632 lines
18 KiB
Rust
632 lines
18 KiB
Rust
//! RGA (Replicated Growable Array) CRDT implementation
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//!
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//! This module provides a conflict-free replicated sequence that maintains
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//! consistent ordering across concurrent insert and delete operations.
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//!
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//! ## RGA Semantics
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//!
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//! - **Causal ordering**: Elements inserted after position P stay after P
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//! - **Concurrent inserts**: Resolved by timestamp + node ID tiebreaker
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//! - **Tombstones**: Deleted elements remain in structure to preserve positions
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//! - **Unique operation IDs**: Each insert gets a UUID for referencing
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//!
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//! ## Example
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//!
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//! ```
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//! use lib::networking::Rga;
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//! use uuid::Uuid;
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//!
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//! let node1 = Uuid::new_v4();
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//! let node2 = Uuid::new_v4();
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//!
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//! // Node 1 creates sequence: [A, B]
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//! let mut seq1: Rga<char> = Rga::new();
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//! let (id_a, _) = seq1.insert_at_beginning('A', node1);
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//! let (id_b, _) = seq1.insert_after(Some(id_a), 'B', node1);
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//!
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//! // Node 2 concurrently inserts C after A
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//! let mut seq2 = seq1.clone();
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//! seq2.insert_after(Some(id_a), 'C', node2);
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//!
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//! // Node 1 inserts D after A
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//! seq1.insert_after(Some(id_a), 'D', node1);
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//!
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//! // Merge - concurrent inserts after A are ordered by timestamp + node ID
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//! seq1.merge(&seq2);
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//!
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//! let values: Vec<char> = seq1.values().copied().collect();
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//! assert_eq!(values.len(), 4); // A, (C or D), (D or C), B
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//! ```
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use std::collections::HashMap;
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use bevy::prelude::*;
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use serde::{
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Deserialize,
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Serialize,
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};
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use crate::networking::vector_clock::{
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NodeId,
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VectorClock,
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};
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/// An element in an RGA sequence
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///
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/// Each element has a unique ID and tracks its logical position in the sequence
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/// via the "after" pointer.
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#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
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pub struct RgaElement<T> {
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/// Unique ID for this element
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pub id: uuid::Uuid,
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/// The actual value
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pub value: T,
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/// ID of the element this was inserted after (None = beginning)
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pub after_id: Option<uuid::Uuid>,
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/// Node that performed the insert
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pub inserting_node: NodeId,
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/// Vector clock when inserted (for ordering concurrent inserts)
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pub vector_clock: VectorClock,
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/// Whether this element has been deleted (tombstone)
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pub is_deleted: bool,
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}
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/// RGA (Replicated Growable Array) CRDT
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///
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/// A replicated sequence supporting concurrent insert/delete with consistent
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/// ordering based on causal relationships.
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///
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/// # Type Parameters
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///
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/// - `T`: The element type (must be Clone, Serialize, Deserialize)
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///
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/// # Internal Structure
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///
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/// Elements are stored in a HashMap by ID. Each element tracks which element
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/// it was inserted after, forming a linked list structure. Deleted elements
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/// remain as tombstones to preserve positions for concurrent operations.
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#[derive(Debug, Clone, Serialize, Deserialize)]
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pub struct Rga<T> {
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/// Map from element ID to element
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elements: HashMap<uuid::Uuid, RgaElement<T>>,
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}
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impl<T> Rga<T>
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where
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T: Clone + Serialize + for<'de> Deserialize<'de>,
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{
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/// Create a new empty RGA sequence
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pub fn new() -> Self {
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Self {
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elements: HashMap::new(),
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}
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}
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/// Insert an element at the beginning of the sequence
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///
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/// Returns (element_id, position) where position is the index in the
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/// visible sequence.
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///
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/// # Example
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///
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/// ```
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/// use lib::networking::Rga;
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/// use uuid::Uuid;
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///
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/// let node = Uuid::new_v4();
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/// let mut seq: Rga<char> = Rga::new();
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///
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/// let (id, pos) = seq.insert_at_beginning('A', node);
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/// assert_eq!(pos, 0);
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/// ```
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pub fn insert_at_beginning(&mut self, value: T, node_id: NodeId) -> (uuid::Uuid, usize) {
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let id = uuid::Uuid::new_v4();
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let mut clock = VectorClock::new();
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clock.increment(node_id);
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let element = RgaElement {
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id,
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value,
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after_id: None,
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inserting_node: node_id,
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vector_clock: clock,
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is_deleted: false,
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};
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self.elements.insert(id, element);
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(id, 0)
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}
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/// Insert an element after a specific element ID
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///
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/// If after_id is None, inserts at the beginning.
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///
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/// Returns (element_id, position) where position is the index in the
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/// visible sequence.
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///
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/// # Example
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///
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/// ```
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/// use lib::networking::Rga;
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/// use uuid::Uuid;
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///
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/// let node = Uuid::new_v4();
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/// let mut seq: Rga<char> = Rga::new();
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///
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/// let (id_a, _) = seq.insert_at_beginning('A', node);
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/// let (id_b, pos) = seq.insert_after(Some(id_a), 'B', node);
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/// assert_eq!(pos, 1);
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///
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/// let values: Vec<char> = seq.values().copied().collect();
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/// assert_eq!(values, vec!['A', 'B']);
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/// ```
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pub fn insert_after(
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&mut self,
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after_id: Option<uuid::Uuid>,
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value: T,
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node_id: NodeId,
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) -> (uuid::Uuid, usize) {
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let id = uuid::Uuid::new_v4();
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let mut clock = VectorClock::new();
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clock.increment(node_id);
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let element = RgaElement {
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id,
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value,
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after_id,
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inserting_node: node_id,
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vector_clock: clock,
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is_deleted: false,
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};
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self.elements.insert(id, element);
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// Calculate position
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let position = self.calculate_position(id);
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(id, position)
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}
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/// Insert an element with explicit vector clock
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///
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/// This is used when applying remote operations that already have
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/// a vector clock.
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pub fn insert_with_clock(
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&mut self,
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id: uuid::Uuid,
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after_id: Option<uuid::Uuid>,
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value: T,
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node_id: NodeId,
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vector_clock: VectorClock,
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) -> usize {
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let element = RgaElement {
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id,
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value,
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after_id,
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inserting_node: node_id,
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vector_clock,
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is_deleted: false,
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};
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self.elements.insert(id, element);
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self.calculate_position(id)
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}
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/// Delete an element by ID
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///
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/// The element becomes a tombstone - it remains in the structure but
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/// is hidden from the visible sequence.
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///
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/// # Example
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///
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/// ```
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/// use lib::networking::Rga;
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/// use uuid::Uuid;
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///
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/// let node = Uuid::new_v4();
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/// let mut seq: Rga<char> = Rga::new();
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///
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/// let (id, _) = seq.insert_at_beginning('A', node);
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/// assert_eq!(seq.len(), 1);
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///
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/// seq.delete(id);
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/// assert_eq!(seq.len(), 0);
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/// assert!(seq.is_deleted(id));
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/// ```
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pub fn delete(&mut self, element_id: uuid::Uuid) {
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if let Some(element) = self.elements.get_mut(&element_id) {
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element.is_deleted = true;
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}
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}
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/// Check if an element is deleted
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pub fn is_deleted(&self, element_id: uuid::Uuid) -> bool {
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self.elements
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.get(&element_id)
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.map(|e| e.is_deleted)
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.unwrap_or(false)
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}
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/// Get the visible length of the sequence (excluding tombstones)
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pub fn len(&self) -> usize {
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self.elements.values().filter(|e| !e.is_deleted).count()
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}
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/// Check if the sequence is empty (no visible elements)
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pub fn is_empty(&self) -> bool {
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self.len() == 0
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}
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/// Get all visible values in order
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///
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/// Returns an iterator over the values in their proper sequence order.
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pub fn values(&self) -> impl Iterator<Item = &T> {
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let ordered = self.get_ordered_elements();
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ordered.into_iter().filter_map(move |id| {
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self.elements
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.get(&id)
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.and_then(|e| if !e.is_deleted { Some(&e.value) } else { None })
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})
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}
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/// Get all visible elements with their IDs in order
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pub fn elements_with_ids(&self) -> Vec<(uuid::Uuid, &T)> {
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let ordered = self.get_ordered_elements();
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ordered
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.into_iter()
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.filter_map(|id| {
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self.elements.get(&id).and_then(|e| {
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if !e.is_deleted {
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Some((id, &e.value))
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} else {
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None
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}
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})
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})
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.collect()
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}
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/// Merge another RGA into this one
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///
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/// Implements CRDT merge by combining all elements from both sequences
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/// and resolving positions based on causal ordering.
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///
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/// # Example
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///
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/// ```
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/// use lib::networking::Rga;
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/// use uuid::Uuid;
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///
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/// let node1 = Uuid::new_v4();
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/// let node2 = Uuid::new_v4();
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///
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/// let mut seq1: Rga<char> = Rga::new();
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/// seq1.insert_at_beginning('A', node1);
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///
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/// let mut seq2: Rga<char> = Rga::new();
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/// seq2.insert_at_beginning('B', node2);
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///
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/// seq1.merge(&seq2);
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/// assert_eq!(seq1.len(), 2);
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/// ```
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pub fn merge(&mut self, other: &Rga<T>) {
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for (id, element) in &other.elements {
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// Insert or update element
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self.elements
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.entry(*id)
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.and_modify(|existing| {
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// If other's element is deleted, mark ours as deleted too
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if element.is_deleted {
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existing.is_deleted = true;
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}
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})
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.or_insert_with(|| element.clone());
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}
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}
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/// Clear the sequence
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///
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/// Removes all elements and tombstones.
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pub fn clear(&mut self) {
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self.elements.clear();
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}
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/// Garbage collect tombstones
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///
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/// Removes deleted elements that have no children (nothing inserted after
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/// them). This is safe because if no element references a tombstone as
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/// its parent, it can be removed without affecting the sequence.
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pub fn garbage_collect(&mut self) {
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// Find all IDs that are referenced as after_id
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let mut referenced_ids = std::collections::HashSet::new();
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for element in self.elements.values() {
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if let Some(after_id) = element.after_id {
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referenced_ids.insert(after_id);
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}
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}
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// Remove deleted elements that aren't referenced
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self.elements
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.retain(|id, element| !element.is_deleted || referenced_ids.contains(id));
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}
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/// Get ordered list of element IDs
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///
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/// This builds the proper sequence order by following the after_id pointers
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/// and resolving concurrent inserts using vector clocks + node IDs.
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fn get_ordered_elements(&self) -> Vec<uuid::Uuid> {
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// Build a map of after_id -> list of elements inserted after it
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let mut children: HashMap<Option<uuid::Uuid>, Vec<uuid::Uuid>> = HashMap::new();
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for (id, element) in &self.elements {
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children
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.entry(element.after_id)
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.or_insert_with(Vec::new)
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.push(*id);
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}
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// Sort children by vector clock, then node ID (for deterministic ordering)
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for child_list in children.values_mut() {
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child_list.sort_by(|a, b| {
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let elem_a = &self.elements[a];
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let elem_b = &self.elements[b];
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// Compare vector clocks
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match elem_a.vector_clock.compare(&elem_b.vector_clock) {
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| Ok(std::cmp::Ordering::Less) => std::cmp::Ordering::Less,
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| Ok(std::cmp::Ordering::Greater) => std::cmp::Ordering::Greater,
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| Ok(std::cmp::Ordering::Equal) | Err(_) => {
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// If clocks are equal or concurrent, use node ID as tiebreaker
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elem_a.inserting_node.cmp(&elem_b.inserting_node)
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},
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}
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});
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}
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// Build ordered list by traversing from None (beginning)
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let mut result = Vec::new();
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let mut to_visit = vec![None];
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while let Some(current_id) = to_visit.pop() {
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if let Some(child_ids) = children.get(¤t_id) {
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// Visit children in reverse order (since we're using a stack)
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for child_id in child_ids.iter().rev() {
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result.push(*child_id);
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to_visit.push(Some(*child_id));
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}
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}
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}
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result
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}
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/// Calculate the visible position of an element
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fn calculate_position(&self, element_id: uuid::Uuid) -> usize {
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let ordered = self.get_ordered_elements();
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ordered.iter().position(|id| id == &element_id).unwrap_or(0)
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}
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}
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impl<T> Default for Rga<T>
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where
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T: Clone + Serialize + for<'de> Deserialize<'de>,
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{
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fn default() -> Self {
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Self::new()
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn test_rga_new() {
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let seq: Rga<char> = Rga::new();
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assert!(seq.is_empty());
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assert_eq!(seq.len(), 0);
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}
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#[test]
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fn test_rga_insert_at_beginning() {
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let node = uuid::Uuid::new_v4();
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let mut seq: Rga<char> = Rga::new();
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let (_, pos) = seq.insert_at_beginning('A', node);
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assert_eq!(pos, 0);
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assert_eq!(seq.len(), 1);
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let values: Vec<char> = seq.values().copied().collect();
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assert_eq!(values, vec!['A']);
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}
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#[test]
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fn test_rga_insert_after() {
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let node = uuid::Uuid::new_v4();
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let mut seq: Rga<char> = Rga::new();
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let (id_a, _) = seq.insert_at_beginning('A', node);
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let (_, pos_b) = seq.insert_after(Some(id_a), 'B', node);
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assert_eq!(pos_b, 1);
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let values: Vec<char> = seq.values().copied().collect();
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assert_eq!(values, vec!['A', 'B']);
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}
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#[test]
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fn test_rga_delete() {
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let node = uuid::Uuid::new_v4();
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let mut seq: Rga<char> = Rga::new();
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let (id_a, _) = seq.insert_at_beginning('A', node);
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let (id_b, _) = seq.insert_after(Some(id_a), 'B', node);
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assert_eq!(seq.len(), 2);
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seq.delete(id_a);
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assert_eq!(seq.len(), 1);
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assert!(seq.is_deleted(id_a));
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let values: Vec<char> = seq.values().copied().collect();
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assert_eq!(values, vec!['B']);
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}
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#[test]
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fn test_rga_insert_delete_insert() {
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let node = uuid::Uuid::new_v4();
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let mut seq: Rga<char> = Rga::new();
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let (id_a, _) = seq.insert_at_beginning('A', node);
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seq.delete(id_a);
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assert_eq!(seq.len(), 0);
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seq.insert_at_beginning('B', node);
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assert_eq!(seq.len(), 1);
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let values: Vec<char> = seq.values().copied().collect();
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assert_eq!(values, vec!['B']);
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}
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#[test]
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fn test_rga_merge_simple() {
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let node1 = uuid::Uuid::new_v4();
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let node2 = uuid::Uuid::new_v4();
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let mut seq1: Rga<char> = Rga::new();
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seq1.insert_at_beginning('A', node1);
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let mut seq2: Rga<char> = Rga::new();
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seq2.insert_at_beginning('B', node2);
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seq1.merge(&seq2);
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assert_eq!(seq1.len(), 2);
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}
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#[test]
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fn test_rga_merge_preserves_order() {
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let node = uuid::Uuid::new_v4();
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let mut seq1: Rga<char> = Rga::new();
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let (id_a, _) = seq1.insert_at_beginning('A', node);
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let (id_b, _) = seq1.insert_after(Some(id_a), 'B', node);
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seq1.insert_after(Some(id_b), 'C', node);
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let seq2 = seq1.clone();
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seq1.merge(&seq2);
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let values: Vec<char> = seq1.values().copied().collect();
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assert_eq!(values, vec!['A', 'B', 'C']);
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}
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#[test]
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fn test_rga_merge_deletion() {
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let node = uuid::Uuid::new_v4();
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let mut seq1: Rga<char> = Rga::new();
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let (id_a, _) = seq1.insert_at_beginning('A', node);
|
|
seq1.insert_after(Some(id_a), 'B', node);
|
|
|
|
let mut seq2 = seq1.clone();
|
|
seq2.delete(id_a);
|
|
|
|
seq1.merge(&seq2);
|
|
|
|
let values: Vec<char> = seq1.values().copied().collect();
|
|
assert_eq!(values, vec!['B']);
|
|
}
|
|
|
|
#[test]
|
|
fn test_rga_concurrent_inserts() {
|
|
let node1 = uuid::Uuid::new_v4();
|
|
let node2 = uuid::Uuid::new_v4();
|
|
|
|
// Both start with [A]
|
|
let mut seq1: Rga<char> = Rga::new();
|
|
let (id_a, _) = seq1.insert_at_beginning('A', node1);
|
|
|
|
let mut seq2 = seq1.clone();
|
|
|
|
// seq1 inserts B after A
|
|
seq1.insert_after(Some(id_a), 'B', node1);
|
|
|
|
// seq2 inserts C after A (concurrent)
|
|
seq2.insert_after(Some(id_a), 'C', node2);
|
|
|
|
// Merge
|
|
seq1.merge(&seq2);
|
|
|
|
// Should have A followed by B and C in some deterministic order
|
|
assert_eq!(seq1.len(), 3);
|
|
|
|
let values: Vec<char> = seq1.values().copied().collect();
|
|
assert_eq!(values[0], 'A');
|
|
assert!(values.contains(&'B'));
|
|
assert!(values.contains(&'C'));
|
|
}
|
|
|
|
#[test]
|
|
fn test_rga_clear() {
|
|
let node = uuid::Uuid::new_v4();
|
|
let mut seq: Rga<char> = Rga::new();
|
|
|
|
seq.insert_at_beginning('A', node);
|
|
seq.insert_at_beginning('B', node);
|
|
assert_eq!(seq.len(), 2);
|
|
|
|
seq.clear();
|
|
assert!(seq.is_empty());
|
|
}
|
|
|
|
#[test]
|
|
fn test_rga_garbage_collect() {
|
|
let node = uuid::Uuid::new_v4();
|
|
let mut seq: Rga<char> = Rga::new();
|
|
|
|
let (id_a, _) = seq.insert_at_beginning('A', node);
|
|
let (id_b, _) = seq.insert_after(Some(id_a), 'B', node);
|
|
let (_, _) = seq.insert_after(Some(id_b), 'C', node);
|
|
|
|
// Delete A (has child B, so should be kept)
|
|
seq.delete(id_a);
|
|
|
|
// Delete B (has child C, so should be kept)
|
|
seq.delete(id_b);
|
|
|
|
assert_eq!(seq.elements.len(), 3);
|
|
|
|
seq.garbage_collect();
|
|
|
|
// A and B should still be there (referenced by children)
|
|
// Only C is visible
|
|
assert_eq!(seq.len(), 1);
|
|
assert!(seq.elements.contains_key(&id_a));
|
|
assert!(seq.elements.contains_key(&id_b));
|
|
}
|
|
|
|
#[test]
|
|
fn test_rga_serialization() -> bincode::Result<()> {
|
|
let node = uuid::Uuid::new_v4();
|
|
let mut seq: Rga<String> = Rga::new();
|
|
|
|
let (id_a, _) = seq.insert_at_beginning("foo".to_string(), node);
|
|
seq.insert_after(Some(id_a), "bar".to_string(), node);
|
|
|
|
let bytes = bincode::serialize(&seq)?;
|
|
let deserialized: Rga<String> = bincode::deserialize(&bytes)?;
|
|
|
|
assert_eq!(deserialized.len(), 2);
|
|
let values: Vec<String> = deserialized.values().cloned().collect();
|
|
assert_eq!(values, vec!["foo".to_string(), "bar".to_string()]);
|
|
|
|
Ok(())
|
|
}
|
|
}
|