Create new store and add storage API

libraries/persistent_store/src/lib.rs

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+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+mod storage;
+
+pub use self::storage::{Storage, StorageError, StorageIndex, StorageResult};

libraries/persistent_store/src/storage.rs

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+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/// Represents a byte position in a storage
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct StorageIndex {
+    pub page: usize,
+    pub byte: usize,
+}
+
+/// Represents a possible storage error
+#[derive(Debug, PartialEq, Eq)]
+pub enum StorageError {
+    /// Arguments are not correctly aligned
+    NotAligned,
+
+    /// Arguments are out of bounds
+    OutOfBounds,
+
+    /// Implementation-specific error
+    CustomError,
+}
+
+pub type StorageResult<T> = Result<T, StorageError>;
+
+/// Abstracts a flash storage
+pub trait Storage {
+    /// The size of a word in bytes
+    fn word_size(&self) -> usize;
+
+    /// The size of a page in bytes
+    fn page_size(&self) -> usize;
+
+    /// The number of pages in the storage
+    fn num_pages(&self) -> usize;
+
+    /// Maximum number of times a word can be written between page erasures
+    fn max_word_writes(&self) -> usize;
+
+    /// Maximum number of times a page can be erased
+    fn max_page_erases(&self) -> usize;
+
+    /// Reads a byte slice from the storage
+    ///
+    /// The `index` must designate `length` bytes in the storage.
+    fn read_slice(&self, index: StorageIndex, length: usize) -> StorageResult<&[u8]>;
+
+    /// Writes a word slice to the storage
+    ///
+    /// The following pre-conditions must hold:
+    /// - The `index` must designate `value.len()` bytes in the storage.
+    /// - Both `index` and `value.len()` must be word-aligned.
+    /// - The written words should not have been written too many times since last page erasure.
+    fn write_slice(&mut self, index: StorageIndex, value: &[u8]) -> StorageResult<()>;
+
+    /// Erases a page of the storage
+    ///
+    /// The `page` must be in the storage.
+    fn erase_page(&mut self, page: usize) -> StorageResult<()>;
+}
+
+impl StorageIndex {
+    /// Whether a slice fits in a storage page
+    fn is_valid(self, length: usize, storage: &impl Storage) -> bool {
+        let page_size = storage.page_size();
+        self.page < storage.num_pages() && length <= page_size && self.byte <= page_size - length
+    }
+
+    /// Returns the range of a valid slice
+    ///
+    /// The range starts at `self` with `length` bytes.
+    pub fn range(
+        self,
+        length: usize,
+        storage: &impl Storage,
+    ) -> StorageResult<core::ops::Range<usize>> {
+        if self.is_valid(length, storage) {
+            let start = self.page * storage.page_size() + self.byte;
+            Ok(start..start + length)
+        } else {
+            Err(StorageError::OutOfBounds)
+        }
+    }
+}