let Some((pos, assoc)) = positions.peek_mut() else { return; };
                         if **pos < old_pos {
                             // Positions are not sorted, revert to the last Operation that
                             // contains this position and continue iterating from there.