Cargo.toml

@@ -41,12 +41,12 @@ lto = true
 opt-level = 3
 strip="debuginfo"  # Keep some info for better panics
 
-[workspace.lints.rust]
+[lints.rust]
 unsafe_code = "forbid"
 rust_2018_idioms = { level = "warn", priority = -1 }
 missing_debug_implementations = "warn"
 
-[workspace.lints.clippy]
+[lints.clippy]
 await_holding_lock = "warn"
 dbg_macro = "warn"
 empty_enum = "warn"

README.md

@@ -1,26 +1,22 @@
 # VaultLink self-hosted Obsidian plugin for file syncing
 
-[![Check](https://github.com/schmelczer/vault-link/actions/workflows/check.yml/badge.svg)](https://github.com/schmelczer/vault-link/actions/workflows/check.yml)
-[![E2E tests](https://github.com/schmelczer/vault-link/actions/workflows/e2e.yml/badge.svg)](https://github.com/schmelczer/vault-link/actions/workflows/e2e.yml)
-[![Publish server Docker image](https://github.com/schmelczer/vault-link/actions/workflows/publish-docker.yml/badge.svg)](https://github.com/schmelczer/vault-link/actions/workflows/publish-docker.yml)
-[![Publish Obsidian plugin](https://github.com/schmelczer/vault-link/actions/workflows/publish-plugin.yml/badge.svg)](https://github.com/schmelczer/vault-link/actions/workflows/publish-plugin.yml)
-
+[![Check](https://github.com/schmelczer/reconcile/actions/workflows/check.yml/badge.svg)](https://github.com/schmelczer/reconcile/actions/workflows/check.yml)
+[![Publish to GitHub Pages](https://github.com/schmelczer/reconcile/actions/workflows/gh-pages.yml/badge.svg)](https://github.com/schmelczer/reconcile/actions/workflows/gh-pages.yml)
 
 ## Develop
 
 ### Install [nvm](https://github.com/nvm-sh/nvm)
 
-- `curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.1/install.sh | bash`
-- `nvm install 22`
-- `nvm use 22`
-- Optionally set the system-wide default: `nvm alias default 22`
-
+-   `curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.1/install.sh | bash`
+-   `nvm install 22`
+-   `nvm use 22`
+-   Optionally set the system-wide default: `nvm alias default 22`
 
 ### Set up Rust
 
-- Install [`rustup`](https://rustup.rs): `curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh`
-- Install [`wasm-pack`](https://rustwasm.github.io/wasm-pack/installer): `curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh`
-- `cargo install cargo-insta sqlx-cli cargo-edit`
+-   Install [`rustup`](https://rustup.rs): `curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh`
+-   Install [`wasm-pack`](https://rustwasm.github.io/wasm-pack/installer): `curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh`
+-   `cargo install cargo-insta sqlx-cli cargo-edit`
 
 ### Install Obsidian on Linux
 
@@ -45,7 +41,6 @@ scripts/update-api-types.sh
 scripts/bump-version.sh patch
 ```
 
-
 #### Run E2E tests
 
 ```sh
@@ -56,4 +51,4 @@ And to clean up the logs & database files, run `scripts/clean-up.sh`
 
 ## Projects
 
-- [Sync server](./backend/sync_server/README.md)
+-   [Sync server](./backend/sync_server/README.md)

src/operation_transformation.rs

@@ -1,8 +1,6 @@
 mod cursor;
 mod edited_text;
-mod merge_context;
 mod operation;
-mod ordered_operation;
 mod utils;
 
 pub use cursor::{CursorPosition, TextWithCursors};
@@ -111,8 +109,8 @@ mod test {
                     }, // inside of "s|ample" because "text" got replaced by "sample"
                     CursorPosition {
                         id: 3,
-                        char_index: 43
-                    }, // before "cursor movements"
+                        char_index: 30
+                    }, // after "complex sample"
                 ]
             )
         );

src/operation_transformation/edited_text.rs

@@ -1,12 +1,11 @@
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
-use super::{CursorPosition, Operation, TextWithCursors, ordered_operation::OrderedOperation};
+use super::{CursorPosition, Operation, TextWithCursors};
 use crate::{
     diffs::{myers::diff, raw_operation::RawOperation},
-    operation_transformation::{
-        merge_context::MergeContext,
-        utils::{cook_operations::cook_operations, elongate_operations::elongate_operations},
+    operation_transformation::utils::{
+        cook_operations::cook_operations, elongate_operations::elongate_operations,
     },
     tokenizer::{Tokenizer, word_tokenizer::word_tokenizer},
     utils::{side::Side, string_builder::StringBuilder},
@@ -31,7 +30,7 @@ where
     T: PartialEq + Clone + std::fmt::Debug,
 {
     text: &'a str,
-    operations: Vec<OrderedOperation<T>>,
+    operations: Vec<Operation<T>>,
     pub(crate) cursors: Vec<CursorPosition>,
 }
 
@@ -77,23 +76,7 @@ where
     /// Create a new `EditedText` with the given operations.
     /// The operations must be in the order in which they are meant to be
     /// applied. The operations must not overlap.
-    fn new(
-        text: &'a str,
-        operations: Vec<OrderedOperation<T>>,
-        mut cursors: Vec<CursorPosition>,
-    ) -> Self {
-        operations
-            .iter()
-            .zip(operations.iter().skip(1))
-            .for_each(|(previous, next)| {
-                debug_assert!(
-                    previous.operation.start_index() <= next.operation.start_index(),
-                    "{} must not come before {} yet it does",
-                    previous.operation,
-                    next.operation
-                );
-            });
-
+    fn new(text: &'a str, operations: Vec<Operation<T>>, mut cursors: Vec<CursorPosition>) -> Self {
         cursors.sort_by_key(|cursor| cursor.char_index);
 
         Self {
@@ -110,14 +93,11 @@ where
             "`EditedText`-s must be derived from the same text to be mergable"
         );
 
-        let mut left_merge_context = MergeContext::default();
-        let mut right_merge_context = MergeContext::default();
-
         let mut merged_cursors = Vec::with_capacity(self.cursors.len() + other.cursors.len());
         let mut left_cursors = self.cursors.into_iter().peekable();
         let mut right_cursors = other.cursors.into_iter().peekable();
 
-        let mut merged_operations: Vec<OrderedOperation<T>> =
+        let mut merged_operations: Vec<Operation<T>> =
             Vec::with_capacity(self.operations.len() + other.operations.len());
 
         let mut left_iter = self.operations.into_iter();
@@ -126,89 +106,107 @@ where
         let mut maybe_left_op = left_iter.next();
         let mut maybe_right_op = right_iter.next();
 
+        let mut seen_left_length: usize = 0;
+        let mut seen_right_length: usize = 0;
+        let mut merged_length: usize = 0;
+
+        let mut last_left_op = None;
+        let mut last_right_op = None;
+
         loop {
-            let (side, OrderedOperation { operation, order }) =
+            let (side, operation, mut last_other_op) =
                 match (maybe_left_op.clone(), maybe_right_op.clone()) {
                     (Some(left_op), Some(right_op)) => {
-                        if left_op < right_op {
-                            (Side::Left, left_op)
+                        if left_op
+                            .get_sort_key(seen_left_length)
+                            .partial_cmp(&right_op.get_sort_key(seen_right_length))
+                            == Some(std::cmp::Ordering::Less)
+                        {
+                            (Side::Left, left_op, last_right_op.clone())
                         } else {
-                            (Side::Right, right_op)
+                            (Side::Right, right_op, last_left_op.clone())
                         }
                     }
 
-                    (Some(left_op), None) => (Side::Left, left_op),
-                    (None, Some(right_op)) => (Side::Right, right_op),
+                    (Some(left_op), None) => (Side::Left, left_op, last_right_op.clone()),
+                    (None, Some(right_op)) => (Side::Right, right_op, last_left_op.clone()),
                     (None, None) => break,
                 };
 
-            if side == Side::Left {
-                maybe_left_op = left_iter.next();
-            } else {
-                maybe_right_op = right_iter.next();
-            }
+            let is_advancing_operation = matches!(
+                operation,
+                Operation::Insert { .. } | Operation::Equal { .. }
+            );
 
-            let original_start = operation.start_index() as i64;
-            let original_end = operation.end_index();
             let original_length = operation.len() as i64;
-
             let result = match side {
-                Side::Left => operation.merge_operations_with_context(
-                    &mut right_merge_context,
-                    &mut left_merge_context,
-                ),
-                Side::Right => operation.merge_operations_with_context(
-                    &mut left_merge_context,
-                    &mut right_merge_context,
-                ),
+                Side::Left => {
+                    let result = operation.merge_operations(&mut last_other_op);
+
+                    if let ref op @ (Operation::Insert { .. } | Operation::Equal { .. }) = result {
+                        let shift = merged_length as i64 - seen_left_length as i64
+                            + op.len() as i64
+                            - original_length;
+
+                        while let Some(cursor) = left_cursors.next_if(|cursor| {
+                            cursor.char_index <= seen_left_length + original_length as usize
+                        }) {
+                            merged_cursors.push(
+                                cursor.with_index((cursor.char_index as i64 + shift) as usize),
+                            );
+                        }
+                    }
+
+                    if is_advancing_operation {
+                        seen_left_length += original_length as usize;
+                    }
+
+                    maybe_left_op = left_iter.next();
+                    last_left_op = Some(result.clone());
+
+                    result
+                }
+                Side::Right => {
+                    let result = operation.merge_operations(&mut last_other_op);
+
+                    if let ref op @ (Operation::Insert { .. } | Operation::Equal { .. }) = result {
+                        let shift = merged_length as i64 - seen_right_length as i64
+                            + op.len() as i64
+                            - original_length;
+
+                        while let Some(cursor) = right_cursors.next_if(|cursor| {
+                            cursor.char_index <= seen_right_length + original_length as usize
+                        }) {
+                            merged_cursors.push(
+                                cursor.with_index((cursor.char_index as i64 + shift) as usize),
+                            );
+                        }
+                    }
+
+                    if is_advancing_operation {
+                        seen_right_length += original_length as usize;
+                    }
+
+                    maybe_right_op = right_iter.next();
+                    last_right_op = Some(result.clone());
+
+                    result
+                }
             };
 
-            if let Some(ref op @ (Operation::Insert { .. } | Operation::Equal { .. })) = result {
-                let shift =
-                    op.start_index() as i64 - original_start + op.len() as i64 - original_length;
-                match side {
-                    Side::Left => {
-                        while let Some(cursor) =
-                            left_cursors.next_if(|cursor| cursor.char_index <= original_end + 1)
-                        {
-                            merged_cursors.push(cursor.with_index(
-                                (op.start_index() as i64).max(cursor.char_index as i64 + shift)
-                                    as usize,
-                            ));
-                        }
-                    }
-                    Side::Right => {
-                        while let Some(cursor) =
-                            right_cursors.next_if(|cursor| cursor.char_index <= original_end + 1)
-                        {
-                            merged_cursors.push(cursor.with_index(
-                                (op.start_index() as i64).max(cursor.char_index as i64 + shift)
-                                    as usize,
-                            ));
-                        }
-                    }
-                }
+            if result.len() == 0 {
+                continue;
             }
 
-            merged_operations.extend(result.into_iter().map(|op| OrderedOperation {
-                order,
-                operation: op,
-            }));
+            if is_advancing_operation {
+                merged_length += result.len();
+            }
+
+            merged_operations.push(result);
         }
 
-        let last_index = merged_operations
-            .iter()
-            .filter(|operation| {
-                matches!(
-                    operation.operation,
-                    Operation::Insert { .. } | Operation::Equal { .. }
-                )
-            })
-            .next_back()
-            .map_or(0, |op| op.operation.end_index());
-
         for cursor in left_cursors.chain(right_cursors) {
-            merged_cursors.push(cursor.with_index(last_index));
+            merged_cursors.push(cursor.with_index(merged_length));
         }
 
         Self::new(self.text, merged_operations, merged_cursors)
@@ -219,7 +217,7 @@ where
     pub fn apply(&self) -> String {
         let mut builder: StringBuilder<'_> = StringBuilder::new(self.text);
 
-        for OrderedOperation { operation, .. } in &self.operations {
+        for operation in &self.operations {
             builder = operation.apply(builder);
         }
 
@@ -229,8 +227,6 @@ where
 
 #[cfg(test)]
 mod tests {
-    use std::env;
-
     use insta::assert_debug_snapshot;
     use pretty_assertions::assert_eq;
 

src/operation_transformation/merge_context.rs

@@ -1,73 +0,0 @@
-use core::fmt::Debug;
-
-use crate::operation_transformation::Operation;
-
-#[derive(Clone, Debug)]
-pub struct MergeContext<T>
-where
-    T: PartialEq + Clone + std::fmt::Debug,
-{
-    last_operation: Option<Operation<T>>,
-    pub shift: i64,
-}
-
-impl<T> Default for MergeContext<T>
-where
-    T: PartialEq + Clone + std::fmt::Debug,
-{
-    fn default() -> Self {
-        MergeContext {
-            last_operation: None,
-            shift: 0,
-        }
-    }
-}
-
-impl<T> MergeContext<T>
-where
-    T: PartialEq + Clone + std::fmt::Debug,
-{
-    pub fn last_operation(&self) -> Option<&Operation<T>> { self.last_operation.as_ref() }
-
-    pub fn replace_last_operation(&mut self, operation: Option<Operation<T>>) {
-        self.last_operation = operation;
-    }
-
-    /// Replace the last delete operation (if there was one) with a new one
-    /// while applying it to the `shift` in case the last operation
-    /// was a delete.
-    pub fn consume_and_replace_last_operation(&mut self, operation: Option<Operation<T>>) {
-        if let Some(Operation::Delete {
-            deleted_character_count,
-            ..
-        }) = self.last_operation.take()
-        {
-            self.shift -= deleted_character_count as i64;
-        }
-
-        self.last_operation = operation;
-    }
-
-    /// Remove the last operation (if there was one) in case it is behind the
-    /// threshold operation. This updates the `shift` in case the last operation
-    /// was a delete.
-    pub fn consume_last_operation_if_it_is_too_behind(&mut self, threshold_index: i64) {
-        if let Some(last_operation) = self.last_operation.as_ref() {
-            if let Operation::Delete {
-                deleted_character_count,
-                ..
-            } = last_operation
-            {
-                if threshold_index + self.shift > last_operation.end_index() as i64 {
-                    self.shift -= *deleted_character_count as i64;
-                    self.last_operation = None;
-                }
-            } else if let Operation::Insert { .. } = last_operation
-                && threshold_index + self.shift - last_operation.len() as i64
-                    > last_operation.end_index() as i64
-            {
-                self.last_operation = None;
-            }
-        }
-    }
-}

src/operation_transformation/operation.rs

@@ -1,10 +1,8 @@
 use core::fmt::{Debug, Display};
-use std::ops::Range;
 
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
-use super::merge_context::MergeContext;
 use crate::{
     Token,
     utils::{
@@ -21,7 +19,7 @@ where
     T: PartialEq + Clone + std::fmt::Debug,
 {
     Equal {
-        index: usize,
+        order: usize,
         length: usize,
 
         #[cfg(debug_assertions)]
@@ -29,12 +27,12 @@ where
     },
 
     Insert {
-        index: usize,
+        order: usize,
         text: Vec<Token<T>>,
     },
 
     Delete {
-        index: usize,
+        order: usize,
         deleted_character_count: usize,
 
         #[cfg(debug_assertions)]
@@ -49,74 +47,83 @@ where
     /// Creates an equal operation with the given index.
     /// This operation is used to indicate that the text at the given index
     /// is unchanged.
-    pub fn create_equal(index: usize, length: usize) -> Option<Self> {
-        if length == 0 {
-            return None;
-        }
-
-        Some(Operation::Equal {
-            index,
+    pub fn create_equal(order: usize, length: usize) -> Self {
+        Operation::Equal {
+            order,
             length,
 
             #[cfg(debug_assertions)]
             text: None,
-        })
+        }
     }
 
-    pub fn create_equal_with_text(index: usize, text: String) -> Option<Self> {
-        if text.is_empty() {
-            return None;
-        }
-
-        Some(Operation::Equal {
-            index,
+    pub fn create_equal_with_text(order: usize, text: String) -> Self {
+        Operation::Equal {
+            order,
             length: text.chars().count(),
 
             #[cfg(debug_assertions)]
             text: Some(text),
-        })
+        }
     }
 
     /// Creates an insert operation with the given index and text.
-    /// If the text is empty (meaning that the operation would be a no-op),
-    /// returns None.
-    pub fn create_insert(index: usize, text: Vec<Token<T>>) -> Option<Self> {
-        if text.is_empty() {
-            return None;
-        }
-
-        Some(Operation::Insert { index, text })
+    pub fn create_insert(order: usize, text: Vec<Token<T>>) -> Self {
+        Operation::Insert { order, text }
     }
 
     /// Creates a delete operation with the given index and number of
-    /// to-be-deleted characters. If the operation would delete 0 (meaning
-    /// that the operation would be a no-op), returns None.
-    pub fn create_delete(index: usize, deleted_character_count: usize) -> Option<Self> {
-        if deleted_character_count == 0 {
-            return None;
-        }
-
-        Some(Operation::Delete {
-            index,
+    /// to-be-deleted characters.
+    pub fn create_delete(order: usize, deleted_character_count: usize) -> Self {
+        Operation::Delete {
+            order,
             deleted_character_count,
 
             #[cfg(debug_assertions)]
             deleted_text: None,
-        })
+        }
     }
 
-    pub fn create_delete_with_text(index: usize, text: String) -> Option<Self> {
-        if text.is_empty() {
-            return None;
-        }
-
-        Some(Operation::Delete {
-            index,
+    pub fn create_delete_with_text(order: usize, text: String) -> Self {
+        Operation::Delete {
+            order,
             deleted_character_count: text.chars().count(),
 
             #[cfg(debug_assertions)]
             deleted_text: Some(text),
-        })
+        }
+    }
+
+    fn order(&self) -> usize {
+        match self {
+            Operation::Equal { order, .. }
+            | Operation::Insert { order, .. }
+            | Operation::Delete { order, .. } => *order,
+        }
+    }
+
+    pub fn get_sort_key(&self, insertion_index: usize) -> (usize, usize, usize, String) {
+        (
+            self.order(),
+            match self {
+                Operation::Delete { .. } => 1,
+                Operation::Insert { .. } => 2,
+                Operation::Equal { .. } => 3,
+            },
+            insertion_index,
+            // Make sure that the ordering is deterministic regardless of which text
+            // is left or right.
+            match self {
+                Operation::Equal { length, .. } => length.to_string(),
+                Operation::Insert { text, .. } => {
+                    text.iter().map(Token::original).collect::<String>()
+                }
+                Operation::Delete {
+                    deleted_character_count,
+                    ..
+                } => deleted_character_count.to_string(),
+            },
+        )
     }
 
     /// Applies the operation to the given `StringBuilder`, returning the
@@ -129,63 +136,47 @@ where
             Operation::Equal {
                 #[cfg(debug_assertions)]
                 text,
+                length,
                 ..
             } => {
                 #[cfg(debug_assertions)]
                 debug_assert!(
                     text.as_ref()
-                        .is_none_or(|text| builder.get_slice(self.range()) == *text),
-                    "Text which is supposed to be equal does not match the text in the range"
+                        .is_none_or(|text| builder.get_slice_from_remaining(self.len()) == *text),
+                    "Text (`{}`) which is supposed to be equal does not match the text in the \
+                     range: `{}`",
+                    text.as_ref().unwrap_or(&String::new()),
+                    builder.get_slice_from_remaining(self.len())
                 );
 
-                return builder;
+                builder.retain(*length);
+            }
+            Operation::Insert { text, .. } => {
+                builder.insert(&text.iter().map(Token::original).collect::<String>());
             }
-            Operation::Insert { text, .. } => builder.insert(
-                self.start_index(),
-                &text.iter().map(Token::original).collect::<String>(),
-            ),
             Operation::Delete {
                 #[cfg(debug_assertions)]
                 deleted_text,
+                deleted_character_count,
                 ..
             } => {
                 #[cfg(debug_assertions)]
                 debug_assert!(
                     deleted_text
                         .as_ref()
-                        .is_none_or(|text| builder.get_slice(self.range()) == *text),
-                    "Text to delete does not match the text in the range"
+                        .is_none_or(|text| builder.get_slice_from_remaining(self.len()) == *text),
+                    "Text to-be-deleted `{}` does not match the text in the range: `{}`",
+                    deleted_text.as_ref().unwrap_or(&String::new()),
+                    builder.get_slice_from_remaining(self.len())
                 );
 
-                builder.delete(self.range());
+                builder.delete(*deleted_character_count);
             }
         }
 
         builder
     }
 
-    /// Returns the index of the first character that the operation affects.
-    pub fn start_index(&self) -> usize {
-        match self {
-            Operation::Equal { index, .. }
-            | Operation::Insert { index, .. }
-            | Operation::Delete { index, .. } => *index,
-        }
-    }
-
-    /// Returns the index of the last character that the operation affects.
-    pub fn end_index(&self) -> usize {
-        debug_assert!(
-            self.len() > 0,
-            " len() must be greater than 0 because operations must be non-empty"
-        );
-        self.start_index() + self.len() - 1
-    }
-
-    /// Returns the range of indices of characters that the operation affects.
-    #[allow(clippy::range_plus_one)]
-    pub fn range(&self) -> Range<usize> { self.start_index()..self.end_index() + 1 }
-
     /// Returns the number of affected characters. It is always greater than 0
     /// because empty operations cannot be created.
     pub fn len(&self) -> usize {
@@ -199,76 +190,17 @@ where
         }
     }
 
-    /// Creates a new operation with the same type and text but with the given
-    /// index.
-    pub fn with_index(self, index: usize) -> Self {
-        match self {
-            Operation::Equal {
-                length,
-
-                #[cfg(debug_assertions)]
-                text,
-                ..
-            } => Operation::Equal {
-                index,
-                length,
-
-                #[cfg(debug_assertions)]
-                text,
-            },
-            Operation::Insert { text, .. } => Operation::Insert { index, text },
-            Operation::Delete {
-                deleted_character_count,
-
-                #[cfg(debug_assertions)]
-                deleted_text,
-                ..
-            } => Operation::Delete {
-                index,
-                deleted_character_count,
-
-                #[cfg(debug_assertions)]
-                deleted_text,
-            },
-        }
-    }
-
-    /// Creates a new operation with the same type and text but with the index
-    /// shifted by the given offset. The offset can be negative but the
-    /// resulting index must be non-negative.
-    ///
-    /// # Panics
-    ///
-    /// In debug mode, panics if the resulting index is negative.
-    pub fn with_shifted_index(self, offset: i64) -> Self {
-        let index = self.start_index() as i64 + offset;
-        debug_assert!(index >= 0, "Shifted index must be non-negative");
-
-        self.with_index(index as usize)
-    }
-
     /// Merges the operation with the given context, producing a new operation
     /// and updating the context. This implements a comples FSM that handles
     /// the merging of operations in a way that is consistent with the text.
     /// The contexts are updated in-place.
     #[allow(clippy::too_many_lines)]
-    pub fn merge_operations_with_context(
-        self,
-        affecting_context: &mut MergeContext<T>,
-        produced_context: &mut MergeContext<T>,
-    ) -> Option<Operation<T>> {
-        affecting_context.consume_last_operation_if_it_is_too_behind(self.start_index() as i64);
-        let operation = self.with_shifted_index(affecting_context.shift);
-
-        match (operation, affecting_context.last_operation()) {
-            (operation @ Operation::Insert { .. }, None | Some(Operation::Equal { .. })) => {
-                produced_context.shift += operation.len() as i64;
-                produced_context.consume_and_replace_last_operation(Some(operation.clone()));
-                Some(operation)
-            }
+    pub fn merge_operations(self, previous_operation: &mut Option<Self>) -> Operation<T> {
+        let operation = self;
 
+        match (operation, previous_operation) {
             (
-                Operation::Insert { text, index },
+                Operation::Insert { order, text },
                 Some(Operation::Insert {
                     text: previous_inserted_text,
                     ..
@@ -279,129 +211,113 @@ where
                 // This way, we don't end up duplicating text.
                 let offset_in_tokens =
                     find_longest_prefix_contained_within(previous_inserted_text, &text);
-                let offset_in_length = text
-                    .iter()
-                    .take(offset_in_tokens)
-                    .map(Token::get_original_length)
-                    .sum::<usize>();
-                let trimmed_operation =
-                    Operation::create_insert(index, text[offset_in_tokens..].to_vec());
 
-                affecting_context.shift -= offset_in_length as i64;
-                produced_context.shift += trimmed_operation
-                    .as_ref()
-                    .map(Operation::len)
-                    .unwrap_or_default() as i64;
-                produced_context.consume_and_replace_last_operation(trimmed_operation.clone());
-
-                trimmed_operation
+                Operation::create_insert(order, text[offset_in_tokens..].to_vec())
             }
 
             (
-                operation @ Operation::Delete { .. },
-                None | Some(Operation::Insert { .. } | Operation::Equal { .. }),
-            ) => {
-                produced_context.consume_and_replace_last_operation(Some(operation.clone()));
-                Some(operation)
-            }
+                Operation::Delete {
+                    order,
+                    deleted_character_count,
 
-            (
-                operation @ Operation::Insert { .. },
-                Some(last_delete @ Operation::Delete { .. }),
+                    #[cfg(debug_assertions)]
+                    deleted_text,
+                },
+                Some(Operation::Delete {
+                    order: last_delete_order,
+                    deleted_character_count: last_delete_deleted_character_count,
+                    ..
+                }),
             ) => {
-                produced_context.shift += operation.len() as i64;
+                let operation_end_index = order + deleted_character_count;
+                let last_delete_end_index =
+                    *last_delete_order + *last_delete_deleted_character_count;
 
-                debug_assert!(
-                    last_delete.range().contains(&operation.start_index()),
-                    "There is a last delete ({last_delete}) but the operation ({operation}) is \
-                     not contained in it"
+                let new_length = deleted_character_count
+                    .min(0.max(operation_end_index as i64 - last_delete_end_index as i64) as usize);
+
+                let overlap = deleted_character_count - new_length;
+
+                #[cfg(debug_assertions)]
+                let updated_delete = deleted_text.as_ref().map_or_else(
+                    || Operation::create_delete(order + overlap, new_length),
+                    |text| {
+                        Operation::create_delete_with_text(
+                            order + overlap,
+                            text.chars()
+                                .skip(deleted_character_count - new_length)
+                                .collect::<String>(),
+                        )
+                    },
                 );
 
-                let difference = operation.start_index() as i64 - last_delete.start_index() as i64;
-
-                let moved_operation = operation.with_index(last_delete.start_index());
-
-                affecting_context.replace_last_operation(Operation::create_delete(
-                    moved_operation.end_index() + 1,
-                    (last_delete.len() as i64 - difference) as usize,
-                ));
-                affecting_context.shift -= difference;
-
-                produced_context.consume_and_replace_last_operation(Some(moved_operation.clone()));
-
-                Some(moved_operation)
-            }
-
-            (
-                operation @ Operation::Delete { .. },
-                Some(last_delete @ Operation::Delete { .. }),
-            ) => {
-                debug_assert!(
-                    last_delete.range().contains(&operation.start_index()),
-                    "There is a last delete ({last_delete}) but the operation ({operation}) is \
-                     not contained in it"
-                );
-
-                let difference = operation.start_index() as i64 - last_delete.start_index() as i64;
-
-                let updated_delete = Operation::create_delete(
-                    last_delete.start_index(),
-                    0.max(operation.end_index() as i64 - last_delete.end_index() as i64) as usize,
-                );
-
-                affecting_context.replace_last_operation(Operation::create_delete(
-                    last_delete.start_index(),
-                    0.max(last_delete.end_index() as i64 - operation.end_index() as i64) as usize,
-                ));
-                affecting_context.shift -= difference;
-
-                produced_context.consume_and_replace_last_operation(updated_delete.clone());
+                #[cfg(not(debug_assertions))]
+                let updated_delete = Operation::create_delete(order + overlap, new_length);
 
                 updated_delete
             }
+
             (
-                ref operation @ Operation::Equal {
+                Operation::Equal {
+                    order,
                     length,
+
                     #[cfg(debug_assertions)]
                     ref text,
-                    ..
                 },
-                Some(last_delete @ Operation::Delete { .. }),
+                Some(Operation::Delete {
+                    order: last_delete_order,
+                    deleted_character_count: last_delete_deleted_character_count,
+                    ..
+                }),
             ) => {
-                debug_assert!(
-                    last_delete.range().contains(&operation.start_index()),
-                    "There is a last delete ({last_delete}) but the operation ({operation}) is \
-                     not contained in it"
-                );
+                let last_delete_end_index =
+                    *last_delete_order + *last_delete_deleted_character_count;
 
-                let overlap = (length as i64)
-                    .min(last_delete.end_index() as i64 - operation.start_index() as i64 + 1);
+                let overlap =
+                    0.max((length as i64).min(last_delete_end_index as i64 - order as i64));
 
                 #[cfg(debug_assertions)]
-                let result = text.as_ref().map_or_else(
+                let updated_equal = text.as_ref().map_or_else(
                     || {
                         Operation::create_equal(
-                            operation.end_index().min(last_delete.end_index()),
+                            order + overlap as usize,
                             (length as i64 - overlap) as usize,
                         )
                     },
                     |text| {
                         Operation::create_equal_with_text(
-                            operation.end_index().min(last_delete.end_index()),
+                            order + overlap as usize,
                             text.chars().skip(overlap as usize).collect::<String>(),
                         )
                     },
                 );
 
                 #[cfg(not(debug_assertions))]
-                let result = Operation::create_equal(
-                    operation.end_index().min(last_delete.end_index()),
+                let updated_equal = Operation::create_equal(
+                    order + overlap as usize,
                     (length as i64 - overlap) as usize,
                 );
 
-                result
+                updated_equal
             }
-            (operation @ Operation::Equal { .. }, _) => Some(operation),
+
+            (
+                ref operation @ Operation::Equal { ref order, .. },
+                Some(Operation::Equal {
+                    order: last_equal_order,
+                    length: last_equal_length,
+                    ..
+                }),
+            ) => {
+                if operation.len() == *last_equal_length && *order == *last_equal_order {
+                    Operation::create_equal(*order, 0)
+                } else {
+                    operation.clone()
+                }
+            }
+
+            (operation, _) => operation,
         }
     }
 }
@@ -413,7 +329,7 @@ where
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         match self {
             Operation::Equal {
-                index,
+                order,
                 length,
 
                 #[cfg(debug_assertions)]
@@ -422,28 +338,29 @@ where
                 #[cfg(debug_assertions)]
                 write!(
                     f,
-                    "<equal {} from index {}>",
+                    "<equal {} from {order}>",
                     text.as_ref()
-                        .map(|text| format!("'{text}'"))
+                        .map(|text| format!("'{}'", text.replace('\n', "\\n")))
                         .unwrap_or(format!("{length} characters")),
-                    index
                 )?;
 
                 #[cfg(not(debug_assertions))]
-                write!(f, "<equal {length} from index {index}>")?;
+                write!(f, "<equal {length} from {order}>")?;
 
                 Ok(())
             }
-            Operation::Insert { index, text } => {
+            Operation::Insert { order, text } => {
                 write!(
                     f,
-                    "<insert '{}' from index {}>",
-                    text.iter().map(Token::original).collect::<String>(),
-                    index
+                    "<insert '{}' at {order}>",
+                    text.iter()
+                        .map(Token::original)
+                        .collect::<String>()
+                        .replace('\n', "\\n"),
                 )
             }
             Operation::Delete {
-                index,
+                order,
                 deleted_character_count,
 
                 #[cfg(debug_assertions)]
@@ -452,18 +369,17 @@ where
                 #[cfg(debug_assertions)]
                 write!(
                     f,
-                    "<delete {} from index {}>",
+                    "<delete {} from {order}>",
                     deleted_text
                         .as_ref()
-                        .map(|text| format!("'{text}'"))
+                        .map(|text| format!("'{}'", text.replace('\n', "\\n")))
                         .unwrap_or(format!("{deleted_character_count} characters")),
-                    index
                 )?;
 
                 #[cfg(not(debug_assertions))]
                 write!(
                     f,
-                    "<delete {deleted_character_count} characters from index {index}>",
+                    "<delete {deleted_character_count} characters from {order}>",
                 )?;
 
                 Ok(())
@@ -485,29 +401,28 @@ mod tests {
 
     use super::*;
 
-    #[test]
-    #[should_panic(expected = "Shifted index must be non-negative")]
-    fn test_shifting_error() {
-        insta::assert_debug_snapshot!(
-            Operation::create_insert(1, vec!["hi".into()])
-                .unwrap()
-                .with_shifted_index(-2)
-        );
-    }
-
     #[test]
     fn test_apply_delete_with_create() {
         let builder = StringBuilder::new("hello world");
-        let operation = Operation::<()>::create_delete_with_text(5, " world".to_owned()).unwrap();
+        let delete_operation = Operation::<()>::create_delete_with_text(0, "hello ".to_owned());
+        let retain_operation = Operation::<()>::create_equal(6, 5);
 
-        assert_eq!(operation.apply(builder).build(), "hello");
+        let mut builder = delete_operation.apply(builder);
+        builder = retain_operation.apply(builder);
+
+        assert_eq!(builder.build(), "world");
     }
 
     #[test]
     fn test_apply_insert() {
         let builder = StringBuilder::new("hello");
-        let operation = Operation::create_insert(5, vec![" my friend".into()]).unwrap();
 
-        assert_eq!(operation.apply(builder).build(), "hello my friend");
+        let retain_operation = Operation::<()>::create_equal(0, 5);
+        let insert_operation = Operation::create_insert(5, vec![" my friend".into()]);
+
+        let mut builder = retain_operation.apply(builder);
+        builder = insert_operation.apply(builder);
+
+        assert_eq!(builder.build(), "hello my friend");
     }
 }

src/operation_transformation/ordered_operation.rs

@@ -1,48 +0,0 @@
-#[cfg(feature = "serde")]
-use serde::{Deserialize, Serialize};
-
-use crate::operation_transformation::Operation;
-
-#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
-#[derive(Debug, Clone, PartialEq)]
-pub struct OrderedOperation<T>
-where
-    T: PartialEq + Clone + std::fmt::Debug,
-{
-    pub order: usize,
-    pub operation: Operation<T>,
-}
-
-impl<T> OrderedOperation<T>
-where
-    T: PartialEq + Clone + std::fmt::Debug,
-{
-    pub fn get_sort_key(&self) -> (usize, usize, String) {
-        (
-            self.order,
-            self.operation.start_index(),
-            // Make sure that the ordering is deterministic regardless of which text
-            // is left or right.
-            match &self.operation {
-                Operation::Equal { index, .. } => index.to_string(),
-                Operation::Insert { text, .. } => text
-                    .iter()
-                    .map(crate::tokenizer::token::Token::original)
-                    .collect::<String>(),
-                Operation::Delete {
-                    deleted_character_count,
-                    ..
-                } => deleted_character_count.to_string(),
-            },
-        )
-    }
-}
-
-impl<T> PartialOrd for OrderedOperation<T>
-where
-    T: PartialEq + Clone + std::fmt::Debug,
-{
-    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
-        self.get_sort_key().partial_cmp(&other.get_sort_key())
-    }
-}

src/operation_transformation/snapshots/reconcile__operation_transformation__edited_text__tests__calculate_operations.snap

@@ -1,43 +1,19 @@
 ---
-source: reconcile/src/operation_transformation/edited_text.rs
+source: src/operation_transformation/edited_text.rs
 expression: operations
 snapshot_kind: text
 ---
 EditedText {
     text: "hello world! How are you?  Adam",
     operations: [
-        OrderedOperation {
-            order: 0,
-            operation: <insert 'Hello, my friend!' from index 0>,
-        },
-        OrderedOperation {
-            order: 0,
-            operation: <delete 'hello world!' from index 17>,
-        },
-        OrderedOperation {
-            order: 12,
-            operation: <equal ' ' from index 17>,
-        },
-        OrderedOperation {
-            order: 13,
-            operation: <equal 'How' from index 18>,
-        },
-        OrderedOperation {
-            order: 16,
-            operation: <equal ' ' from index 21>,
-        },
-        OrderedOperation {
-            order: 17,
-            operation: <equal 'are' from index 22>,
-        },
-        OrderedOperation {
-            order: 20,
-            operation: <insert ' you doing? Albert' from index 25>,
-        },
-        OrderedOperation {
-            order: 20,
-            operation: <delete ' you?  Adam' from index 43>,
-        },
+        <delete 'hello world!' from 0>,
+        <insert 'Hello, my friend!' at 12>,
+        <equal ' ' from 12>,
+        <equal 'How' from 13>,
+        <equal ' ' from 16>,
+        <equal 'are' from 17>,
+        <delete ' you?  Adam' from 20>,
+        <insert ' you doing? Albert' at 31>,
     ],
     cursors: [],
 }

src/operation_transformation/snapshots/reconcile__operation_transformation__edited_text__tests__calculate_operations_with_no_diff.snap

@@ -1,23 +1,14 @@
 ---
-source: reconcile/src/operation_transformation/edited_text.rs
+source: src/operation_transformation/edited_text.rs
 expression: operations
 snapshot_kind: text
 ---
 EditedText {
     text: "hello world!",
     operations: [
-        OrderedOperation {
-            order: 0,
-            operation: <equal 'hello' from index 0>,
-        },
-        OrderedOperation {
-            order: 5,
-            operation: <equal ' ' from index 5>,
-        },
-        OrderedOperation {
-            order: 6,
-            operation: <equal 'world!' from index 6>,
-        },
+        <equal 'hello' from 0>,
+        <equal ' ' from 5>,
+        <equal 'world!' from 6>,
     ],
     cursors: [],
 }

src/operation_transformation/utils/cook_operations.rs

@@ -1,52 +1,44 @@
-use crate::{
-    diffs::raw_operation::RawOperation,
-    operation_transformation::{Operation, ordered_operation::OrderedOperation},
-};
+use crate::{diffs::raw_operation::RawOperation, operation_transformation::Operation};
 
-/// Turn raw operations into ordered operations while keeping track of old & new
-/// indexes.
-pub fn cook_operations<I, T>(raw_operations: I) -> impl Iterator<Item = OrderedOperation<T>>
+/// Turn raw operations into ordered operations while keeping track of the
+/// original token's indexes.
+pub fn cook_operations<I, T>(raw_operations: I) -> impl Iterator<Item = Operation<T>>
 where
     I: IntoIterator<Item = RawOperation<T>>,
     T: PartialEq + Clone + std::fmt::Debug,
 {
-    let mut new_index = 0; // this is the start index of the operation on the new text
-    let mut order = 0; // this is the start index of the operation on the original text
+    let mut original_text_index = 0; // this is the start index of the operation on the original text
 
-    raw_operations.into_iter().filter_map(move |raw_operation| {
+    raw_operations.into_iter().map(move |raw_operation| {
         let length = raw_operation.original_text_length();
 
         match raw_operation {
             RawOperation::Equal(..) => {
                 let op = if cfg!(debug_assertions) {
-                    Operation::create_equal_with_text(new_index, raw_operation.get_original_text())
+                    Operation::create_equal_with_text(
+                        original_text_index,
+                        raw_operation.get_original_text(),
+                    )
                 } else {
-                    Operation::create_equal(new_index, length)
-                }
-                .map(|operation| OrderedOperation { order, operation });
+                    Operation::create_equal(original_text_index, length)
+                };
 
-                new_index += length;
-                order += length;
-
-                op
-            }
-            RawOperation::Insert(tokens) => {
-                let op = Operation::create_insert(new_index, tokens)
-                    .map(|operation| OrderedOperation { order, operation });
-
-                new_index += length;
+                original_text_index += length;
 
                 op
             }
+            RawOperation::Insert(tokens) => Operation::create_insert(original_text_index, tokens),
             RawOperation::Delete(..) => {
                 let op = if cfg!(debug_assertions) {
-                    Operation::create_delete_with_text(new_index, raw_operation.get_original_text())
+                    Operation::create_delete_with_text(
+                        original_text_index,
+                        raw_operation.get_original_text(),
+                    )
                 } else {
-                    Operation::create_delete(new_index, length)
-                }
-                .map(|operation| OrderedOperation { order, operation });
+                    Operation::create_delete(original_text_index, length)
+                };
 
-                order += length;
+                original_text_index += length;
 
                 op
             }

src/operation_transformation/utils/elongate_operations.rs

@@ -17,6 +17,8 @@ where
     let mut maybe_previous_insert: Option<RawOperation<T>> = None;
     let mut maybe_previous_delete: Option<RawOperation<T>> = None;
 
+    // We don't elongate `equals` as they're needed to maintain cursor positions
+    // when merging against deletes.
     let mut result: Vec<RawOperation<T>> = raw_operations
         .into_iter()
         .flat_map(|next| match next {
@@ -41,87 +43,80 @@ where
                 }
             },
             RawOperation::Equal(..) => Box::new(
-                maybe_previous_insert
+                maybe_previous_delete
                     .take()
                     .into_iter()
-                    .chain(maybe_previous_delete.take())
+                    .chain(maybe_previous_insert.take())
                     .chain(iter::once(next)),
             ) as Box<dyn Iterator<Item = RawOperation<T>>>,
         })
         .collect();
 
-    if let Some(prev) = maybe_previous_insert {
+    if let Some(prev) = maybe_previous_delete {
         result.push(prev);
     }
 
-    if let Some(prev) = maybe_previous_delete {
+    if let Some(prev) = maybe_previous_insert {
         result.push(prev);
     }
 
     result
 }
 
-// #[cfg(test)]
-// mod tests {
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::tokenizer::token::Token;
 
-//     use super::*;
+    // Helper constructors for cleaner tests
+    fn ins(texts: &[&str]) -> RawOperation<String> {
+        RawOperation::Insert(texts.iter().map(|t| Token::from(*t)).collect())
+    }
 
-//     #[test]
-//     fn test_elongate_operations_empty() {
-//         let operations: Vec<RawOperation<()>> = vec![];
-//         let result = elongate_operations(operations);
-//         assert_eq!(result, vec![]);
-//     }
+    fn del(texts: &[&str]) -> RawOperation<String> {
+        RawOperation::Delete(texts.iter().map(|t| Token::from(*t)).collect())
+    }
 
-//     #[test]
-//     fn test_elongate_operations_single_operation() {
-//         let operations = vec![RawOperation::Insert(vec!["test".into()])];
-//         let result = elongate_operations(operations);
-//         assert_eq!(result.len(), 1);
-//         assert!(matches!(result[0], RawOperation::Insert(_)));
-//     }
+    fn ins_custom(text: &str, lj: bool, rj: bool) -> RawOperation<String> {
+        RawOperation::Insert(vec![Token::new(text.to_string(), text.to_string(), lj, rj)])
+    }
 
-//     #[test]
-//     fn test_elongate_operations_interleaved() {
-//         let operations = vec![
-//             RawOperation::Insert(vec!["a".into()]),
-//             RawOperation::Delete(vec!["b".into()]),
-//             RawOperation::Insert(vec!["c".into()]),
-//             RawOperation::Delete(vec!["d".into()]),
-//         ];
-//         let result = elongate_operations(operations);
-//         assert_eq!(result.len(), 2);
-//         assert!(matches!(result[0], RawOperation::Insert(_)));
-//         assert!(matches!(result[1], RawOperation::Delete(_)));
-//     }
+    #[test]
+    fn merges_adjacent_joinable_inserts() {
+        let ops = vec![ins(&["a"]), ins(&["b"]), ins(&["c"])];
+        let result = elongate_operations(ops);
+        assert_eq!(result.len(), 1);
+        match &result[0] {
+            RawOperation::Insert(tokens) => {
+                let originals: String = tokens.iter().map(|t| t.original()).collect();
+                assert_eq!(originals, "abc");
+            }
+            _ => panic!("Expected single Insert operation"),
+        }
+    }
 
-//     #[test]
-//     fn test_elongate_operations_with_equal() {
-//         let operations = vec![
-//             RawOperation::Equal(vec!["a".into()]),
-//             RawOperation::Equal(vec!["b".into()]),
-//             RawOperation::Insert(vec!["c".into()]),
-//             RawOperation::Insert(vec!["d".into()]),
-//         ];
-//         let result = elongate_operations(operations);
-//         assert_eq!(result.len(), 2);
-//         assert!(matches!(result[0], RawOperation::Equal(_)));
-//         assert!(matches!(result[1], RawOperation::Insert(_)));
-//     }
+    #[test]
+    fn does_not_merge_when_not_joinable() {
+        let ops = vec![
+            ins_custom("a", true, false), // not right-joinable
+            ins_custom("b", true, true),  // left-joinable but previous isn't right-joinable
+        ];
+        let result = elongate_operations(ops);
+        assert_eq!(
+            result.len(),
+            2,
+            "Operations should remain separate when not joinable"
+        );
+    }
 
-//     #[test]
-//     fn test_elongate_operations_mixed_sequence() {
-//         let operations = vec![
-//             RawOperation::Insert(vec!["a".into()]),
-//             RawOperation::Equal(vec!["b".into()]),
-//             RawOperation::Delete(vec!["c".into()]),
-//             RawOperation::Equal(vec!["d".into()]),
-//         ];
-//         let result = elongate_operations(operations);
-//         assert_eq!(result.len(), 4);
-//         assert!(matches!(result[0], RawOperation::Insert(_)));
-//         assert!(matches!(result[1], RawOperation::Equal(_)));
-//         assert!(matches!(result[2], RawOperation::Delete(_)));
-//         assert!(matches!(result[3], RawOperation::Equal(_)));
-//     }
-// }
+    #[test]
+    fn merges_interleaved_insert_delete_sequences() {
+        // Pattern IDID -> II DD
+        let ops = vec![ins(&["i1"]), del(&["d1"]), ins(&["i2"]), del(&["d2"])];
+        let result = elongate_operations(ops);
+
+        assert_eq!(result.len(), 2);
+        assert!(matches!(result[0], RawOperation::Delete(_)));
+        assert!(matches!(result[1], RawOperation::Insert(_)));
+    }
+}

src/utils/find_longest_prefix_contained_within.rs

@@ -9,20 +9,20 @@ use crate::Token;
 /// old: [0, 1, 9, 0, 2, 5]
 /// new:       [9, 0, 2, 5, 1]
 /// ```
-/// > results in an length of 4
+/// > results in a length of 4
 ///
 ///
 /// ```not_rust
 /// old: [0, 1, 9, 0, 2, 5]
 /// new:          [0, 2]
 /// ```
-/// > results in an length of 2
+/// > results in a length of 2
 ///
 /// ```not_rust
 /// old: [0, 1, 9, 0, 2, 5]
 /// new:          [0, 4]
 /// ```
-/// > results in an length of 1
+/// > results in a length of 1
 pub fn find_longest_prefix_contained_within<T>(old: &[Token<T>], new: &[Token<T>]) -> usize
 where
     T: PartialEq + Clone + std::fmt::Debug,

src/utils/string_builder.rs

@@ -1,83 +1,67 @@
-use core::ops::Range;
+use std::iter::Iterator;
 
-/// A helper for building a string in order based on an original string and a
-/// series of insertions and deletions applied to it. It is safe to use with
-/// UTF-8 strings as all operations are based on character indices.
-#[derive(Debug, Clone)]
+/// A helper for building a string in-order based on an original string and a
+/// series of insertions, deletions, and copies applied to it. It is safe to use
+/// with UTF-8 strings as all operations are based on character indices. The
+/// methods must be called in-order.
 pub struct StringBuilder<'a> {
-    original: &'a str,
-    last_old_char_index: usize,
+    original: Box<dyn Iterator<Item = char> + 'a>,
     buffer: String,
+
+    #[cfg(debug_assertions)]
+    remaining: String,
 }
 
 impl StringBuilder<'_> {
     pub fn new(original: &str) -> StringBuilder<'_> {
         StringBuilder {
-            original,
-            last_old_char_index: 0,
+            original: Box::new(original.chars()),
             buffer: String::with_capacity(original.len()),
+
+            #[cfg(debug_assertions)]
+            remaining: original.to_owned(),
         }
     }
 
-    /// Insert a string at the given index after copying the original string up
-    /// to that index from the last insertion or deletion.
-    pub fn insert(&mut self, from: usize, text: &str) {
-        self.copy_until(from);
-        self.buffer.push_str(text);
+    /// Insert a string at the end of the built buffer.
+    pub fn insert(&mut self, text: &str) { self.buffer.push_str(text); }
+
+    /// Skip copying `length` characters from the original string to the built
+    /// buffer.
+    pub fn delete(&mut self, length: usize) {
+        if length == 0 {
+            return;
+        }
+
+        self.original.nth(length - 1);
+
+        if cfg!(debug_assertions) {
+            self.remaining = self.remaining.chars().skip(length).collect();
+        }
     }
 
-    /// Delete a string at the given index after copying the original string up
-    /// to that index from the last insertion or deletion.
-    pub fn delete(&mut self, range: core::ops::Range<usize>) {
-        self.copy_until(range.start);
-        self.last_old_char_index += range.len();
-    }
+    /// Copy `length` characters from the original string to the built buffer.
+    pub fn retain(&mut self, length: usize) {
+        self.buffer.extend(self.original.by_ref().take(length));
 
-    fn copy_until(&mut self, index: usize) {
-        let current_char_count = self.buffer.chars().count();
-        debug_assert!(
-            index >= current_char_count,
-            "String builder only support building in order"
-        );
-
-        let jump = index - current_char_count;
-
-        self.buffer.push_str(
-            &self
-                .original
-                .chars()
-                .skip(self.last_old_char_index)
-                .take(jump)
-                .collect::<String>(),
-        );
-        self.last_old_char_index += jump;
+        if cfg!(debug_assertions) {
+            self.remaining = self.remaining.chars().skip(length).collect();
+        }
     }
 
     /// Finish building the string after copying the remaining original string
     /// since the last insertion or deletion.
-    pub fn build(mut self) -> String {
-        self.buffer.push_str(
-            &self
-                .original
-                .chars()
-                .skip(self.last_old_char_index)
-                .collect::<String>(),
-        );
+    pub fn build(self) -> String { self.buffer }
 
-        self.buffer
-    }
+    #[cfg(debug_assertions)]
+    /// Get a slice of the remaining original string. The slice starts from
+    /// where the next delete/retain operation would start and is of length
+    /// `length`. The implementation is quite suboptimal but it's only used
+    /// for debugging.
+    pub fn get_slice_from_remaining(&self, length: usize) -> String {
+        let result = self.remaining.chars().take(length).collect::<String>();
 
-    #[allow(dead_code)]
-    pub fn get_slice(&self, range: Range<usize>) -> String {
-        let result = self
-            .buffer
-            .chars()
-            .chain(self.original.chars().skip(self.last_old_char_index))
-            .skip(range.start)
-            .take(range.end - range.start)
-            .collect::<String>();
-
-        debug_assert_eq!(result.chars().count(), range.len(), "Range out of bounds",);
+        debug_assert_eq!(result.chars().count(), length, "Range out of bounds");
 
         result
     }
@@ -85,6 +69,8 @@ impl StringBuilder<'_> {
 
 #[cfg(test)]
 mod tests {
+    use pretty_assertions::assert_eq;
+
     use super::*;
 
     #[test]
@@ -92,20 +78,74 @@ mod tests {
         let original = "aaa bbb ccc";
         let mut builder = StringBuilder::new(original);
 
-        builder.insert(0, "ddd ");
-        builder.delete(4..8);
-        builder.insert(11, " eee");
+        builder.insert("ddd");
+        builder.delete(3);
+        builder.retain(8);
+        builder.insert(" eee");
 
         assert_eq!(builder.build(), "ddd bbb ccc eee");
-    }
 
-    #[test]
-    fn test_string_builder2() {
         let original = "abcde";
         let mut builder = StringBuilder::new(original);
 
-        builder.delete(1..4);
+        builder.retain(1);
+        builder.delete(3);
+        builder.retain(1);
 
         assert_eq!(builder.build(), "ae");
     }
+
+    #[test]
+    fn test_empty_original() {
+        let original = "";
+        let mut builder = StringBuilder::new(original);
+
+        builder.insert("test");
+        assert_eq!(builder.build(), "test");
+    }
+
+    #[test]
+    fn test_unicode_characters() {
+        let original = "こんにちは";
+        let mut builder = StringBuilder::new(original);
+
+        builder.retain(3);
+        builder.insert("世界, "); // Insert "World, "
+        builder.retain(2);
+
+        assert_eq!(builder.build(), "こんに世界, ちは");
+    }
+
+    #[test]
+    fn test_get_slice() {
+        let original = "abcdef";
+        let builder = StringBuilder::new(original);
+
+        // Test getting a slice of the original string
+        assert_eq!(builder.get_slice_from_remaining(3), "abc");
+
+        // Test getting a slice that includes both buffer and remaining original
+        let mut builder = StringBuilder::new(original);
+        builder.retain(2); // "ab" in buffer
+        assert_eq!(builder.get_slice_from_remaining(2), "cd");
+    }
+
+    #[test]
+    fn test_retain_all() {
+        let original = "Hello, world!";
+        let mut builder = StringBuilder::new(original);
+
+        builder.retain(original.len());
+        assert_eq!(builder.build(), original);
+    }
+
+    #[test]
+    fn test_delete_all() {
+        let original = "Hello";
+        let mut builder = StringBuilder::new(original);
+
+        builder.delete(original.len());
+        builder.insert("Hi");
+        assert_eq!(builder.build(), "Hi");
+    }
 }

tests/examples/deletes.yml

@@ -24,6 +24,12 @@ left: long small
 right: long with big and small
 expected: long small
 
+---
+parent: long run of text where one barely has no changes but has cursors
+left: long| run of tex|t where one barely has no |changes but has |cursors
+right: long run one barely has no changes cursors
+expected: long| ru|n one barely has no |changes |cursors
+
 ---
 parent: long text where the cursor has to be clamped after delete
 left: long text where the cursor has to be clamped after delete|

tests/examples/replacing.yml

@@ -2,7 +2,7 @@
 parent: original_1 original_2 original_3
 left: original_1 edit_1| original_3
 right: original_1 original_2| edit_2
-expected: original_1 edit_1|| edit_2
+expected: original_1| edit_1| edit_2
 
 ---
 # Both replace the same token with the same value

tests/examples/various.yml

@@ -43,7 +43,7 @@ expected: Send the |detailed |quarterly |detailed report to the |entire |team
 parent: Ready, Set go
 left: Ready! Set go|
 right: Ready, Set, go!|
-expected: Ready! Set, go!||
+expected: Ready!| Set, go!|
 
 ---
 parent: "Total: $100"
@@ -67,7 +67,7 @@ expected: market| placemarket|space
 parent: A B C D
 left: A X B D|
 right: A B Y|
-expected: A X B |Y|
+expected: A X B| Y|
 
 ---
 parent: Please submit your assignment by Friday