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pub struct EmbeddingStr([usize; 2]);
const STR_INNER_SIZE: usize = std::mem::size_of::<EmbeddingStr>();
#[derive(Eq, PartialEq, Debug)]
pub enum EmbeddingStrMode {
Boxed,
Embedded,
}
impl EmbeddingStr {
pub fn mode(&self) -> EmbeddingStrMode {
if (self.0[0] & 1) == 0 {
EmbeddingStrMode::Boxed
} else {
EmbeddingStrMode::Embedded
}
}
pub fn as_str(&self) -> &str {
match self.mode() {
EmbeddingStrMode::Boxed => unsafe { std::mem::transmute(self.0) },
EmbeddingStrMode::Embedded => {
let embedded = unsafe { &*(self as *const Self as *const EmbeddedStr) };
embedded.as_str()
}
}
}
}
impl Drop for EmbeddingStr {
fn drop(&mut self) {
match self.mode() {
EmbeddingStrMode::Boxed => {
let _boxed: Box<str> = unsafe { std::mem::transmute(self.0) };
}
EmbeddingStrMode::Embedded => {
}
}
}
}
impl std::fmt::Display for EmbeddingStr {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
std::fmt::Display::fmt(self.as_str(), f)
}
}
impl std::fmt::Debug for EmbeddingStr {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "{:?}({:?})", self.mode(), self.as_str())
}
}
impl From<String> for EmbeddingStr {
#[inline]
fn from(s: String) -> Self {
const MAX_SIZE: usize = STR_INNER_SIZE - 1;
Self(if (1..=MAX_SIZE).contains(&s.len()) {
let embedded = EmbeddedStr::from(s.as_str());
unsafe { std::mem::transmute(embedded) }
} else {
let boxed = s.into_boxed_str();
unsafe { std::mem::transmute(boxed) }
})
}
}
impl From<&'static str> for EmbeddingStr {
#[inline]
fn from(s: &str) -> Self {
const MAX_SIZE: usize = STR_INNER_SIZE - 1;
Self(if (1..=MAX_SIZE).contains(&s.len()) {
let embedded = EmbeddedStr::from(s);
unsafe { std::mem::transmute(embedded) }
} else {
let boxed = s.to_owned().into_boxed_str();
unsafe { std::mem::transmute(boxed) }
})
}
}
struct EmbeddedStr([u8; STR_INNER_SIZE]);
impl EmbeddedStr {
fn as_str(&self) -> &str {
let ptr;
let encoded_len;
#[cfg(target_endian = "little")]
unsafe {
ptr = self.0.as_ptr().offset(1);
encoded_len = (*self.0.as_ptr()) as usize;
}
#[cfg(target_endian = "big")]
unsafe {
ptr = self.0.as_ptr();
encoded_len = *self.0.as_ptr().offset(STR_INNER_SIZE - 1);
}
let pair = [ptr as usize, encoded_len as usize >> 1];
unsafe { std::mem::transmute(pair) }
}
}
impl From<&str> for EmbeddedStr {
#[inline]
fn from(s: &str) -> Self {
debug_assert!(s.len() < STR_INNER_SIZE);
let mut new = std::mem::MaybeUninit::<Self>::uninit();
let mut_ptr = new.as_mut_ptr() as *mut u8;
let encoded_len = (s.len() << 1) as u8 + 1;
unsafe {
#[cfg(target_endian = "little")]
{
std::ptr::copy_nonoverlapping(s.as_ptr(), mut_ptr.offset(1), s.len());
mut_ptr.write(encoded_len);
}
#[cfg(target_endian = "big")]
{
std::ptr::copy_nonoverlapping(s.as_ptr(), mut_ptr, s.len());
mut_ptr.offset(MAX_SIZE).write(encoded_len);
}
new.assume_init()
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_lifecycle() {
assert_eq!(STR_INNER_SIZE, 16);
let s = EmbeddingStr::from("");
assert_eq!(s.mode(), EmbeddingStrMode::Embedded);
assert_eq!(s.as_str(), "");
let s = EmbeddingStr::from("a");
assert_eq!(s.mode(), EmbeddingStrMode::Embedded);
assert_eq!(s.as_str(), "a");
let s = EmbeddingStr::from("abcdxyz01");
assert_eq!(s.mode(), EmbeddingStrMode::Embedded);
assert_eq!(s.as_str(), "abcdxyz01");
let s = EmbeddingStr::from("123456789012345");
assert_eq!(s.mode(), EmbeddingStrMode::Embedded);
assert_eq!(s.as_str(), "123456789012345");
let s = EmbeddingStr::from("1234567890123456");
assert_eq!(s.mode(), EmbeddingStrMode::Boxed);
assert_eq!(s.as_str(), "1234567890123456");
let slong = EmbeddingStr::from("something longer than 15 byets");
assert_eq!(slong.mode(), EmbeddingStrMode::Boxed);
assert_eq!(slong.as_str(), "something longer than 15 byets");
}
#[test]
fn test_format() {
let s = EmbeddingStr::from("a");
assert_eq!(format!("{}", s), "a".to_owned());
assert_eq!(format!("{:?}", s), "Embedded(\"a\")".to_owned());
let s = EmbeddingStr::from("1234567890123456");
assert_eq!(format!("{}", s), "1234567890123456".to_owned());
assert_eq!(format!("{:?}", s), "Boxed(\"1234567890123456\")".to_owned());
}
}