From 490e23b645311fef86d8e8045f97f514927d2867 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bla=C5=BE=20Hrastnik?= <blaz@mxxn.io>
Date: Wed, 7 Oct 2020 22:22:50 +0900
Subject: [PATCH] Simplify changeset tracking.

---
 helix-core/src/state.rs       |  7 +++++--
 helix-core/src/transaction.rs | 24 +++++++++++++++++++-----
 helix-view/src/commands.rs    | 28 ++++++++++++++++------------
 3 files changed, 40 insertions(+), 19 deletions(-)

diff --git a/helix-core/src/state.rs b/helix-core/src/state.rs
index d8aa3ed4c..fde6a8666 100644
--- a/helix-core/src/state.rs
+++ b/helix-core/src/state.rs
@@ -25,7 +25,8 @@ pub struct State {
 
     //
     pub syntax: Option<Syntax>,
-    pub changes: Option<ChangeSet>,
+    /// Pending changes since last history commit.
+    pub changes: ChangeSet,
     pub old_state: Option<(Rope, Selection)>,
 }
 
@@ -45,6 +46,8 @@ pub enum Granularity {
 impl State {
     #[must_use]
     pub fn new(doc: Rope) -> Self {
+        let changes = ChangeSet::new(&doc);
+
         Self {
             path: None,
             doc,
@@ -52,7 +55,7 @@ impl State {
             mode: Mode::Normal,
             restore_cursor: false,
             syntax: None,
-            changes: None,
+            changes,
             old_state: None,
         }
     }
diff --git a/helix-core/src/transaction.rs b/helix-core/src/transaction.rs
index dfb1bbe1e..86fc0bc81 100644
--- a/helix-core/src/transaction.rs
+++ b/helix-core/src/transaction.rs
@@ -342,6 +342,22 @@ pub struct Transaction {
     // scroll_into_view
 }
 
+/// Like std::mem::replace() except it allows the replacement value to be mapped from the
+/// original value.
+pub fn take_with<T, F>(mut_ref: &mut T, closure: F)
+where
+    F: FnOnce(T) -> T,
+{
+    use std::{panic, ptr};
+
+    unsafe {
+        let old_t = ptr::read(mut_ref);
+        let new_t = panic::catch_unwind(panic::AssertUnwindSafe(|| closure(old_t)))
+            .unwrap_or_else(|_| ::std::process::abort());
+        ptr::write(mut_ref, new_t);
+    }
+}
+
 impl Transaction {
     /// Create a new, empty transaction.
     pub fn new(state: &mut State) -> Self {
@@ -364,11 +380,9 @@ impl Transaction {
             }
 
             // Compose this transaction with the previous one
-            let old_changes = state.changes.take();
-            state.changes = Some(old_changes.map_or_else(
-                || self.changes.clone(),
-                |changes| changes.compose(self.changes.clone()).unwrap(),
-            ));
+            take_with(&mut state.changes, |changes| {
+                changes.compose(self.changes.clone()).unwrap()
+            });
 
             if let Some(syntax) = &mut state.syntax {
                 // TODO: no unwrap
diff --git a/helix-view/src/commands.rs b/helix-view/src/commands.rs
index aafc6202f..e5e3244d0 100644
--- a/helix-view/src/commands.rs
+++ b/helix-view/src/commands.rs
@@ -368,22 +368,26 @@ pub fn open_below(view: &mut View, _count: usize) {
 // O inserts a new line before each line with a selection
 
 fn append_changes_to_history(view: &mut View) {
-    if let Some(changes) = view.state.changes.take() {
-        // Instead of doing this messy merge we could always commit, and based on transaction
-        // annotations either add a new layer or compose into the previous one.
-        let transaction = Transaction::from(changes).with_selection(view.state.selection().clone());
-        let (doc, selection) = view.state.old_state.take().unwrap();
-        let mut old_state = State::new(doc);
-        old_state.selection = selection;
-
-        // TODO: take transaction by value?
-        view.history.commit_revision(&transaction, &old_state);
+    if view.state.changes.is_empty() {
+        return;
     }
 
-    // TODO: need to start the state with these vals
+    let new_changeset = ChangeSet::new(view.state.doc());
+    let changes = std::mem::replace(&mut view.state.changes, new_changeset);
+    // Instead of doing this messy merge we could always commit, and based on transaction
+    // annotations either add a new layer or compose into the previous one.
+    let transaction = Transaction::from(changes).with_selection(view.state.selection().clone());
+
+    // HAXX: we need to reconstruct the state as it was before the changes..
+    let (doc, selection) = view.state.old_state.take().unwrap();
+    let mut old_state = State::new(doc);
+    old_state.selection = selection;
 
+    // TODO: take transaction by value?
+    view.history.commit_revision(&transaction, &old_state);
+
+    // TODO: need to start the state with these vals
     // HAXX
-    view.state.changes = Some(ChangeSet::new(view.state.doc()));
     view.state.old_state = Some((view.state.doc().clone(), view.state.selection.clone()));
 }