From d581440210e461eb74f397580ec8a3a154aee18d Mon Sep 17 00:00:00 2001
From: lifning <>
Date: Sat, 27 Nov 2021 01:22:54 -0800
Subject: [PATCH] initial commit

---
 .gitignore  |   2 +
 Cargo.lock  |  16 ++++++
 Cargo.toml  |   7 +++
 src/main.rs | 144 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 169 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 Cargo.lock
 create mode 100644 Cargo.toml
 create mode 100644 src/main.rs

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..3a8cabc
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+/target
+.idea
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..b872342
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,16 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "lazy_static"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
+
+[[package]]
+name = "shortpointer"
+version = "0.1.0"
+dependencies = [
+ "lazy_static",
+]
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..86249af
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "shortpointer"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+lazy_static = "1"
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..099e646
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,144 @@
+use core::alloc::Layout;
+use core::marker::PhantomData;
+use core::mem::size_of;
+use core::ops::{Deref, DerefMut};
+
+use std::fmt::Formatter;
+use std::mem::size_of_val;
+use std::sync::Mutex;
+
+use lazy_static::lazy_static;
+
+#[repr(C, align(65536))]
+struct Pool {
+    data: [u8; 65536],
+    begin: usize,
+    end: usize,
+    last_position: usize,
+}
+
+lazy_static! {
+    static ref POOL: Mutex<Pool> = Mutex::new(Pool { data: [0; 65536], begin: 0, end: 0, last_position: 0 });
+}
+
+// halfassed bump allocator just to prove a point.
+impl Pool {
+    fn alloc(&mut self, layout: Layout) -> u16 {
+        let size = layout.size();
+        let align = layout.align();
+
+        let mut position = self.end + size_of::<u16>(); // space for skip marker
+        let alignmask = align - 1;
+        if position & alignmask != 0 {
+            position += ((position ^ alignmask) & alignmask) + 1;
+        }
+        let new_end = position + size;
+        assert!(new_end < self.data.len(), "ShortPointer memory pool exhausted");
+        // using most significant bit as whether it's been freed
+        assert!(position - self.last_position < 32768, "Alignment caused 32KiB limit overrun");
+
+        let skip_marker = 0x8000_u16.to_ne_bytes();
+        self.data[position - 2..position].copy_from_slice(&skip_marker);
+        self.end = new_end;
+        if self.begin == 0 {
+            self.begin = position;
+        }
+        self.last_position = position;
+        position as u16
+    }
+
+    fn dealloc(&mut self, sptr: u16) {
+        let mut position = sptr as usize;
+        unsafe {
+            let skip_marker_ptr = self.data.as_mut_ptr().add(position - 2) as *mut u16;
+            *skip_marker_ptr &= 0x7fff;
+            // see how much we can free
+            let mut skip_marker = *skip_marker_ptr;
+            let update_begin = self.begin == position;
+            while skip_marker & 0x8000 == 0 {
+                if skip_marker == 0 {
+                    // last object, we can reclaim memory back to sptr
+                    self.end = sptr as usize;
+                    break;
+                } else {
+                    position += skip_marker as usize;
+                }
+                skip_marker = *(self.data.as_ptr().add(position - 2) as *const u16);
+            }
+            if update_begin {
+                self.begin = position;
+            }
+        }
+        if self.begin == self.end {
+            // all objects freed, we can reset the entire state
+            self.begin = 0;
+            self.end = 0;
+            self.last_position = 0;
+        }
+    }
+}
+
+impl std::fmt::Debug for Pool {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f, "Pool {{ data: [_; 65536], begin: {}, end: {}, last_position: {} }}",
+            self.begin,
+            self.end,
+            self.last_position,
+        )
+    }
+}
+
+pub struct ShortPointer<T>(u16, PhantomData<T>);
+
+impl<T> Drop for ShortPointer<T> {
+    fn drop(&mut self) {
+        POOL.lock().unwrap().dealloc(self.0)
+    }
+}
+
+impl<T> ShortPointer<T> {
+    fn deref_inner(&self) -> &mut T {
+        unsafe {
+            ((((POOL.lock().unwrap().data.as_mut_ptr()) as usize) + (self.0 as usize)) as *mut T).as_mut().unwrap()
+        }
+    }
+
+    pub fn new(obj: T) -> Self {
+        let layout = Layout::for_value(&obj);
+        let mut sptr = Self(POOL.lock().unwrap().alloc(layout), PhantomData::default());
+        let uninitialized = core::mem::replace(sptr.deref_mut(), obj);
+        core::mem::forget(uninitialized);
+        sptr
+    }
+}
+
+impl<T> Deref for ShortPointer<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        self.deref_inner()
+    }
+}
+
+impl<T> DerefMut for ShortPointer<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.deref_inner()
+    }
+}
+
+#[allow(unused_must_use)] // dbg! statements
+fn main() {
+    dbg!(POOL.lock().unwrap());
+    {
+        let foo = ShortPointer::new([1, 2, 3, 4, 5]);
+        dbg!(POOL.lock().unwrap());
+        {
+            let bar = ShortPointer::new(0x900dCafeDa7e5_u64);
+            dbg!(POOL.lock().unwrap());
+            println!("{} | {:?} | {:x}", size_of_val(&foo), *foo, *bar);
+        }
+        dbg!(POOL.lock().unwrap());
+    }
+    dbg!(POOL.lock().unwrap());
+}