Preview the latest value for regular registers

This fixes a discrepancy between regular registers which are used for
yanking multiple values (for example via `"ay`) and regular registers
that store a history of values (for example `"a*`).

Previously, the preview shown in `select_register`'s infobox would show
the oldest value in history. It's intuitive and useful to see the most
recent value pushed to the history though.

We cannot simply switch the preview line from `values.first()`
to `values.last()`: that would fix the preview for registers
used for history but break the preview for registers used to yank
multiple values. We could push to the beginning of the values with
`Registers::push` but this is wasteful from a performance perspective.
Instead we can have `Registers::read` return an iterator that
returns elements in the reverse order and reverse the values in
`Register::write`. This effectively means that `push` adds elements to
the beginning of the register's values. For the sake of the preview, we
can switch to `values.last()` and that is then correct for both usage-
styles. This also needs a change to call-sites that read the latest
history value to switch from `last` to `first`.

3 files changed, 24 insertions, 16 deletions

diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index 329b7ab7..6f10848a 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -1911,7 +1911,7 @@ fn search_next_or_prev_impl(cx: &mut Context, movement: Movement, direction: Dir
     let register = cx.register.unwrap_or('/');
     let config = cx.editor.config();
     let scrolloff = config.scrolloff;
-    if let Some(query) = cx.editor.registers.last(register, cx.editor) {
+    if let Some(query) = cx.editor.registers.first(register, cx.editor) {
         let doc = doc!(cx.editor);
         let contents = doc.text().slice(..).to_string();
         let search_config = &config.search;
@@ -1983,7 +1983,7 @@ fn search_selection(cx: &mut Context) {
 
 fn make_search_word_bounded(cx: &mut Context) {
     let register = cx.register.unwrap_or('/');
-    let regex = match cx.editor.registers.last(register, cx.editor) {
+    let regex = match cx.editor.registers.first(register, cx.editor) {
         Some(regex) => regex,
         None => return,
     };
diff --git a/helix-term/src/ui/prompt.rs b/helix-term/src/ui/prompt.rs
index 8dc2906a..702a6e67 100644
--- a/helix-term/src/ui/prompt.rs
+++ b/helix-term/src/ui/prompt.rs
@@ -307,7 +307,7 @@ impl Prompt {
     ) {
         (self.callback_fn)(cx, &self.line, PromptEvent::Abort);
         let mut values = match cx.editor.registers.read(register, cx.editor) {
-            Some(values) if values.len() > 0 => values,
+            Some(values) if values.len() > 0 => values.rev(),
             _ => return,
         };
 
@@ -473,7 +473,7 @@ impl Prompt {
             // Show the most recently entered value as a suggestion.
             if let Some(suggestion) = self
                 .history_register
-                .and_then(|reg| cx.editor.registers.last(reg, cx.editor))
+                .and_then(|reg| cx.editor.registers.first(reg, cx.editor))
             {
                 surface.set_string(line_area.x, line_area.y, suggestion, suggestion_color);
             }
@@ -570,7 +570,7 @@ impl Component for Prompt {
                 } else {
                     let last_item = self
                         .history_register
-                        .and_then(|reg| cx.editor.registers.last(reg, cx.editor))
+                        .and_then(|reg| cx.editor.registers.first(reg, cx.editor))
                         .map(|entry| entry.to_string())
                         .unwrap_or_else(|| String::from(""));
 
diff --git a/helix-view/src/register.rs b/helix-view/src/register.rs
index e82051dd..61912378 100644
--- a/helix-view/src/register.rs
+++ b/helix-view/src/register.rs
@@ -23,6 +23,10 @@ use crate::{
 /// * Primary clipboard (`+`)
 #[derive(Debug)]
 pub struct Registers {
+    /// The mapping of register to values.
+    /// Values are stored in reverse order when inserted with `Registers::write`.
+    /// The order is reversed again in `Registers::read`. This allows us to
+    /// efficiently prepend new values in `Registers::push`.
     inner: HashMap<char, Vec<String>>,
     clipboard_provider: Box<dyn ClipboardProvider>,
 }
@@ -77,11 +81,11 @@ impl Registers {
             _ => self
                 .inner
                 .get(&name)
-                .map(|values| RegisterValues::new(values.iter().map(Cow::from))),
+                .map(|values| RegisterValues::new(values.iter().map(Cow::from).rev())),
         }
     }
 
-    pub fn write(&mut self, name: char, values: Vec<String>) -> Result<()> {
+    pub fn write(&mut self, name: char, mut values: Vec<String>) -> Result<()> {
         match name {
             '_' => Ok(()),
             '#' | '.' | '%' => Err(anyhow::anyhow!("Register {name} does not support writing")),
@@ -94,17 +98,19 @@ impl Registers {
                         _ => unreachable!(),
                     },
                 )?;
+                values.reverse();
                 self.inner.insert(name, values);
                 Ok(())
             }
             _ => {
+                values.reverse();
                 self.inner.insert(name, values);
                 Ok(())
             }
         }
     }
 
-    pub fn push(&mut self, name: char, value: String) -> Result<()> {
+    pub fn push(&mut self, name: char, mut value: String) -> Result<()> {
         match name {
             '_' => Ok(()),
             '#' | '.' | '%' => Err(anyhow::anyhow!("Register {name} does not support pushing")),
@@ -121,11 +127,13 @@ impl Registers {
                     anyhow::bail!("Failed to push to register {name}: clipboard does not match register contents");
                 }
 
-                saved_values.push(value);
-                self.clipboard_provider.set_contents(
-                    saved_values.join(NATIVE_LINE_ENDING.as_str()),
-                    clipboard_type,
-                )?;
+                saved_values.push(value.clone());
+                if !contents.is_empty() {
+                    value.push_str(NATIVE_LINE_ENDING.as_str());
+                }
+                value.push_str(&contents);
+                self.clipboard_provider
+                    .set_contents(value, clipboard_type)?;
 
                 Ok(())
             }
@@ -150,7 +158,7 @@ impl Registers {
             .filter(|(name, _)| !matches!(name, '*' | '+'))
             .map(|(name, values)| {
                 let preview = values
-                    .first()
+                    .last()
                     .and_then(|s| s.lines().next())
                     .unwrap_or("<empty>");
 
@@ -222,7 +230,7 @@ fn read_from_clipboard<'a>(
             let Some(values) = saved_values else { return RegisterValues::new(iter::once(contents.into())) };
 
             if contents_are_saved(values, &contents) {
-                RegisterValues::new(values.iter().map(Cow::from))
+                RegisterValues::new(values.iter().map(Cow::from).rev())
             } else {
                 RegisterValues::new(iter::once(contents.into()))
             }
@@ -243,7 +251,7 @@ fn read_from_clipboard<'a>(
 
 fn contents_are_saved(saved_values: &[String], mut contents: &str) -> bool {
     let line_ending = NATIVE_LINE_ENDING.as_str();
-    let mut values = saved_values.iter();
+    let mut values = saved_values.iter().rev();
 
     match values.next() {
         Some(first) if contents.starts_with(first) => {