Merge pull request #192 from ia0/v2_lib

Clarify different integer types

libraries/persistent_store/src/format.rs

@@ -15,68 +15,104 @@
 // TODO(ia0): Remove when the module is used.
 #![allow(dead_code)]
 
-use crate::bitfield::*;
+#[macro_use]
-use crate::{Storage, StorageIndex, StoreError, StoreResult};
+mod bitfield;
+
+use self::bitfield::*;
+use crate::{usize_to_nat, Nat, Storage, StorageIndex, StoreError, StoreResult};
 use alloc::vec::Vec;
 use core::cmp::min;
+use core::convert::TryFrom;
 
+/// Internal representation of a word in flash.
+///
+/// Currently, the store only supports storages where a word is 32 bits.
 type WORD = u32;
 
+/// Abstract representation of a word in flash.
+///
+/// This type is kept abstract to avoid possible confusion with `Nat` if they happen to have the
+/// same representation. This is because they have different semantics, `Nat` represents natural
+/// numbers while `Word` represents sequences of bits (and thus has no arithmetic).
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Word(WORD);
+
+/// Byte slice representation of a word in flash.
+///
+/// The slice is in little-endian representation.
+pub type WordSlice = [u8; core::mem::size_of::<WORD>()];
+
+impl Word {
+    /// Converts a byte slice into a word.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `slice.len() != WORD_SIZE`.
+    pub fn from_slice(slice: &[u8]) -> Word {
+        Word(WORD::from_le_bytes(<WordSlice>::try_from(slice).unwrap()))
+    }
+
+    /// Converts a word into a byte slice.
+    pub fn as_slice(self) -> WordSlice {
+        self.0.to_le_bytes()
+    }
+}
+
 /// Size of a word in bytes.
 ///
 /// Currently, the store only supports storages where a word is 4 bytes.
-const WORD_SIZE: usize = core::mem::size_of::<WORD>();
+const WORD_SIZE: Nat = core::mem::size_of::<WORD>() as Nat;
 
 /// Minimum number of words per page.
 ///
 /// Currently, the store only supports storages where pages have at least 8 words.
-const MIN_NUM_WORDS_PER_PAGE: usize = 8;
+const MIN_NUM_WORDS_PER_PAGE: Nat = 8;
 
 /// Maximum size of a page in bytes.
 ///
 /// Currently, the store only supports storages where pages are between 8 and 1024 [words].
 ///
 /// [words]: constant.WORD_SIZE.html
-const MAX_PAGE_SIZE: usize = 4096;
+const MAX_PAGE_SIZE: Nat = 4096;
 
 /// Maximum number of erase cycles.
 ///
 /// Currently, the store only supports storages where the maximum number of erase cycles fits on 16
 /// bits.
-const MAX_ERASE_CYCLE: usize = 65535;
+const MAX_ERASE_CYCLE: Nat = 65535;
 
 /// Minimum number of pages.
 ///
 /// Currently, the store only supports storages with at least 3 pages.
-const MIN_NUM_PAGES: usize = 3;
+const MIN_NUM_PAGES: Nat = 3;
 
 /// Maximum page index.
 ///
 /// Thus the maximum number of pages is one more than this number. Currently, the store only
 /// supports storages where the number of pages is between 3 and 64.
-const MAX_PAGE_INDEX: usize = 63;
+const MAX_PAGE_INDEX: Nat = 63;
 
 /// Maximum key index.
 ///
 /// Thus the number of keys is one more than this number. Currently, the store only supports 4096
 /// keys.
-const MAX_KEY_INDEX: usize = 4095;
+const MAX_KEY_INDEX: Nat = 4095;
 
 /// Maximum length in bytes of a user payload.
 ///
 /// Currently, the store only supports values smaller than 1024 bytes.
-const MAX_VALUE_LEN: usize = 1023;
+const MAX_VALUE_LEN: Nat = 1023;
 
 /// Maximum number of updates per transaction.
 ///
 /// Currently, the store only supports transactions with at most 31 updates.
-const MAX_UPDATES: usize = 31;
+const MAX_UPDATES: Nat = 31;
 
 /// Maximum number of words per virtual page.
-const MAX_VIRT_PAGE_SIZE: usize = div_ceil(MAX_PAGE_SIZE, WORD_SIZE) - CONTENT_WORD;
+const MAX_VIRT_PAGE_SIZE: Nat = div_ceil(MAX_PAGE_SIZE, WORD_SIZE) - CONTENT_WORD;
 
 /// Word with all bits set to one.
-const ERASED_WORD: WORD = !(0 as WORD);
+const ERASED_WORD: Word = Word(!(0 as WORD));
 
 /// Helpers for a given storage configuration.
 #[derive(Clone, Debug)]
@@ -88,7 +124,7 @@ pub struct Format {
     /// - Words divide a page evenly.
     /// - There are at least 8 words in a page.
     /// - There are at most `MAX_PAGE_SIZE` bytes in a page.
-    page_size: usize,
+    page_size: Nat,
 
     /// The number of pages in the storage.
     ///
@@ -96,14 +132,14 @@ pub struct Format {
     ///
     /// - There are at least 3 pages.
     /// - There are at most `MAX_PAGE_INDEX + 1` pages.
-    num_pages: usize,
+    num_pages: Nat,
 
     /// The maximum number of times a page can be erased.
     ///
     /// # Invariant
     ///
     /// - A page can be erased at most `MAX_ERASE_CYCLE` times.
-    max_page_erases: usize,
+    max_page_erases: Nat,
 }
 
 impl Format {
@@ -115,9 +151,9 @@ impl Format {
     pub fn new<S: Storage>(storage: &S) -> Option<Format> {
         if Format::is_storage_supported(storage) {
             Some(Format {
-                page_size: storage.page_size(),
+                page_size: usize_to_nat(storage.page_size()),
-                num_pages: storage.num_pages(),
+                num_pages: usize_to_nat(storage.num_pages()),
-                max_page_erases: storage.max_page_erases(),
+                max_page_erases: usize_to_nat(storage.max_page_erases()),
             })
         } else {
             None
@@ -143,11 +179,11 @@ impl Format {
     /// [`MAX_PAGE_INDEX`]: constant.MAX_PAGE_INDEX.html
     /// [`MAX_ERASE_CYCLE`]: constant.MAX_ERASE_CYCLE.html
     fn is_storage_supported<S: Storage>(storage: &S) -> bool {
-        let word_size = storage.word_size();
+        let word_size = usize_to_nat(storage.word_size());
-        let page_size = storage.page_size();
+        let page_size = usize_to_nat(storage.page_size());
-        let num_pages = storage.num_pages();
+        let num_pages = usize_to_nat(storage.num_pages());
-        let max_word_writes = storage.max_word_writes();
+        let max_word_writes = usize_to_nat(storage.max_word_writes());
-        let max_page_erases = storage.max_page_erases();
+        let max_page_erases = usize_to_nat(storage.max_page_erases());
         word_size == WORD_SIZE
             && page_size % word_size == 0
             && (MIN_NUM_WORDS_PER_PAGE * word_size <= page_size && page_size <= MAX_PAGE_SIZE)
@@ -157,45 +193,45 @@ impl Format {
     }
 
     /// The size of a word in bytes.
-    pub fn word_size(&self) -> usize {
+    pub fn word_size(&self) -> Nat {
         WORD_SIZE
     }
 
     /// The size of a page in bytes.
     ///
     /// We have `MIN_NUM_WORDS_PER_PAGE * self.word_size() <= self.page_size() <= MAX_PAGE_SIZE`.
-    pub fn page_size(&self) -> usize {
+    pub fn page_size(&self) -> Nat {
         self.page_size
     }
 
     /// The number of pages in the storage, denoted by `N`.
     ///
     /// We have `MIN_NUM_PAGES <= N <= MAX_PAGE_INDEX + 1`.
-    pub fn num_pages(&self) -> usize {
+    pub fn num_pages(&self) -> Nat {
         self.num_pages
     }
 
     /// The maximum page index.
     ///
     /// We have `2 <= self.max_page() <= MAX_PAGE_INDEX`.
-    pub fn max_page(&self) -> usize {
+    pub fn max_page(&self) -> Nat {
         self.num_pages - 1
     }
 
     /// The maximum number of times a page can be erased, denoted by `E`.
     ///
     /// We have `E <= MAX_ERASE_CYCLE`.
-    pub fn max_page_erases(&self) -> usize {
+    pub fn max_page_erases(&self) -> Nat {
         self.max_page_erases
     }
 
     /// The maximum key.
-    pub fn max_key(&self) -> usize {
+    pub fn max_key(&self) -> Nat {
         MAX_KEY_INDEX
     }
 
     /// The maximum number of updates per transaction.
-    pub fn max_updates(&self) -> usize {
+    pub fn max_updates(&self) -> Nat {
         MAX_UPDATES
     }
 
@@ -204,7 +240,7 @@ impl Format {
     /// A virtual page is stored in a physical page after the page header.
     ///
     /// We have `MIN_NUM_WORDS_PER_PAGE - 2 <= Q <= MAX_VIRT_PAGE_SIZE`.
-    pub fn virt_page_size(&self) -> usize {
+    pub fn virt_page_size(&self) -> Nat {
         self.page_size() / self.word_size() - CONTENT_WORD
     }
 
@@ -212,7 +248,7 @@ impl Format {
     ///
     /// We have `(MIN_NUM_WORDS_PER_PAGE - 3) * self.word_size() <= self.max_value_len() <=
     /// MAX_VALUE_LEN`.
-    pub fn max_value_len(&self) -> usize {
+    pub fn max_value_len(&self) -> Nat {
         min(
             (self.virt_page_size() - 1) * self.word_size(),
             MAX_VALUE_LEN,
@@ -225,7 +261,7 @@ impl Format {
     /// virtual page. This happens because entries may overlap up to 2 virtual pages.
     ///
     /// We have `MIN_NUM_WORDS_PER_PAGE - 3 <= M < Q`.
-    pub fn max_prefix_len(&self) -> usize {
+    pub fn max_prefix_len(&self) -> Nat {
         self.bytes_to_words(self.max_value_len())
     }
 
@@ -239,7 +275,7 @@ impl Format {
     /// - `V >= (N - 1) * (Q - 1) - (Q - 1)` from `V` definition
     ///
     /// [`M`]: struct.Format.html#method.max_prefix_len
-    pub fn virt_size(&self) -> usize {
+    pub fn virt_size(&self) -> Nat {
         (self.num_pages() - 1) * (self.virt_page_size() - 1) - self.max_prefix_len()
     }
 
@@ -253,7 +289,7 @@ impl Format {
     /// - `(N - 2) * (Q - 1) - N = (N - 2) * (Q - 2) - 2` by calculus
     ///
     /// [`V`]: struct.Format.html#method.virt_size
-    pub fn total_capacity(&self) -> usize {
+    pub fn total_capacity(&self) -> Nat {
         // From the virtual capacity, we reserve N - 1 words for `Erase` entries and 1 word for a
         // `Clear` entry.
         self.virt_size() - self.num_pages()
@@ -270,18 +306,21 @@ impl Format {
     ///
     /// The init info of the page must be provided to know where the first entry of the page
     /// starts.
-    pub fn page_head(&self, init: InitInfo, page: usize) -> Position {
+    pub fn page_head(&self, init: InitInfo, page: Nat) -> Position {
         Position::new(self, init.cycle, page, init.prefix)
     }
 
     /// Returns the storage index of the init info of a page.
-    pub fn index_init(&self, page: usize) -> StorageIndex {
+    pub fn index_init(&self, page: Nat) -> StorageIndex {
         let byte = INIT_WORD * self.word_size();
-        StorageIndex { page, byte }
+        StorageIndex {
+            page: page as usize,
+            byte: byte as usize,
+        }
     }
 
     /// Parses the init info of a page from its storage representation.
-    pub fn parse_init(&self, word: WORD) -> StoreResult<WordState<InitInfo>> {
+    pub fn parse_init(&self, word: Word) -> StoreResult<WordState<InitInfo>> {
         Ok(if word == ERASED_WORD {
             WordState::Erased
         } else if WORD_CHECKSUM.get(word)? != 0 {
@@ -297,22 +336,25 @@ impl Format {
     }
 
     /// Builds the storage representation of an init info.
-    pub fn build_init(&self, init: InitInfo) -> [u8; WORD_SIZE] {
+    pub fn build_init(&self, init: InitInfo) -> WordSlice {
         let mut word = ERASED_WORD;
         INIT_CYCLE.set(&mut word, init.cycle);
         INIT_PREFIX.set(&mut word, init.prefix);
         WORD_CHECKSUM.set(&mut word, 0);
-        word.to_ne_bytes()
+        word.as_slice()
     }
 
     /// Returns the storage index of the compact info of a page.
-    pub fn index_compact(&self, page: usize) -> StorageIndex {
+    pub fn index_compact(&self, page: Nat) -> StorageIndex {
         let byte = COMPACT_WORD * self.word_size();
-        StorageIndex { page, byte }
+        StorageIndex {
+            page: page as usize,
+            byte: byte as usize,
+        }
     }
 
     /// Parses the compact info of a page from its storage representation.
-    pub fn parse_compact(&self, word: WORD) -> StoreResult<WordState<CompactInfo>> {
+    pub fn parse_compact(&self, word: Word) -> StoreResult<WordState<CompactInfo>> {
         Ok(if word == ERASED_WORD {
             WordState::Erased
         } else if WORD_CHECKSUM.get(word)? != 0 {
@@ -327,15 +369,15 @@ impl Format {
     }
 
     /// Builds the storage representation of a compact info.
-    pub fn build_compact(&self, compact: CompactInfo) -> [u8; WORD_SIZE] {
+    pub fn build_compact(&self, compact: CompactInfo) -> WordSlice {
         let mut word = ERASED_WORD;
         COMPACT_TAIL.set(&mut word, compact.tail);
         WORD_CHECKSUM.set(&mut word, 0);
-        word.to_ne_bytes()
+        word.as_slice()
     }
 
     /// Builds the storage representation of an internal entry.
-    pub fn build_internal(&self, internal: InternalEntry) -> [u8; WORD_SIZE] {
+    pub fn build_internal(&self, internal: InternalEntry) -> WordSlice {
         let mut word = ERASED_WORD;
         match internal {
             InternalEntry::Erase { page } => {
@@ -356,11 +398,11 @@ impl Format {
             }
         }
         WORD_CHECKSUM.set(&mut word, 0);
-        word.to_ne_bytes()
+        word.as_slice()
     }
 
     /// Parses the first word of an entry from its storage representation.
-    pub fn parse_word(&self, word: WORD) -> StoreResult<WordState<ParsedWord>> {
+    pub fn parse_word(&self, word: Word) -> StoreResult<WordState<ParsedWord>> {
         let valid = if ID_PADDING.check(word) {
             ParsedWord::Padding(Padding { length: 0 })
         } else if ID_HEADER.check(word) {
@@ -418,32 +460,35 @@ impl Format {
     }
 
     /// Builds the storage representation of a user entry.
-    pub fn build_user(&self, key: usize, value: &[u8]) -> Vec<u8> {
+    pub fn build_user(&self, key: Nat, value: &[u8]) -> Vec<u8> {
-        let length = value.len();
+        let length = usize_to_nat(value.len());
         let word_size = self.word_size();
         let footer = self.bytes_to_words(length);
-        let mut result = vec![0xff; (1 + footer) * word_size];
+        let mut result = vec![0xff; ((1 + footer) * word_size) as usize];
-        result[word_size..][..length].copy_from_slice(value);
+        result[word_size as usize..][..length as usize].copy_from_slice(value);
         let mut word = ERASED_WORD;
         ID_HEADER.set(&mut word);
-        if footer > 0 && is_erased(&result[footer * word_size..]) {
+        if footer > 0 && is_erased(&result[(footer * word_size) as usize..]) {
             HEADER_FLIPPED.set(&mut word);
             *result.last_mut().unwrap() = 0x7f;
         }
         HEADER_LENGTH.set(&mut word, length);
         HEADER_KEY.set(&mut word, key);
-        HEADER_CHECKSUM.set(&mut word, count_zeros(&result[footer * word_size..]));
+        HEADER_CHECKSUM.set(
-        result[..word_size].copy_from_slice(&word.to_ne_bytes());
+            &mut word,
+            count_zeros(&result[(footer * word_size) as usize..]),
+        );
+        result[..word_size as usize].copy_from_slice(&word.as_slice());
         result
     }
 
     /// Sets the padding bit in the first word of a user entry.
-    pub fn set_padding(&self, word: &mut WORD) {
+    pub fn set_padding(&self, word: &mut Word) {
         ID_PADDING.set(word);
     }
 
     /// Sets the deleted bit in the first word of a user entry.
-    pub fn set_deleted(&self, word: &mut WORD) {
+    pub fn set_deleted(&self, word: &mut Word) {
         HEADER_DELETED.set(word);
     }
 
@@ -452,21 +497,21 @@ impl Format {
     /// # Preconditions
     ///
     /// - `bytes + self.word_size()` does not overflow.
-    pub fn bytes_to_words(&self, bytes: usize) -> usize {
+    pub fn bytes_to_words(&self, bytes: Nat) -> Nat {
         div_ceil(bytes, self.word_size())
     }
 }
 
 /// The word index of the init info in a page.
-const INIT_WORD: usize = 0;
+const INIT_WORD: Nat = 0;
 
 /// The word index of the compact info in a page.
-const COMPACT_WORD: usize = 1;
+const COMPACT_WORD: Nat = 1;
 
 /// The word index of the content of a page.
 ///
 /// Since a page is at least 8 words, there is always at least 6 words of content.
-const CONTENT_WORD: usize = 2;
+const CONTENT_WORD: Nat = 2;
 
 /// The checksum for a single word.
 ///
@@ -619,27 +664,36 @@ bitfield! {
 ///
 /// Then the position of a word is `(c*N + p)*Q + w`. This position monotonically increases and
 /// represents the consumed lifetime of the storage.
+///
+/// This type is kept abstract to avoid possible confusion with `Nat` and `Word` if they happen to
+/// have the same representation. Here is an overview of their semantics:
+///
+/// | Name       | Semantics                   | Arithmetic operations | Bit-wise operations |
+/// | ---------- | --------------------------- | --------------------- | ------------------- |
+/// | `Nat`      | Natural numbers             | Yes (no overflow)     | No                  |
+/// | `Word`     | Word in flash               | No                    | Yes                 |
+/// | `Position` | Position in virtual storage | Yes (no overflow)     | No                  |
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
-pub struct Position(usize);
+pub struct Position(Nat);
 
-impl core::ops::Add<usize> for Position {
+impl core::ops::Add<Nat> for Position {
     type Output = Position;
 
-    fn add(self, delta: usize) -> Position {
+    fn add(self, delta: Nat) -> Position {
         Position(self.0 + delta)
     }
 }
 
 impl core::ops::Sub<Position> for Position {
-    type Output = usize;
+    type Output = Nat;
 
-    fn sub(self, base: Position) -> usize {
+    fn sub(self, base: Position) -> Nat {
         self.0 - base.0
     }
 }
 
-impl core::ops::AddAssign<usize> for Position {
+impl core::ops::AddAssign<Nat> for Position {
-    fn add_assign(&mut self, delta: usize) {
+    fn add_assign(&mut self, delta: Nat) {
         self.0 += delta;
     }
 }
@@ -651,12 +705,12 @@ impl Position {
     /// - Its word index in its page.
     /// - Its page index in the storage.
     /// - The number of times that page was erased.
-    pub fn new(format: &Format, cycle: usize, page: usize, word: usize) -> Position {
+    pub fn new(format: &Format, cycle: Nat, page: Nat, word: Nat) -> Position {
         Position((cycle * format.num_pages() + page) * format.virt_page_size() + word)
     }
 
     /// Accesses the underlying position as a natural number.
-    pub fn get(self) -> usize {
+    pub fn get(self) -> Nat {
         self.0
     }
 
@@ -665,7 +719,10 @@ impl Position {
         let page = self.page(format);
         let word = CONTENT_WORD + self.word(format);
         let byte = word * format.word_size();
-        StorageIndex { page, byte }
+        StorageIndex {
+            page: page as usize,
+            byte: byte as usize,
+        }
     }
 
     /// Returns the beginning of the current virtual page.
@@ -681,17 +738,17 @@ impl Position {
     }
 
     /// Returns the number of times the current page was erased.
-    pub fn cycle(self, format: &Format) -> usize {
+    pub fn cycle(self, format: &Format) -> Nat {
         (self.0 / format.virt_page_size()) / format.num_pages()
     }
 
     /// Returns the current page index.
-    pub fn page(self, format: &Format) -> usize {
+    pub fn page(self, format: &Format) -> Nat {
         (self.0 / format.virt_page_size()) % format.num_pages()
     }
 
     /// Returns the current word index in the page.
-    pub fn word(self, format: &Format) -> usize {
+    pub fn word(self, format: &Format) -> Nat {
         self.0 % format.virt_page_size()
     }
 }
@@ -711,16 +768,16 @@ pub enum WordState<T> {
 /// Information for an initialized page.
 pub struct InitInfo {
     /// The number of times this page has been erased.
-    pub cycle: usize,
+    pub cycle: Nat,
 
     /// The word index of the first entry in this virtual page.
-    pub prefix: usize,
+    pub prefix: Nat,
 }
 
 /// Information for a page being compacted.
 pub struct CompactInfo {
     /// The distance in words between head and tail at compaction.
-    pub tail: usize,
+    pub tail: Nat,
 }
 
 /// The first word of an entry.
@@ -740,7 +797,7 @@ pub enum ParsedWord {
 #[derive(Debug)]
 pub struct Padding {
     /// The number of following padding words after the first word of the padding entry.
-    pub length: usize,
+    pub length: Nat,
 }
 
 /// Header of a user entry.
@@ -750,13 +807,13 @@ pub struct Header {
     pub flipped: bool,
 
     /// The length in bytes of the user data.
-    pub length: usize,
+    pub length: Nat,
 
     /// The key of the user entry.
-    pub key: usize,
+    pub key: Nat,
 
     /// The checksum of the user entry.
-    pub checksum: usize,
+    pub checksum: Nat,
 }
 
 impl Header {
@@ -775,13 +832,13 @@ pub enum InternalEntry {
     /// Indicates that a page should be erased.
     Erase {
         /// The page to be erased.
-        page: usize,
+        page: Nat,
     },
 
     /// Indicates that user entries with high key should be deleted.
     Clear {
         /// The minimum key a user entry should have to be deleted.
-        min_key: usize,
+        min_key: Nat,
     },
 
     /// Marks the start of a transaction.
@@ -790,7 +847,7 @@ pub enum InternalEntry {
     /// entries.
     Marker {
         /// The number of updates in the transaction.
-        count: usize,
+        count: Nat,
     },
 
     /// Indicates that a user entry should be removed.
@@ -799,7 +856,7 @@ pub enum InternalEntry {
     /// already atomic.
     Remove {
         /// The key of the user entry to be removed.
-        key: usize,
+        key: Nat,
     },
 }
 
@@ -815,7 +872,7 @@ pub fn is_erased(slice: &[u8]) -> bool {
 /// # Preconditions
 ///
 /// - `x + m` does not overflow.
-const fn div_ceil(x: usize, m: usize) -> usize {
+const fn div_ceil(x: Nat, m: Nat) -> Nat {
     (x + m - 1) / m
 }
 
@@ -825,7 +882,7 @@ mod tests {
 
     #[test]
     fn size_of_format() {
-        assert_eq!(std::mem::size_of::<Format>(), 24);
+        assert_eq!(std::mem::size_of::<Format>(), 12);
     }
 
     #[test]
@@ -921,6 +978,18 @@ mod tests {
         assert_eq!(LEN_REMOVE.pos, 17);
     }
 
+    #[test]
+    fn word_from_slice_ok() {
+        assert_eq!(
+            Word::from_slice(&[0x04, 0x03, 0x02, 0x01]),
+            Word(0x01020304)
+        );
+        assert_eq!(
+            Word::from_slice(&[0x1e, 0x3c, 0x78, 0xf0]),
+            Word(0xf0783c1e)
+        );
+    }
+
     #[test]
     fn is_erased_ok() {
         assert!(is_erased(&[]));

libraries/persistent_store/src/format/bitfield.rs

@@ -12,9 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//! Helps manipulate bit fields in 32-bits words.
+//! Helps manipulate words as bit fields.
+//!
+//! This module assumes that `Word` and `Nat` are both represented as `u32`.
 
-use crate::{StoreError, StoreResult};
+use crate::format::Word;
+use crate::{Nat, StoreError, StoreResult};
 
 /// Represents a bit field.
 ///
@@ -25,16 +28,16 @@ use crate::{StoreError, StoreResult};
 /// - The bit field must fit in a 32-bits word: `pos + len < 32`.
 pub struct Field {
     /// The position of the bit field.
-    pub pos: usize,
+    pub pos: Nat,
 
     /// The length of the bit field.
-    pub len: usize,
+    pub len: Nat,
 }
 
 impl Field {
     /// Reads the value of a bit field.
-    pub fn get(&self, word: u32) -> usize {
+    pub fn get(&self, word: Word) -> Nat {
-        ((word >> self.pos) & self.mask()) as usize
+        (word.0 >> self.pos) & self.mask()
     }
 
     /// Sets the value of a bit field.
@@ -43,12 +46,11 @@ impl Field {
     ///
     /// - The value must fit in the bit field: `num_bits(value) < self.len`.
     /// - The value must only change bits from 1 to 0: `self.get(*word) & value == value`.
-    pub fn set(&self, word: &mut u32, value: usize) {
+    pub fn set(&self, word: &mut Word, value: Nat) {
-        let value = value as u32;
         debug_assert_eq!(value & self.mask(), value);
         let mask = !(self.mask() << self.pos);
-        *word &= mask | (value << self.pos);
+        word.0 &= mask | (value << self.pos);
-        debug_assert_eq!(self.get(*word), value as usize);
+        debug_assert_eq!(self.get(*word), value);
     }
 
     /// Returns a bit mask the length of the bit field.
@@ -70,17 +72,17 @@ pub struct ConstField {
     pub field: Field,
 
     /// The constant value.
-    pub value: usize,
+    pub value: Nat,
 }
 
 impl ConstField {
     /// Checks that the bit field has its value.
-    pub fn check(&self, word: u32) -> bool {
+    pub fn check(&self, word: Word) -> bool {
         self.field.get(word) == self.value
     }
 
     /// Sets the bit field to its value.
-    pub fn set(&self, word: &mut u32) {
+    pub fn set(&self, word: &mut Word) {
         self.field.set(word, self.value);
     }
 }
@@ -92,18 +94,18 @@ impl ConstField {
 /// - The bit must fit in a 32-bits word: `pos < 32`.
 pub struct Bit {
     /// The position of the bit.
-    pub pos: usize,
+    pub pos: Nat,
 }
 
 impl Bit {
     /// Returns whether the value of the bit is zero.
-    pub fn get(&self, word: u32) -> bool {
+    pub fn get(&self, word: Word) -> bool {
-        word & (1 << self.pos) == 0
+        word.0 & (1 << self.pos) == 0
     }
 
     /// Sets the value of the bit to zero.
-    pub fn set(&self, word: &mut u32) {
+    pub fn set(&self, word: &mut Word) {
-        *word &= !(1 << self.pos);
+        word.0 &= !(1 << self.pos);
     }
 }
 
@@ -123,9 +125,9 @@ impl Checksum {
     /// # Errors
     ///
     /// Returns `InvalidStorage` if the external increment would be negative.
-    pub fn get(&self, word: u32) -> StoreResult<usize> {
+    pub fn get(&self, word: Word) -> StoreResult<Nat> {
         let checksum = self.field.get(word);
-        let zeros = word.count_zeros() as usize - (self.field.len - checksum.count_ones() as usize);
+        let zeros = word.0.count_zeros() - (self.field.len - checksum.count_ones());
         checksum
             .checked_sub(zeros)
             .ok_or(StoreError::InvalidStorage)
@@ -139,9 +141,9 @@ impl Checksum {
     ///   self.field.mask()`.
     /// - The checksum value should fit in the checksum bit field: `num_bits(word.count_zeros() +
     ///   value) < self.field.len`.
-    pub fn set(&self, word: &mut u32, value: usize) {
+    pub fn set(&self, word: &mut Word, value: Nat) {
-        debug_assert_eq!(self.field.get(*word), self.field.mask() as usize);
+        debug_assert_eq!(self.field.get(*word), self.field.mask());
-        self.field.set(word, word.count_zeros() as usize + value);
+        self.field.set(word, word.0.count_zeros() + value);
     }
 }
 
@@ -153,7 +155,7 @@ impl Checksum {
 #[cfg(any(doc, test))]
 pub struct Length {
     /// The position of the next available bit.
-    pub pos: usize,
+    pub pos: Nat,
 }
 
 /// Helps defining contiguous bit fields.
@@ -266,13 +268,13 @@ macro_rules! bitfield_impl {
 }
 
 /// Counts the number of bits equal to zero in a byte slice.
-pub fn count_zeros(slice: &[u8]) -> usize {
+pub fn count_zeros(slice: &[u8]) -> Nat {
-    slice.iter().map(|&x| x.count_zeros() as usize).sum()
+    slice.iter().map(|&x| x.count_zeros()).sum()
 }
 
 /// Returns the number of bits necessary to represent a number.
-pub const fn num_bits(x: usize) -> usize {
+pub const fn num_bits(x: Nat) -> Nat {
-    8 * core::mem::size_of::<usize>() - x.leading_zeros() as usize
+    8 * core::mem::size_of::<Nat>() as Nat - x.leading_zeros()
 }
 
 #[cfg(test)]
@@ -282,14 +284,14 @@ mod tests {
     #[test]
     fn field_ok() {
         let field = Field { pos: 3, len: 5 };
-        assert_eq!(field.get(0x00000000), 0);
+        assert_eq!(field.get(Word(0x00000000)), 0);
-        assert_eq!(field.get(0x00000007), 0);
+        assert_eq!(field.get(Word(0x00000007)), 0);
-        assert_eq!(field.get(0x00000008), 1);
+        assert_eq!(field.get(Word(0x00000008)), 1);
-        assert_eq!(field.get(0x000000f8), 0x1f);
+        assert_eq!(field.get(Word(0x000000f8)), 0x1f);
-        assert_eq!(field.get(0x0000ff37), 6);
+        assert_eq!(field.get(Word(0x0000ff37)), 6);
-        let mut word = 0xffffffff;
+        let mut word = Word(0xffffffff);
         field.set(&mut word, 3);
-        assert_eq!(word, 0xffffff1f);
+        assert_eq!(word, Word(0xffffff1f));
     }
 
     #[test]
@@ -298,25 +300,25 @@ mod tests {
             field: Field { pos: 3, len: 5 },
             value: 9,
         };
-        assert!(!field.check(0x00000000));
+        assert!(!field.check(Word(0x00000000)));
-        assert!(!field.check(0x0000ffff));
+        assert!(!field.check(Word(0x0000ffff)));
-        assert!(field.check(0x00000048));
+        assert!(field.check(Word(0x00000048)));
-        assert!(field.check(0x0000ff4f));
+        assert!(field.check(Word(0x0000ff4f)));
-        let mut word = 0xffffffff;
+        let mut word = Word(0xffffffff);
         field.set(&mut word);
-        assert_eq!(word, 0xffffff4f);
+        assert_eq!(word, Word(0xffffff4f));
     }
 
     #[test]
     fn bit_ok() {
         let bit = Bit { pos: 3 };
-        assert!(bit.get(0x00000000));
+        assert!(bit.get(Word(0x00000000)));
-        assert!(bit.get(0xfffffff7));
+        assert!(bit.get(Word(0xfffffff7)));
-        assert!(!bit.get(0x00000008));
+        assert!(!bit.get(Word(0x00000008)));
-        assert!(!bit.get(0xffffffff));
+        assert!(!bit.get(Word(0xffffffff)));
-        let mut word = 0xffffffff;
+        let mut word = Word(0xffffffff);
         bit.set(&mut word);
-        assert_eq!(word, 0xfffffff7);
+        assert_eq!(word, Word(0xfffffff7));
     }
 
     #[test]
@@ -324,15 +326,15 @@ mod tests {
         let field = Checksum {
             field: Field { pos: 3, len: 5 },
         };
-        assert_eq!(field.get(0x00000000), Err(StoreError::InvalidStorage));
+        assert_eq!(field.get(Word(0x00000000)), Err(StoreError::InvalidStorage));
-        assert_eq!(field.get(0xffffffff), Ok(31));
+        assert_eq!(field.get(Word(0xffffffff)), Ok(31));
-        assert_eq!(field.get(0xffffff07), Ok(0));
+        assert_eq!(field.get(Word(0xffffff07)), Ok(0));
-        assert_eq!(field.get(0xffffff0f), Ok(1));
+        assert_eq!(field.get(Word(0xffffff0f)), Ok(1));
-        assert_eq!(field.get(0x00ffff67), Ok(4));
+        assert_eq!(field.get(Word(0x00ffff67)), Ok(4));
-        assert_eq!(field.get(0x7fffff07), Err(StoreError::InvalidStorage));
+        assert_eq!(field.get(Word(0x7fffff07)), Err(StoreError::InvalidStorage));
-        let mut word = 0x0fffffff;
+        let mut word = Word(0x0fffffff);
         field.set(&mut word, 4);
-        assert_eq!(word, 0x0fffff47);
+        assert_eq!(word, Word(0x0fffff47));
     }
 
     #[test]

libraries/persistent_store/src/lib.rs

@@ -348,8 +348,6 @@
 #[macro_use]
 extern crate alloc;
 
-#[macro_use]
-mod bitfield;
 mod buffer;
 mod format;
 #[cfg(feature = "std")]
@@ -362,3 +360,25 @@ pub use self::buffer::{BufferCorruptFunction, BufferOptions, BufferStorage};
 pub use self::model::{StoreModel, StoreOperation};
 pub use self::storage::{Storage, StorageError, StorageIndex, StorageResult};
 pub use self::store::{StoreError, StoreRatio, StoreResult, StoreUpdate};
+
+/// Internal representation of natural numbers.
+///
+/// In Rust natural numbers are represented as `usize`. However, internally we represent them as
+/// `u32`. This is done to preserve semantics across different targets. This is useful when tests
+/// run with `usize = u64` while the actual target has `usize = u32`.
+///
+/// To avoid too many conversions between `usize` and `Nat` which are necessary when interfacing
+/// with Rust, `usize` is used instead of `Nat` in code meant only for tests.
+///
+/// Currently, the store only supports targets with `usize = u32`.
+type Nat = u32;
+
+/// Returns the internal representation of a Rust natural number.
+///
+/// # Panics
+///
+/// Panics if the conversion overflows.
+fn usize_to_nat(x: usize) -> Nat {
+    use core::convert::TryFrom;
+    Nat::try_from(x).unwrap()
+}

libraries/persistent_store/src/model.rs

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 use crate::format::Format;
-use crate::{StoreError, StoreRatio, StoreResult, StoreUpdate};
+use crate::{usize_to_nat, StoreError, StoreRatio, StoreResult, StoreUpdate};
 use std::collections::{HashMap, HashSet};
 
 /// Models the mutable operations of a store.
@@ -79,14 +79,14 @@ impl StoreModel {
     /// Returns the capacity according to the model.
     pub fn capacity(&self) -> StoreRatio {
         let total = self.format.total_capacity();
-        let used: usize = self.content.values().map(|x| self.entry_size(x)).sum();
+        let used = usize_to_nat(self.content.values().map(|x| self.entry_size(x)).sum());
         StoreRatio { used, total }
     }
 
     /// Applies a transaction.
     fn transaction(&mut self, updates: Vec<StoreUpdate>) -> StoreResult<()> {
         // Fail if too many updates.
-        if updates.len() > self.format.max_updates() {
+        if updates.len() > self.format.max_updates() as usize {
             return Err(StoreError::InvalidArgument);
         }
         // Fail if an update is invalid.
@@ -130,7 +130,7 @@ impl StoreModel {
 
     /// Applies a clear operation.
     fn clear(&mut self, min_key: usize) -> StoreResult<()> {
-        if min_key > self.format.max_key() {
+        if min_key > self.format.max_key() as usize {
             return Err(StoreError::InvalidArgument);
         }
         self.content.retain(|&k, _| k < min_key);
@@ -155,14 +155,14 @@ impl StoreModel {
 
     /// Returns the word capacity of an entry.
     fn entry_size(&self, value: &[u8]) -> usize {
-        1 + self.format.bytes_to_words(value.len())
+        1 + self.format.bytes_to_words(usize_to_nat(value.len())) as usize
     }
 
     /// Returns whether an update is valid.
     fn update_valid(&self, update: &StoreUpdate) -> bool {
-        update.key() <= self.format.max_key()
+        update.key() <= self.format.max_key() as usize
             && update
                 .value()
-                .map_or(true, |x| x.len() <= self.format.max_value_len())
+                .map_or(true, |x| x.len() <= self.format.max_value_len() as usize)
     }
 }

libraries/persistent_store/src/storage.rs

@@ -37,9 +37,13 @@ pub type StorageResult<T> = Result<T, StorageError>;
 /// Abstracts a flash storage.
 pub trait Storage {
     /// The size of a word in bytes.
+    ///
+    /// A word is the smallest unit of writable flash.
     fn word_size(&self) -> usize;
 
     /// The size of a page in bytes.
+    ///
+    /// A page is the smallest unit of erasable flash.
     fn page_size(&self) -> usize;
 
     /// The number of pages in the storage.

libraries/persistent_store/src/store.rs

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use crate::StorageError;
+use crate::{Nat, StorageError};
 use alloc::vec::Vec;
 
 /// Errors returned by store operations.
@@ -78,26 +78,26 @@ pub type StoreResult<T> = Result<T, StoreError>;
 #[derive(Copy, Clone, PartialEq, Eq)]
 pub struct StoreRatio {
     /// How much of the metric is used.
-    pub(crate) used: usize,
+    pub(crate) used: Nat,
 
     /// How much of the metric can be used at most.
-    pub(crate) total: usize,
+    pub(crate) total: Nat,
 }
 
 impl StoreRatio {
     /// How much of the metric is used.
     pub fn used(self) -> usize {
-        self.used
+        self.used as usize
     }
 
     /// How much of the metric can be used at most.
     pub fn total(self) -> usize {
-        self.total
+        self.total as usize
     }
 
     /// How much of the metric is remaining.
     pub fn remaining(self) -> usize {
-        self.total - self.used
+        (self.total - self.used) as usize
     }
 }