Add :earlier and :later commands that can be used to navigate the full edit history. (#194)

* Disable deleting from an empty buffer which can cause a crash.

* Improve on the fix for deleting from the end of the buffer.

* Clean up leftover log.

* Avoid theoretical underflow.

* Implement :before which accepts a time interval and moves the editor to
the closest history state to the commit of the current time minus that
interval. Current time is now by default, or the commit time if :before
has just been used.

* Add :earlier an :later commands that can move through
the edit history and retrieve changes hidded by undoing
and commiting new changes. The commands accept a number
of steps or a time period relative to the currrent change.

* Fix clippy lint error.

* Remove the dependency on parse_duration, add a custom parser instead.

* Fix clippy errors.

* Make helix_core::history a public module.

* Use the helper for getting the current document and view.

* Handled some PR comments.

* Fix the logic in :later n.

Co-authored-by: Ivan Tham <pickfire@riseup.net>

* Add an alias for :earlier.

Co-authored-by: Ivan Tham <pickfire@riseup.net>

* Add an alias for later.

Co-authored-by: Ivan Tham <pickfire@riseup.net>

* Run cargo fmt.

* Add some tests for earlier and later.

* Add more tests and restore the fix for later that diappeared somehow.

* Use ? instead of a match on an option.

Co-authored-by: Ivan Tham <pickfire@riseup.net>

* Rename to UndoKind.

* Remove the leftover match.

* Handle a bunch of review comments.

* More systemd.time compliant time units and additional description for the new commands.

* A more concise rewrite of the time span parser using ideas from PR discussion.

* Replace a match with map_err().

Co-authored-by: Ivan Tham <pickfire@riseup.net>

Co-authored-by: Jakub Bartodziej <jqb@google.com>
Co-authored-by: Ivan Tham <pickfire@riseup.net>

4 files changed, 502 insertions, 45 deletions

diff --git a/helix-core/src/history.rs b/helix-core/src/history.rs
index aa3bf193..23680275 100644
--- a/helix-core/src/history.rs
+++ b/helix-core/src/history.rs
@@ -1,20 +1,61 @@
 use crate::{ChangeSet, Rope, State, Transaction};
+use once_cell::sync::Lazy;
+use regex::Regex;
 use smallvec::{smallvec, SmallVec};
+use std::num::NonZeroUsize;
+use std::time::{Duration, Instant};
 
-/// Undo-tree style history store.
+// Stores the history of changes to a buffer.
+//
+// Currently the history is represented as a vector of revisions. The vector
+// always has at least one element: the empty root revision. Each revision
+// with the exception of the root has a parent revision, a [Transaction]
+// that can be applied to its parent to transition from the parent to itself,
+// and an inversion of that transaction to transition from the parent to its
+// latest child.
+//
+// When using `u` to undo a change, an inverse of the stored transaction will
+// be applied which will transition the buffer to the parent state.
+//
+// Each revision with the exception of the last in the vector also has a
+// last child revision. When using `U` to redo a change, the last child transaction
+// will be applied to the current state of the buffer.
+//
+// The current revision is the one currently displayed in the buffer.
+//
+// Commiting a new revision to the history will update the last child of the
+// current revision, and push a new revision to the end of the vector.
+//
+// Revisions are commited with a timestamp. :earlier and :later can be used
+// to jump to the closest revision to a moment in time relative to the timestamp
+// of the current revision plus (:later) or minus (:earlier) the duration
+// given to the command. If a single integer is given, the editor will instead
+// jump the given number of revisions in the vector.
+//
+// Limitations:
+//  * Changes in selections currently don't commit history changes. The selection
+//    will only be updated to the state after a commited buffer change.
+//  * The vector of history revisions is currently unbounded. This might
+//    cause the memory consumption to grow significantly large during long
+//    editing sessions.
+//  * Because delete transactions currently don't store the text that they
+//    delete, we also store an inversion of the transaction.
 #[derive(Debug)]
 pub struct History {
     revisions: Vec<Revision>,
-    cursor: usize,
+    current: usize,
 }
 
+// A single point in history. See [History] for more information.
 #[derive(Debug)]
 struct Revision {
     parent: usize,
-    children: SmallVec<[(usize, Transaction); 1]>,
-    /// The transaction to revert to previous state.
-    revert: Transaction,
-    // selection before, selection after?
+    last_child: Option<NonZeroUsize>,
+    transaction: Transaction,
+    // We need an inversion for undos because delete transactions don't store
+    // the deleted text.
+    inversion: Transaction,
+    timestamp: Instant,
 }
 
 impl Default for History {
@@ -23,72 +64,253 @@ impl Default for History {
         Self {
             revisions: vec![Revision {
                 parent: 0,
-                children: SmallVec::new(),
-                revert: Transaction::from(ChangeSet::new(&Rope::new())),
+                last_child: None,
+                transaction: Transaction::from(ChangeSet::new(&Rope::new())),
+                inversion: Transaction::from(ChangeSet::new(&Rope::new())),
+                timestamp: Instant::now(),
             }],
-            cursor: 0,
+            current: 0,
         }
     }
 }
 
 impl History {
     pub fn commit_revision(&mut self, transaction: &Transaction, original: &State) {
-        // TODO: could store a single transaction, if deletes also stored the text they delete
-        let revert = transaction
+        self.commit_revision_at_timestamp(transaction, original, Instant::now());
+    }
+
+    pub fn commit_revision_at_timestamp(
+        &mut self,
+        transaction: &Transaction,
+        original: &State,
+        timestamp: Instant,
+    ) {
+        let inversion = transaction
             .invert(&original.doc)
             // Store the current cursor position
             .with_selection(original.selection.clone());
 
-        let new_cursor = self.revisions.len();
+        let new_current = self.revisions.len();
+        self.revisions[self.current].last_child = NonZeroUsize::new(new_current);
         self.revisions.push(Revision {
-            parent: self.cursor,
-            children: SmallVec::new(),
-            revert,
+            parent: self.current,
+            last_child: None,
+            transaction: transaction.clone(),
+            inversion,
+            timestamp,
         });
-
-        // add a reference to the parent
-        self.revisions
-            .get_mut(self.cursor)
-            .unwrap() // TODO: get_unchecked_mut
-            .children
-            .push((new_cursor, transaction.clone()));
-
-        self.cursor = new_cursor;
+        self.current = new_current;
     }
 
     #[inline]
     pub fn current_revision(&self) -> usize {
-        self.cursor
+        self.current
     }
 
     #[inline]
     pub const fn at_root(&self) -> bool {
-        self.cursor == 0
+        self.current == 0
     }
 
     pub fn undo(&mut self) -> Option<&Transaction> {
         if self.at_root() {
-            // We're at the root of undo, nothing to do.
             return None;
         }
 
-        let current_revision = &self.revisions[self.cursor];
+        let current_revision = &self.revisions[self.current];
+        self.current = current_revision.parent;
+        Some(&current_revision.inversion)
+    }
 
-        self.cursor = current_revision.parent;
+    pub fn redo(&mut self) -> Option<&Transaction> {
+        let current_revision = &self.revisions[self.current];
+        let last_child = current_revision.last_child?;
+        self.current = last_child.get();
 
-        Some(&current_revision.revert)
+        let last_child_revision = &self.revisions[last_child.get()];
+        Some(&self.revisions[last_child.get()].transaction)
     }
 
-    pub fn redo(&mut self) -> Option<&Transaction> {
-        let current_revision = &self.revisions[self.cursor];
+    fn lowest_common_ancestor(&self, mut a: usize, mut b: usize) -> usize {
+        use std::collections::HashSet;
+        let mut a_path_set = HashSet::new();
+        let mut b_path_set = HashSet::new();
+        loop {
+            a_path_set.insert(a);
+            b_path_set.insert(b);
+            if a_path_set.contains(&b) {
+                return b;
+            }
+            if b_path_set.contains(&a) {
+                return a;
+            }
+            a = self.revisions[a].parent; // Relies on the parent of 0 being 0.
+            b = self.revisions[b].parent; // Same as above.
+        }
+    }
+
+    // List of nodes on the way from `n` to 'a`. Doesn`t include `a`.
+    // Includes `n` unless `a == n`. `a` must be an ancestor of `n`.
+    fn path_up(&self, mut n: usize, a: usize) -> Vec<usize> {
+        let mut path = Vec::new();
+        while n != a {
+            path.push(n);
+            n = self.revisions[n].parent;
+        }
+        path
+    }
 
-        // for now, simply pick the latest child (linear undo / redo)
-        if let Some((index, transaction)) = current_revision.children.last() {
-            self.cursor = *index;
+    fn jump_to(&mut self, to: usize) -> Vec<Transaction> {
+        let lca = self.lowest_common_ancestor(self.current, to);
+        let up = self.path_up(self.current, lca);
+        let down = self.path_up(to, lca);
+        self.current = to;
+        let up_txns = up.iter().map(|&n| self.revisions[n].inversion.clone());
+        let down_txns = down
+            .iter()
+            .rev()
+            .map(|&n| self.revisions[n].transaction.clone());
+        up_txns.chain(down_txns).collect()
+    }
+
+    fn jump_backward(&mut self, delta: usize) -> Vec<Transaction> {
+        self.jump_to(self.current.saturating_sub(delta))
+    }
+
+    fn jump_forward(&mut self, delta: usize) -> Vec<Transaction> {
+        self.jump_to(
+            self.current
+                .saturating_add(delta)
+                .min(self.revisions.len() - 1),
+        )
+    }
+
+    // Helper for a binary search case below.
+    fn revision_closer_to_instant(&self, i: usize, instant: Instant) -> usize {
+        let dur_im1 = instant.duration_since(self.revisions[i - 1].timestamp);
+        let dur_i = self.revisions[i].timestamp.duration_since(instant);
+        use std::cmp::Ordering::*;
+        match dur_im1.cmp(&dur_i) {
+            Less => i - 1,
+            Equal | Greater => i,
+        }
+    }
+
+    fn jump_instant(&mut self, instant: Instant) -> Vec<Transaction> {
+        let search_result = self
+            .revisions
+            .binary_search_by(|rev| rev.timestamp.cmp(&instant));
+        let revision = match search_result {
+            Ok(revision) => revision,
+            Err(insert_point) => match insert_point {
+                0 => 0,
+                n if n == self.revisions.len() => n - 1,
+                i => self.revision_closer_to_instant(i, instant),
+            },
+        };
+        self.jump_to(revision)
+    }
+
+    fn jump_duration_backward(&mut self, duration: Duration) -> Vec<Transaction> {
+        match self.revisions[self.current].timestamp.checked_sub(duration) {
+            Some(instant) => self.jump_instant(instant),
+            None => self.jump_to(0),
+        }
+    }
 
-            return Some(&transaction);
+    fn jump_duration_forward(&mut self, duration: Duration) -> Vec<Transaction> {
+        match self.revisions[self.current].timestamp.checked_add(duration) {
+            Some(instant) => self.jump_instant(instant),
+            None => self.jump_to(self.revisions.len() - 1),
+        }
+    }
+
+    pub fn earlier(&mut self, uk: UndoKind) -> Vec<Transaction> {
+        use UndoKind::*;
+        match uk {
+            Steps(n) => self.jump_backward(n),
+            TimePeriod(d) => self.jump_duration_backward(d),
+        }
+    }
+
+    pub fn later(&mut self, uk: UndoKind) -> Vec<Transaction> {
+        use UndoKind::*;
+        match uk {
+            Steps(n) => self.jump_forward(n),
+            TimePeriod(d) => self.jump_duration_forward(d),
+        }
+    }
+}
+
+#[derive(Debug, PartialEq)]
+pub enum UndoKind {
+    Steps(usize),
+    TimePeriod(std::time::Duration),
+}
+
+// A subset of sytemd.time time span syntax units.
+const TIME_UNITS: &[(&[&str], &str, u64)] = &[
+    (&["seconds", "second", "sec", "s"], "seconds", 1),
+    (&["minutes", "minute", "min", "m"], "minutes", 60),
+    (&["hours", "hour", "hr", "h"], "hours", 60 * 60),
+    (&["days", "day", "d"], "days", 24 * 60 * 60),
+];
+
+static DURATION_VALIDATION_REGEX: Lazy<Regex> =
+    Lazy::new(|| Regex::new(r"^(?:\d+\s*[a-z]+\s*)+$").unwrap());
+
+static NUMBER_UNIT_REGEX: Lazy<Regex> = Lazy::new(|| Regex::new(r"(\d+)\s*([a-z]+)").unwrap());
+
+fn parse_human_duration(s: &str) -> Result<Duration, String> {
+    if !DURATION_VALIDATION_REGEX.is_match(s) {
+        return Err("duration should be composed \
+        of positive integers followed by time units"
+            .to_string());
+    }
+
+    let mut specified = [false; TIME_UNITS.len()];
+    let mut seconds = 0u64;
+    for cap in NUMBER_UNIT_REGEX.captures_iter(s) {
+        let (n, unit_str) = (&cap[1], &cap[2]);
+
+        let n: u64 = n.parse().map_err(|_| format!("integer too large: {}", n))?;
+
+        let time_unit = TIME_UNITS
+            .iter()
+            .enumerate()
+            .find(|(_, (forms, _, _))| forms.iter().any(|f| f == &unit_str));
+
+        if let Some((i, (_, unit, mul))) = time_unit {
+            if specified[i] {
+                return Err(format!("{} specified more than once", unit));
+            }
+            specified[i] = true;
+
+            let new_seconds = n.checked_mul(*mul).and_then(|s| seconds.checked_add(s));
+            match new_seconds {
+                Some(ns) => seconds = ns,
+                None => return Err("duration too large".to_string()),
+            }
+        } else {
+            return Err(format!("incorrect time unit: {}", unit_str));
+        }
+    }
+
+    Ok(Duration::from_secs(seconds))
+}
+
+impl std::str::FromStr for UndoKind {
+    type Err = String;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let s = s.trim();
+        if s.is_empty() {
+            Ok(Self::Steps(1usize))
+        } else if let Ok(n) = s.parse::<usize>() {
+            Ok(UndoKind::Steps(n))
+        } else {
+            Ok(Self::TimePeriod(parse_human_duration(s)?))
         }
-        None
     }
 }
 
@@ -144,4 +366,191 @@ mod test {
         undo(&mut history, &mut state);
         assert_eq!("hello", state.doc);
     }
+
+    #[test]
+    fn test_earlier_later() {
+        let mut history = History::default();
+        let doc = Rope::from("a\n");
+        let mut state = State::new(doc);
+
+        fn undo(history: &mut History, state: &mut State) {
+            if let Some(transaction) = history.undo() {
+                transaction.apply(&mut state.doc);
+            }
+        };
+
+        fn earlier(history: &mut History, state: &mut State, uk: UndoKind) {
+            let txns = history.earlier(uk);
+            for txn in txns {
+                txn.apply(&mut state.doc);
+            }
+        };
+
+        fn later(history: &mut History, state: &mut State, uk: UndoKind) {
+            let txns = history.later(uk);
+            for txn in txns {
+                txn.apply(&mut state.doc);
+            }
+        };
+
+        fn commit_change(
+            history: &mut History,
+            state: &mut State,
+            change: crate::transaction::Change,
+            instant: Instant,
+        ) {
+            let txn = Transaction::change(&state.doc, vec![change.clone()].into_iter());
+            history.commit_revision_at_timestamp(&txn, &state, instant);
+            txn.apply(&mut state.doc);
+        };
+
+        let t0 = Instant::now();
+        let t = |n| t0.checked_add(Duration::from_secs(n)).unwrap();
+
+        commit_change(&mut history, &mut state, (1, 1, Some(" b".into())), t(0));
+        assert_eq!("a b\n", state.doc);
+
+        commit_change(&mut history, &mut state, (3, 3, Some(" c".into())), t(10));
+        assert_eq!("a b c\n", state.doc);
+
+        commit_change(&mut history, &mut state, (5, 5, Some(" d".into())), t(20));
+        assert_eq!("a b c d\n", state.doc);
+
+        undo(&mut history, &mut state);
+        assert_eq!("a b c\n", state.doc);
+
+        commit_change(&mut history, &mut state, (5, 5, Some(" e".into())), t(30));
+        assert_eq!("a b c e\n", state.doc);
+
+        undo(&mut history, &mut state);
+        undo(&mut history, &mut state);
+        assert_eq!("a b\n", state.doc);
+
+        commit_change(&mut history, &mut state, (1, 3, None), t(40));
+        assert_eq!("a\n", state.doc);
+
+        commit_change(&mut history, &mut state, (1, 1, Some(" f".into())), t(50));
+        assert_eq!("a f\n", state.doc);
+
+        use UndoKind::*;
+
+        earlier(&mut history, &mut state, Steps(3));
+        assert_eq!("a b c d\n", state.doc);
+
+        later(&mut history, &mut state, TimePeriod(Duration::new(20, 0)));
+        assert_eq!("a\n", state.doc);
+
+        earlier(&mut history, &mut state, TimePeriod(Duration::new(19, 0)));
+        assert_eq!("a b c d\n", state.doc);
+
+        earlier(
+            &mut history,
+            &mut state,
+            TimePeriod(Duration::new(10000, 0)),
+        );
+        assert_eq!("a\n", state.doc);
+
+        later(&mut history, &mut state, Steps(50));
+        assert_eq!("a f\n", state.doc);
+
+        earlier(&mut history, &mut state, Steps(4));
+        assert_eq!("a b c\n", state.doc);
+
+        later(&mut history, &mut state, TimePeriod(Duration::new(1, 0)));
+        assert_eq!("a b c\n", state.doc);
+
+        later(&mut history, &mut state, TimePeriod(Duration::new(5, 0)));
+        assert_eq!("a b c d\n", state.doc);
+
+        later(&mut history, &mut state, TimePeriod(Duration::new(6, 0)));
+        assert_eq!("a b c e\n", state.doc);
+
+        later(&mut history, &mut state, Steps(1));
+        assert_eq!("a\n", state.doc);
+    }
+
+    #[test]
+    fn test_parse_undo_kind() {
+        use UndoKind::*;
+
+        // Default is one step.
+        assert_eq!("".parse(), Ok(Steps(1)));
+
+        // An integer means the number of steps.
+        assert_eq!("1".parse(), Ok(Steps(1)));
+        assert_eq!("  16 ".parse(), Ok(Steps(16)));
+
+        // Duration has a strict format.
+        let validation_err = Err("duration should be composed \
+         of positive integers followed by time units"
+            .to_string());
+        assert_eq!("  16 33".parse::<UndoKind>(), validation_err);
+        assert_eq!("  seconds 22  ".parse::<UndoKind>(), validation_err);
+        assert_eq!("  -4 m".parse::<UndoKind>(), validation_err);
+        assert_eq!("5s 3".parse::<UndoKind>(), validation_err);
+
+        // Units are u64.
+        assert_eq!(
+            "18446744073709551616minutes".parse::<UndoKind>(),
+            Err("integer too large: 18446744073709551616".to_string())
+        );
+
+        // Units are validated.
+        assert_eq!(
+            "1 millenium".parse::<UndoKind>(),
+            Err("incorrect time unit: millenium".to_string())
+        );
+
+        // Units can't be specified twice.
+        assert_eq!(
+            "2 seconds 6s".parse::<UndoKind>(),
+            Err("seconds specified more than once".to_string())
+        );
+
+        // Various formats are correctly handled.
+        assert_eq!(
+            "4s".parse::<UndoKind>(),
+            Ok(TimePeriod(Duration::from_secs(4)))
+        );
+        assert_eq!(
+            "2m".parse::<UndoKind>(),
+            Ok(TimePeriod(Duration::from_secs(120)))
+        );
+        assert_eq!(
+            "5h".parse::<UndoKind>(),
+            Ok(TimePeriod(Duration::from_secs(5 * 60 * 60)))
+        );
+        assert_eq!(
+            "3d".parse::<UndoKind>(),
+            Ok(TimePeriod(Duration::from_secs(3 * 24 * 60 * 60)))
+        );
+        assert_eq!(
+            "1m30s".parse::<UndoKind>(),
+            Ok(TimePeriod(Duration::from_secs(90)))
+        );
+        assert_eq!(
+            "1m 20 seconds".parse::<UndoKind>(),
+            Ok(TimePeriod(Duration::from_secs(80)))
+        );
+        assert_eq!(
+            "  2 minute 1day".parse::<UndoKind>(),
+            Ok(TimePeriod(Duration::from_secs(24 * 60 * 60 + 2 * 60)))
+        );
+        assert_eq!(
+            "3 d 2hour 5 minutes 30sec".parse::<UndoKind>(),
+            Ok(TimePeriod(Duration::from_secs(
+                3 * 24 * 60 * 60 + 2 * 60 * 60 + 5 * 60 + 30
+            )))
+        );
+
+        // Sum overflow is handled.
+        assert_eq!(
+            "18446744073709551615minutes".parse::<UndoKind>(),
+            Err("duration too large".to_string())
+        );
+        assert_eq!(
+            "1 minute 18446744073709551615 seconds".parse::<UndoKind>(),
+            Err("duration too large".to_string())
+        );
+    }
 }
diff --git a/helix-core/src/lib.rs b/helix-core/src/lib.rs
index 8f4239b1..a9792562 100644
--- a/helix-core/src/lib.rs
+++ b/helix-core/src/lib.rs
@@ -3,7 +3,7 @@ pub mod auto_pairs;
 pub mod comment;
 pub mod diagnostic;
 pub mod graphemes;
-mod history;
+pub mod history;
 pub mod indent;
 pub mod macros;
 pub mod match_brackets;
@@ -89,7 +89,6 @@ pub use smallvec::SmallVec;
 pub use syntax::Syntax;
 
 pub use diagnostic::Diagnostic;
-pub use history::History;
 pub use state::State;
 
 pub use transaction::{Assoc, Change, ChangeSet, Operation, Transaction};
diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index 07034214..3cb1288e 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -980,6 +980,30 @@ mod cmd {
         doc.format(view.id)
     }
 
+    fn earlier(editor: &mut Editor, args: &[&str], event: PromptEvent) {
+        let uk = match args.join(" ").parse::<helix_core::history::UndoKind>() {
+            Ok(uk) => uk,
+            Err(msg) => {
+                editor.set_error(msg);
+                return;
+            }
+        };
+        let (view, doc) = editor.current();
+        doc.earlier(view.id, uk)
+    }
+
+    fn later(editor: &mut Editor, args: &[&str], event: PromptEvent) {
+        let uk = match args.join(" ").parse::<helix_core::history::UndoKind>() {
+            Ok(uk) => uk,
+            Err(msg) => {
+                editor.set_error(msg);
+                return;
+            }
+        };
+        let (view, doc) = editor.current();
+        doc.later(view.id, uk)
+    }
+
     pub const COMMAND_LIST: &[Command] = &[
         Command {
             name: "quit",
@@ -1023,6 +1047,20 @@ mod cmd {
             fun: format,
             completer: None,
         },
+        Command {
+            name: "earlier",
+            alias: Some("ear"),
+            doc: "Jump back to an earlier point in edit history. Accepts a number of steps or a time span.",
+            fun: earlier,
+            completer: None,
+        },
+        Command {
+            name: "later",
+            alias: Some("lat"),
+            doc: "Jump to a later point in edit history. Accepts a number of steps or a time span.",
+            fun: later,
+            completer: None,
+        },
     ];
 
     pub static COMMANDS: Lazy<HashMap<&'static str, &'static Command>> = Lazy::new(|| {
diff --git a/helix-view/src/document.rs b/helix-view/src/document.rs
index 6a687955..75a0e9fa 100644
--- a/helix-view/src/document.rs
+++ b/helix-view/src/document.rs
@@ -5,8 +5,9 @@ use std::path::{Component, Path, PathBuf};
 use std::sync::Arc;
 
 use helix_core::{
+    history::History,
     syntax::{LanguageConfiguration, LOADER},
-    ChangeSet, Diagnostic, History, Rope, Selection, State, Syntax, Transaction,
+    ChangeSet, Diagnostic, Rope, Selection, State, Syntax, Transaction,
 };
 
 use crate::{DocumentId, ViewId};
@@ -387,7 +388,7 @@ impl Document {
         success
     }
 
-    pub fn undo(&mut self, view_id: ViewId) -> bool {
+    pub fn undo(&mut self, view_id: ViewId) {
         let mut history = self.history.take();
         let success = if let Some(transaction) = history.undo() {
             self._apply(&transaction, view_id)
@@ -400,11 +401,9 @@ impl Document {
             // reset changeset to fix len
             self.changes = ChangeSet::new(self.text());
         }
-
-        success
     }
 
-    pub fn redo(&mut self, view_id: ViewId) -> bool {
+    pub fn redo(&mut self, view_id: ViewId) {
         let mut history = self.history.take();
         let success = if let Some(transaction) = history.redo() {
             self._apply(&transaction, view_id)
@@ -417,8 +416,20 @@ impl Document {
             // reset changeset to fix len
             self.changes = ChangeSet::new(self.text());
         }
+    }
 
-        false
+    pub fn earlier(&mut self, view_id: ViewId, uk: helix_core::history::UndoKind) {
+        let txns = self.history.get_mut().earlier(uk);
+        for txn in txns {
+            self._apply(&txn, view_id);
+        }
+    }
+
+    pub fn later(&mut self, view_id: ViewId, uk: helix_core::history::UndoKind) {
+        let txns = self.history.get_mut().later(uk);
+        for txn in txns {
+            self._apply(&txn, view_id);
+        }
     }
 
     pub fn append_changes_to_history(&mut self, view_id: ViewId) {