Registers: Store a bounded history of yanks

This doesn't fundamentally change `Registers` - it extends the
underlying storage of each regular `Register` (a sequence of `String`)
to support a history of yanked values by storing a limited set of past
yanks and tracking the length of each yank.

This commit also changes `Registers::write` to accept an IntoIterator of
the values. But otherwise this compiles with no changes necessary to
callers and no behavior changes visible from outside the module.

History is limited at 10 entries to set a rough bounds on memory usage.

helix-view/src/register.rs

@@ -1,4 +1,9 @@
-use std::{borrow::Cow, collections::HashMap, iter};
+use std::{
+    borrow::Cow,
+    collections::{HashMap, VecDeque},
+    iter,
+    num::NonZeroUsize,
+};
 
 use anyhow::Result;
 use helix_core::NATIVE_LINE_ENDING;
@@ -9,6 +14,71 @@ use crate::{
     Editor,
 };
 
+/// Standard registers store up to this many yanks worth of history.
+/// Once a register hits this many yanks, it discards the oldest values to
+/// make space for new yanks.
+const MAX_REGISTER_HISTORY_LEN: usize = 100;
+
+#[derive(Debug, Default)]
+struct Register {
+    /// The values held by the register.
+    ///
+    /// When yanking to a register, all values are pushed into this `VecDeque`. The length
+    /// of those values is stored in `length`. So each yank is stored in this flat sequence,
+    /// but this `VecDeque` holds the sequence of all yanks.
+    ///
+    /// This `VecDeque` should only be empty when constructing `Register` via `Default`, which
+    /// we do in `Registers` for simplicity. (Note that it should be impossible to `write`
+    /// with an empty set of values.)
+    ///
+    /// Yanks are stored least to most recent. Within each yank, values are stored in order.
+    values: VecDeque<String>,
+    /// The length of each yank into the register.
+    lengths: VecDeque<NonZeroUsize>,
+}
+
+impl Register {
+    fn latest_value(&self) -> Option<&String> {
+        self.values.back()
+    }
+
+    fn values(&self) -> RegisterValues<'_> {
+        let length = self.lengths.back().map(|len| len.get()).unwrap_or_default();
+        RegisterValues::new(
+            self.values
+                .iter()
+                .rev()
+                .take(length)
+                .rev()
+                .map(|s| Cow::Borrowed(s.as_str())),
+        )
+    }
+
+    fn write<I: IntoIterator<Item = String>>(&mut self, values: I) {
+        // If the register is full, discard the oldest yank in history.
+        if self.lengths.len() > MAX_REGISTER_HISTORY_LEN {
+            // Greater than max length implies non-empty.
+            let oldest_len = self.lengths.pop_front().unwrap();
+            self.values.drain(..oldest_len.get());
+        }
+
+        let pre_yank_len = self.values.len();
+        self.values.extend(values.into_iter());
+        let yank_len = NonZeroUsize::new(self.values.len() - pre_yank_len)
+            .expect("writes to registers must not be empty");
+        self.lengths.push_back(yank_len);
+    }
+
+    fn push(&mut self, value: String) {
+        self.values.push_back(value);
+        if let Some(last_length) = self.lengths.back_mut() {
+            *last_length = NonZeroUsize::new(last_length.get() + 1).unwrap();
+        } else {
+            self.lengths.push_back(NonZeroUsize::new(1).unwrap());
+        }
+    }
+}
+
 /// A key-value store for saving sets of values.
 ///
 /// Each register corresponds to a `char`. Most chars can be used to store any set of
@@ -24,10 +94,8 @@ use crate::{
 #[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>>,
+    /// This contains non-special registers plus '+' and '*'.
+    inner: HashMap<char, Register>,
     clipboard_provider: Box<dyn ClipboardProvider>,
     pub last_search_register: char,
 }
@@ -80,33 +148,35 @@ impl Registers {
                     _ => unreachable!(),
                 },
             )),
-            _ => self
-                .inner
-                .get(&name)
-                .map(|values| RegisterValues::new(values.iter().map(Cow::from).rev())),
+            _ => self.inner.get(&name).map(Register::values),
         }
     }
 
-    pub fn write(&mut self, name: char, mut values: Vec<String>) -> Result<()> {
+    pub fn write<I: IntoIterator<Item = String>>(&mut self, name: char, values: I) -> Result<()> {
         match name {
             '_' => Ok(()),
             '#' | '.' | '%' => Err(anyhow::anyhow!("Register {name} does not support writing")),
             '*' | '+' => {
+                self.inner.entry(name).or_default().write(values);
+                let mut contents = String::new();
+                for val in self.inner[&name].values() {
+                    if !contents.is_empty() {
+                        contents.push_str(NATIVE_LINE_ENDING.as_str());
+                    }
+                    contents.push_str(&val);
+                }
                 self.clipboard_provider.set_contents(
-                    values.join(NATIVE_LINE_ENDING.as_str()),
+                    contents,
                     match name {
                         '+' => ClipboardType::Clipboard,
                         '*' => ClipboardType::Selection,
                         _ => unreachable!(),
                     },
                 )?;
-                values.reverse();
-                self.inner.insert(name, values);
                 Ok(())
             }
             _ => {
-                values.reverse();
-                self.inner.insert(name, values);
+                self.inner.entry(name).or_default().write(values);
                 Ok(())
             }
         }
@@ -123,13 +193,13 @@ impl Registers {
                     _ => unreachable!(),
                 };
                 let contents = self.clipboard_provider.get_contents(clipboard_type)?;
-                let saved_values = self.inner.entry(name).or_default();
+                let register = self.inner.entry(name).or_default();
 
-                if !contents_are_saved(saved_values, &contents) {
+                if !contents_are_saved(register.values(), &contents) {
                     anyhow::bail!("Failed to push to register {name}: clipboard does not match register contents");
                 }
 
-                saved_values.push(value.clone());
+                register.push(value.clone());
                 if !contents.is_empty() {
                     value.push_str(NATIVE_LINE_ENDING.as_str());
                 }
@@ -158,9 +228,9 @@ impl Registers {
         self.inner
             .iter()
             .filter(|(name, _)| !matches!(name, '*' | '+'))
-            .map(|(name, values)| {
-                let preview = values
-                    .last()
+            .map(|(name, register)| {
+                let preview = register
+                    .latest_value()
                     .and_then(|s| s.lines().next())
                     .unwrap_or("<empty>");
 
@@ -225,7 +295,7 @@ impl Registers {
 
 fn read_from_clipboard<'a>(
     provider: &dyn ClipboardProvider,
-    saved_values: Option<&'a Vec<String>>,
+    register: Option<&'a Register>,
     clipboard_type: ClipboardType,
 ) -> RegisterValues<'a> {
     match provider.get_contents(clipboard_type) {
@@ -233,12 +303,12 @@ fn read_from_clipboard<'a>(
             // If we're pasting the same values that we just yanked, re-use
             // the saved values. This allows pasting multiple selections
             // even when yanked to a clipboard.
-            let Some(values) = saved_values else {
+            let Some(register) = register else {
                 return RegisterValues::new(iter::once(contents.into()));
             };
 
-            if contents_are_saved(values, &contents) {
-                RegisterValues::new(values.iter().map(Cow::from).rev())
+            if contents_are_saved(register.values(), &contents) {
+                register.values()
             } else {
                 RegisterValues::new(iter::once(contents.into()))
             }
@@ -257,12 +327,11 @@ fn read_from_clipboard<'a>(
     }
 }
 
-fn contents_are_saved(saved_values: &[String], mut contents: &str) -> bool {
+fn contents_are_saved(mut values: RegisterValues<'_>, mut contents: &str) -> bool {
     let line_ending = NATIVE_LINE_ENDING.as_str();
-    let mut values = saved_values.iter().rev();
 
     match values.next() {
-        Some(first) if contents.starts_with(first) => {
+        Some(first) if contents.starts_with(&*first) => {
             contents = &contents[first.len()..];
         }
         None if contents.is_empty() => return true,
@@ -270,7 +339,7 @@ fn contents_are_saved(saved_values: &[String], mut contents: &str) -> bool {
     }
 
     for value in values {
-        if contents.starts_with(line_ending) && contents[line_ending.len()..].starts_with(value) {
+        if contents.starts_with(line_ending) && contents[line_ending.len()..].starts_with(&*value) {
             contents = &contents[line_ending.len() + value.len()..];
         } else {
             return false;