use serde::{Deserialize, Serialize};
use garage_util::data::now_msec;
use crate::crdt::crdt::*;
/// Last Write Win Map
///
/// This types defines a CRDT for a map from keys to values.
/// The values have an associated timestamp, such that the last written value
/// takes precedence over previous ones. As for the simpler `LWW` type, the value
/// type `V` is also required to implement the CRDT trait.
/// We do not encourage mutating the values associated with a given key
/// without updating the timestamp, in fact at the moment we do not provide a `.get_mut()`
/// method that would allow that.
///
/// Internally, the map is stored as a vector of keys and values, sorted by ascending key order.
/// This is why the key type `K` must implement `Ord` (and also to ensure a unique serialization,
/// such that two values can be compared for equality based on their hashes). As a consequence,
/// insertions take `O(n)` time. This means that LWWMap should be used for reasonably small maps.
/// However, note that even if we were using a more efficient data structure such as a `BTreeMap`,
/// the serialization cost `O(n)` would still have to be paid at each modification, so we are
/// actually not losing anything here.
#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
pub struct LWWMap<K, V> {
vals: Vec<(K, u64, V)>,
}
impl<K, V> LWWMap<K, V>
where
K: Ord,
V: CRDT,
{
/// Create a new empty map CRDT
pub fn new() -> Self {
Self { vals: vec![] }
}
/// Used to migrate from a map defined in an incompatible format. This produces
/// a map that contains a single item with the specified timestamp (copied from
/// the incompatible format). Do this as many times as you have items to migrate,
/// and put them all together using the CRDT merge operator.
pub fn migrate_from_raw_item(k: K, ts: u64, v: V) -> Self {
Self {
vals: vec![(k, ts, v)],
}
}
/// Returns a map that contains a single mapping from the specified key to the specified value.
/// This map is a mutator, or a delta-CRDT, such that when it is merged with the original map,
/// the previous value will be replaced with the one specified here.
/// The timestamp in the provided mutator is set to the maximum of the current system's clock
/// and 1 + the previous value's timestamp (if there is one), so that the new value will always
/// take precedence (LWW rule).
///
/// Typically, to update the value associated to a key in the map, you would do the following:
///
/// ```ignore
/// let my_update = my_crdt.update_mutator(key_to_modify, new_value);
/// my_crdt.merge(&my_update);
/// ```
///
/// However extracting the mutator on its own and only sending that on the network is very
/// interesting as it is much smaller than the whole map.
pub fn update_mutator(&self, k: K, new_v: V) -> Self {
let new_vals = match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
Ok(i) => {
let (_, old_ts, _) = self.vals[i];
let new_ts = std::cmp::max(old_ts + 1, now_msec());
vec![(k, new_ts, new_v)]
}
Err(_) => vec![(k, now_msec(), new_v)],
};
Self { vals: new_vals }
}
/// Takes all of the values of the map and returns them. The current map is reset to the
/// empty map. This is very usefull to produce in-place a new map that contains only a delta
/// that modifies a certain value:
///
/// ```ignore
/// let mut a = get_my_crdt_value();
/// let old_a = a.take_and_clear();
/// a.merge(&old_a.update_mutator(key_to_modify, new_value));
/// put_my_crdt_value(a);
/// ```
///
/// Of course in this simple example we could have written simply
/// `pyt_my_crdt_value(a.update_mutator(key_to_modify, new_value))`,
/// but in the case where the map is a field in a struct for instance (as is always the case),
/// this becomes very handy:
///
/// ```ignore
/// let mut a = get_my_crdt_value();
/// let old_a_map = a.map_field.take_and_clear();
/// a.map_field.merge(&old_a_map.update_mutator(key_to_modify, new_value));
/// put_my_crdt_value(a);
/// ```
pub fn take_and_clear(&mut self) -> Self {
let vals = std::mem::replace(&mut self.vals, vec![]);
Self { vals }
}
/// Removes all values from the map
pub fn clear(&mut self) {
self.vals.clear();
}
/// Get a reference to the value assigned to a key
pub fn get(&self, k: &K) -> Option<&V> {
match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
Ok(i) => Some(&self.vals[i].2),
Err(_) => None,
}
}
/// Gets a reference to all of the items, as a slice. Usefull to iterate on all map values.
/// In most case you will want to ignore the timestamp (second item of the tuple).
pub fn items(&self) -> &[(K, u64, V)] {
&self.vals[..]
}
/// Returns the number of items in the map
pub fn len(&self) -> usize {
self.vals.len()
}
}
impl<K, V> CRDT for LWWMap<K, V>
where
K: Clone + Ord,
V: Clone + CRDT,
{
fn merge(&mut self, other: &Self) {
for (k, ts2, v2) in other.vals.iter() {
match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
Ok(i) => {
let (_, ts1, _v1) = &self.vals[i];
if ts2 > ts1 {
self.vals[i].1 = *ts2;
self.vals[i].2 = v2.clone();
} else if ts1 == ts2 {
self.vals[i].2.merge(&v2);
}
}
Err(i) => {
self.vals.insert(i, (k.clone(), *ts2, v2.clone()));
}
}
}
}
}
