StoreBackend::backup_space(): add for statically allocated memory

Cargo.toml

@@ -11,5 +11,11 @@ categories = ["embedded", "database"]
 license = "GPL-3"
 edition = "2018"
 
+[features]
+default = ["postcard-values"]
+postcard-values = ["postcard", "serde"]
+
 [dependencies]
-byteorder = { version = "1.0", default-features = false }
+byteorder = { version = "1", default-features = false }
+serde = { version = "1.0", default-features = false, optional = true }
+postcard = { version = "0.5", optional = true }

README.md

@@ -11,15 +11,7 @@ automatically erasing and rewriting when full.
 
 For details see `trait StoreBackend`.
 
-## TODO
+## Ideas
-
-* NoFlash backend
-* read_int()
-* write_str()
-* automatic value coercion
-* support for floats
-
-### Ideas
 
 * iterator (quadratic)
 * compaction to a second backend instead on stack

src/error.rs

@@ -1,9 +1,13 @@
 use core::{fmt, str};
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Error<B> {
     Read(ReadError),
     Write(WriteError<B>),
+    #[cfg(feature = "postcard-values")]
+    Encode(postcard::Error),
+    #[cfg(feature = "postcard-values")]
+    Decode(postcard::Error),
 }
 
 impl<B> From<ReadError> for Error<B> {
@@ -54,11 +58,11 @@ pub enum WriteError<B> {
 
 impl<B: fmt::Display> fmt::Display for WriteError<B> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        match self {
+        match &self {
             &WriteError::SpaceExhausted =>
                 write!(f, "space exhausted"),
-            &WriteError::Backend(ref e) =>
+            &WriteError::Backend(err) =>
-                e.fmt(f),
+                write!(f, "{}", err)
         }
     }
 }

src/fmt.rs

@@ -1,30 +0,0 @@
-use core::fmt;
-
-pub struct FmtWrapper<'a> {
-    buf: &'a mut [u8],
-    offset: usize,
-}
-
-impl<'a> FmtWrapper<'a> {
-    pub fn new(buf: &'a mut [u8]) -> Self {
-        FmtWrapper {
-            buf: buf,
-            offset: 0,
-        }
-    }
-
-    pub fn contents(&self) -> &[u8] {
-        &self.buf[..self.offset]
-    }
-}
-
-impl<'a> fmt::Write for FmtWrapper<'a> {
-    fn write_str(&mut self, s: &str) -> fmt::Result {
-        let bytes = s.as_bytes();
-        let remainder = &mut self.buf[self.offset..];
-        let remainder = &mut remainder[..bytes.len()];
-        remainder.copy_from_slice(bytes);
-        self.offset += bytes.len();
-        Ok(())
-    }
-}

src/lib.rs

@@ -1,19 +1,21 @@
-#![no_std]
+//#![no_std]
 
-use core::{fmt::Write, mem::size_of, str};
+use core::{mem::size_of, str};
 use byteorder::{ByteOrder, BigEndian};
+#[cfg(feature = "postcard-values")]
+use serde::{Deserialize, Serialize};
 
 mod error;
 pub use error::{Error, ReadError, WriteError};
 mod iter;
 use iter::Iter;
-mod fmt;
+pub mod no_flash;
-use fmt::FmtWrapper;
 mod test;
 
+/// Backend interface for `Store`
 pub trait StoreBackend {
-    type Data: Sized + AsRef<[u8]> + Clone;
+    type Data: Sized + AsRef<[u8]>;
-    
+
     /// Memory-mapped
     fn data(&self) -> &Self::Data;
     fn len(&self) -> usize {
@@ -21,10 +23,17 @@ pub trait StoreBackend {
     }
 
     type Error;
+    /// erase flash
     fn erase(&mut self) -> Result<(), Self::Error>;
+    /// program flash
     fn program(&mut self, offset: usize, payload: &[u8]) -> Result<(), Self::Error>;
     /// called after repeated `program()` invocations to allow for eg. cache flushing
     fn program_done(&mut self) {}
+
+    /// need for automatic compaction
+    ///
+    /// leaves memory management to the implementer
+    fn backup_space(&self) -> &'static mut [u8];
 }
 
 /// Simple Flash Key Value Store
@@ -59,7 +68,29 @@ impl<B: StoreBackend> Store<B> {
                       .map(|value| str::from_utf8(value)
                            .map_err(ReadError::Utf8Error)
                       )
-                      .transpose())
+                      .transpose()
+            )
+    }
+
+    /// read from `key`, decode with `postcard`
+    #[cfg(feature = "postcard-values")]
+    pub fn read_value<'a, T: Deserialize<'a>>(&'a self, key: &str) -> Result<Option<T>, Error<B::Error>> {
+        self.read(key)?
+            .map(
+                |data| postcard::from_bytes(data)
+                    .map_err(Error::Decode)
+            ).transpose()
+    }
+
+    /// how many bytes are currently used
+    ///
+    /// equally, offset of free space
+    pub fn get_bytes_used(&self) -> Result<usize, ReadError> {
+        let mut iter = Iter::new(self.backend.data().as_ref());
+        while let Some(result) = iter.next() {
+            let _ = result?;
+        }
+        Ok(iter.offset)
     }
 
     unsafe fn append_at(&mut self, mut offset: usize,
@@ -91,7 +122,8 @@ impl<B: StoreBackend> Store<B> {
     }
 
     fn compact(&mut self) -> Result<(), Error<B::Error>> {
-        let old_data = self.backend.data().clone();
+        let old_data = self.backend.backup_space();
+        old_data.copy_from_slice(self.backend.data().as_ref());
 
         self.erase()?;
 
@@ -121,21 +153,14 @@ impl<B: StoreBackend> Store<B> {
     }
 
     fn append(&mut self, key: &str, value: &[u8]) -> Result<(), Error<B::Error>> {
-        let free_offset = {
+        let free_offset = self.get_bytes_used()?;
-            let mut iter = Iter::new(self.backend.data().as_ref());
-            while let Some(result) = iter.next() {
-                let _ = result?;
-            }
-            iter.offset
-        };
-
         unsafe { self.append_at(free_offset, key.as_bytes(), value)? };
-
         Ok(())
     }
 
     /// store a buffer `value` at `key`
-    pub fn write(&mut self, key: &str, value: &[u8]) -> Result<(), Error<B::Error>> {
+    pub fn write<V: AsRef<[u8]>>(&mut self, key: &str, value: V) -> Result<(), Error<B::Error>> {
+        let value = value.as_ref();
         match self.append(key, value) {
             Err(Error::Write(WriteError::SpaceExhausted)) => {
                 self.compact()?;
@@ -145,12 +170,13 @@ impl<B: StoreBackend> Store<B> {
         }
     }
 
-    /// serialize 32-bit as ASCII, store at `key`
+    /// encode with `postcard`, write at `key`
-    pub fn write_int(&mut self, key: &str, value: u32) -> Result<(), Error<B::Error>> {
+    #[cfg(feature = "postcard-values")]
-        let mut buf = [0; 16];
+    pub fn write_value<T: Serialize>(&mut self, key: &str, value: T) -> Result<(), Error<B::Error>> {
-        let mut wrapper = FmtWrapper::new(&mut buf);
+        let mut buf = [0; 64];
-        write!(&mut wrapper, "{}", value).unwrap();
+        let data = postcard::to_slice(&value, &mut buf)
-        self.write(key, wrapper.contents())
+            .map_err(Error::Encode)?;
+        self.write(key, data)
     }
 
     /// store a 0-byte tombstone value at `key`
@@ -158,6 +184,7 @@ impl<B: StoreBackend> Store<B> {
         self.write(key, &[])
     }
 
+    /// invokes erase on the backend (this is no compaction!)
     pub fn erase(&mut self) -> Result<(), WriteError<B::Error>> {
         self.backend.erase()
             .map_err(WriteError::Backend)?;

src/no_flash.rs

@@ -0,0 +1,37 @@
+use crate::StoreBackend;
+use core::fmt;
+
+/// expect this for any write operation to `NoFlash`
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct NoFlashError;
+
+impl fmt::Display for NoFlashError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "no flash")
+    }
+}
+
+/// a valid `StoreBackend` that can be used for configurations lacking a flash
+#[derive(Debug, Clone, Copy)]
+pub struct NoFlash;
+
+impl StoreBackend for NoFlash {
+    type Data = [u8; 0];
+
+    fn data(&self) -> &Self::Data {
+        &[]
+    }
+
+    type Error = NoFlashError;
+    fn erase(&mut self) -> Result<(), Self::Error> {
+        Err(NoFlashError)
+    }
+
+    fn program(&mut self, _: usize, _: &[u8]) -> Result<(), Self::Error> {
+        Err(NoFlashError)
+    }
+
+    fn backup_space(&self) -> &'static mut [u8] {
+        &mut []
+    }
+}

src/test.rs

@@ -1,8 +1,9 @@
 #![cfg(test)]
 
-use crate::{Store, StoreBackend};
+use crate::{Error, no_flash, Store, StoreBackend, WriteError};
 
 const SIZE: usize = 1024;
+static mut BACKUP_SPACE: [u8; SIZE] = [0; SIZE];
 
 pub struct TestBackend {
     data: [u8; SIZE],
@@ -39,6 +40,11 @@ impl StoreBackend for TestBackend {
         self.data[offset..end].copy_from_slice(payload);
         Ok(())
     }
+
+
+    fn backup_space(&self) -> &'static mut [u8] {
+        unsafe { &mut BACKUP_SPACE }
+    }
 }
 
 fn make_store() -> Store<TestBackend> {
@@ -52,19 +58,65 @@ fn empty_read_not_found() {
     assert_eq!(store.read("foo").unwrap(), None)
 }
 
-    
 #[test]
 fn write_read() {
     let mut store = make_store();
     store.write("foo", b"bar").unwrap();
     assert_eq!(store.read("foo").unwrap().unwrap(), b"bar");
 }
-    
+
 #[test]
-fn write_int_read_str() {
+fn write_read_str() {
     let mut store = make_store();
-    store.write_int("foo", 42).unwrap();
+    store.write("foo", "bar").unwrap();
-    assert_eq!(store.read_str("foo").unwrap().unwrap(), "42");
+    assert_eq!(store.read_str("foo").unwrap().unwrap(), "bar");
+}
+
+#[test]
+fn write_read_str_utf8() {
+    let mut store = make_store();
+    store.write("réseau", "point à point").unwrap();
+    assert_eq!(store.read_str("réseau").unwrap().unwrap(), "point à point");
+}
+
+#[test]
+#[cfg(feature = "postcard-values")]
+fn write_read_value_str() {
+    let mut store = make_store();
+    store.write_value("foo", "bar").unwrap();
+    assert_eq!(store.read_value("foo"), Ok(Some("bar")));
+}
+
+#[test]
+#[cfg(feature = "postcard-values")]
+fn write_read_value_u32() {
+    let mut store = make_store();
+    store.write_value("foo", 42005u32).unwrap();
+    assert_eq!(store.read_value("foo"), Ok(Some(42005u32)));
+}
+
+#[test]
+#[cfg(feature = "postcard-values")]
+fn write_read_value_i64() {
+    let mut store = make_store();
+    store.write_value("foo", -99999999i64).unwrap();
+    assert_eq!(store.read_value("foo"), Ok(Some(-99999999i64)));
+}
+
+#[test]
+#[cfg(feature = "postcard-values")]
+fn write_read_value_f32() {
+    let mut store = make_store();
+    store.write_value("foo", 3.75e17f32).unwrap();
+    assert_eq!(store.read_value("foo"), Ok(Some(3.75e17f32)));
+}
+
+#[test]
+#[cfg(feature = "postcard-values")]
+fn write_read_value_f64() {
+    let mut store = make_store();
+    store.write_value("foo", -1.999e-13f64).unwrap();
+    assert_eq!(store.read_value("foo"), Ok(Some(-1.999e-13f64)));
 }
 
 #[test]
@@ -77,3 +129,13 @@ fn write_many() {
     assert_eq!(store.read("foo").unwrap().unwrap(), b"do not compact away");
     assert_eq!(store.read("bar").unwrap().unwrap(), b"SPAM");
 }
+
+#[test]
+fn no_flash() {
+    let mut store = Store::new(no_flash::NoFlash);
+    assert_eq!(store.read("foo").unwrap(), None);
+    assert_eq!(
+        store.write("foo", b"bar"),
+        Err(Error::Write(WriteError::Backend(no_flash::NoFlashError)))
+    );
+}