Add stash functions

src/net.rs

@@ -90,7 +90,18 @@ impl Iterator for PeersIter {
     }
 }
 
-/// Get the peer cirresponding to the local device.
+/// Stash is a serialized binary state of the app that you want to persist
+/// between app runs and to be available in multiplayer.
+///
+/// For single-player purposes, you can save data in a regular file
+/// using [`dump_file`](crate::dump_file). File saved that way can be bigger
+/// (and you can create lots of them) but it cannot be accessed in multiplayer.
+///
+/// It's your job to serialize data into a binary stash and later parse it.
+/// Stash can be saved using [`save_stash`] and later read using [`load_stash`].
+type Stash = [u8];
+
+/// Get the peer corresponding to the local device.
 #[must_use]
 pub fn get_me() -> Peer {
     let me = unsafe { bindings::get_me() };
@@ -104,11 +115,71 @@ pub fn get_peers() -> Peers {
     Peers(peers)
 }
 
+/// Save the given [`Stash`].
+///
+/// When called, the stash for the given peer will be stored in RAM.
+/// Calling [`load_stash`] for the same peer will return that stash.
+/// On exit, the runtime will persist the stash in FS.
+/// Next time the app starts, calling [`load_stash`] will restore the stash
+/// saved earlier.
+pub fn save_stash(peer: Peer, stash: &Stash) {
+    let ptr = stash.as_ptr();
+    let peer = u32::from(peer.0);
+    unsafe {
+        bindings::save_stash(peer, ptr as u32, stash.len() as u32);
+    }
+}
+
+/// Read the stash of the given peer using the passed buffer.
+///
+/// It's the app's responsibility to ensure that the passed buffer is big enough
+/// to fit the stash. If it doesn't fit, runtime will fill the buffer
+/// and discard the rest.
+///
+/// The returned stash is a slice of the passed in buffer of the actual content size
+/// (up to the buffer size).
+///
+/// If there is no stash (which is always true before [`save_stash`]
+/// is called for the first time ever), the result is `None`.
+#[must_use]
+pub fn load_stash(peer: Peer, stash: &mut Stash) -> Option<&mut Stash> {
+    let ptr = stash.as_ptr();
+    let peer = u32::from(peer.0);
+    let size = unsafe { bindings::load_stash(peer, ptr as u32, stash.len() as u32) };
+    if size == 0 {
+        return None;
+    }
+    let size = size as usize;
+    Some(&mut stash[..size])
+}
+
+/// Similar to [`load_stash`] but statically allocates the stash of the right size.
+///
+/// Unlike [`load_stash`], the returned stash size is not adjusted
+/// for the actual content size.
+///
+/// Unlike other `_buf` functions, like [`load_file_buf`][crate::load_file_buf],
+/// this one allocates the buffer statically, not dynamically.
+/// The app must statically know the max stash size.
+#[must_use]
+pub fn load_stash_buf<const N: usize>(peer: Peer) -> Option<[u8; N]> {
+    let stash = [0u8; N];
+    let ptr = stash.as_ptr();
+    let peer = u32::from(peer.0);
+    let size = unsafe { bindings::load_stash(peer, ptr as u32, stash.len() as u32) };
+    if size == 0 {
+        return None;
+    }
+    Some(stash)
+}
+
 /// Internal bindings to the raw runtime functions.
 mod bindings {
     #[link(wasm_import_module = "net")]
     extern {
         pub(crate) fn get_me() -> u32;
         pub(crate) fn get_peers() -> u32;
+        pub(crate) fn save_stash(peer: u32, ptr: u32, len: u32);
+        pub(crate) fn load_stash(peer: u32, ptr: u32, len: u32) -> u32;
     }
 }