Add model for new store

libraries/persistent_store/src/store.rs

@@ -13,7 +13,9 @@
 // limitations under the License.
 
 use crate::StorageError;
+use alloc::vec::Vec;
 
+/// Errors returned by store operations.
 #[derive(Debug, PartialEq, Eq)]
 pub enum StoreError {
     /// Invalid argument.
@@ -60,4 +62,55 @@ impl From<StorageError> for StoreError {
     }
 }
 
+/// Result of store operations.
 pub type StoreResult<T> = Result<T, StoreError>;
+
+/// Progression ratio for store metrics.
+///
+/// This is used for the [capacity] and [lifetime] metrics. Those metrics are measured in words.
+///
+/// [capacity]: struct.Store.html#method.capacity
+/// [lifetime]: struct.Store.html#method.lifetime
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub struct StoreRatio {
+    /// How much of the metric is used.
+    pub used: usize,
+
+    /// How much of the metric can be used at most.
+    pub total: usize,
+}
+
+impl StoreRatio {
+    /// How much of the metric is remaining.
+    pub fn remaining(self) -> usize {
+        self.total - self.used
+    }
+}
+
+/// Represents an update to the store as part of a transaction.
+#[derive(Clone, Debug)]
+pub enum StoreUpdate {
+    /// Inserts or replaces an entry in the store.
+    Insert { key: usize, value: Vec<u8> },
+
+    /// Removes an entry from the store.
+    Remove { key: usize },
+}
+
+impl StoreUpdate {
+    /// Returns the key affected by the update.
+    pub fn key(&self) -> usize {
+        match *self {
+            StoreUpdate::Insert { key, .. } => key,
+            StoreUpdate::Remove { key } => key,
+        }
+    }
+
+    /// Returns the value written by the update.
+    pub fn value(&self) -> Option<&[u8]> {
+        match self {
+            StoreUpdate::Insert { value, .. } => Some(value),
+            StoreUpdate::Remove { .. } => None,
+        }
+    }
+}