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//! `in_place_string_map` is a library for doing string manipulation in place. //! //! Normally in Rust, if you wanted to handle escapes, for example, you'd need to either map to a //! new String, causing allocations, or do `.remove` and `.insert` calls on a String, which //! wouldn't cause reallocations if you never grow the String, but *would* cause slowdowns on large //! strings due to the need to backshift elements on every item. //! //! Here, you can just do //! //! ```rust //! use in_place_string_map::MapInPlace; //! //! fn decode_percent(s: &mut str) -> &mut str { //! let mut m = MapInPlace::new(s); //! //! while let Some(c) = m.pop() { //! match c { //! '%' => { //! let num = m.pop_chars(2).expect("not enough chars"); //! let n = u8::from_str_radix(num, 16).expect("invalid hex"); //! m.push(n as char).expect("no more capacity"); //! } //! _ => { //! m.push(c).expect("no more capacity"); //! } //! } //! } //! //! m.into_mapped() //! } //! //! let mut input = String::from("%54r%61ns %52igh%74%73%21"); //! //! assert_eq!(decode_percent(&mut input), "Trans Rights!"); //! ``` //! //! ## Safety //! //! This library takes care to ensure that the input string is always left in a valid state. //! //! Since [`core::mem::forget`] is safe, no code can soundly rely on users to call destructors. The //! contents of the original borrowed string after any operation is left unspecified generally, but //! it is guaranteed to always be valid UTF-8. // https://twitter.com/reduct_rs/status/1387153973010829315 #![cfg_attr(all(not(test)), no_std)] #![warn(clippy::all)] #![warn(clippy::pedantic)] #![warn(clippy::nursery)] #![warn(clippy::cargo)] #![warn(missing_docs)] /// Safety: Identical to [`core::str::from_utf8_unchecked`], only has a debug assertion that it is /// indeed valid UTF-8. unsafe fn from_utf8_unchecked(input: &[u8]) -> &str { debug_assert!( core::str::from_utf8(input).is_ok(), "{:?} was invalid UTF-8", input ); core::str::from_utf8_unchecked(input) } /// Safety: Identical to [`core::str::from_utf8_unchecked_mut`], only has a debug assertion that it is /// indeed valid UTF-8. unsafe fn from_utf8_unchecked_mut(input: &mut [u8]) -> &mut str { debug_assert!( core::str::from_utf8(input).is_ok(), "{:?} was invalid UTF-8", input ); core::str::from_utf8_unchecked_mut(input) } #[derive(Debug)] /// A mutable reference to a [`str`] that allows for in-place pushes and pops while maintaining /// valid UTF-8 at all times. /// /// Semantically, this creates 2 buffers, a "mapped" buffer, and an "unmapped" buffer. /// /// The mapped buffer starts off empty, and the unmapped buffer starts off with the full contents /// of the `&mut str` given in [`MapInPlace::new`]. /// /// The mapped buffer can be pushed to, and this will append to the end of it. The unmapped buffer /// can be popped from, and this will pop from the start of it. /// /// The size of the mapped buffer, plus the size of the unmapped buffer, can never be bigger than /// the original string size, and you will get errors when you go to push if this is the case. /// /// However, it's free to be smaller, in which case there will be some area in the middle with /// unspecified contents. It will still be valid UTF-8 though, to ensure safety. pub struct MapInPlace<'a> { /// Invariants: /// /// * This must always be valid UTF-8 when the [`MapInPlace`] is exposed to code /// outside this crate. buf: &'a mut [u8], /// Invariants: /// /// * `0..mapped_head` must always be in bounds for [`MapInPlace.buf`] /// * `0..mapped_head` must always be valid UTF-8 (when exposed to code outside this crate) mapped_head: usize, /// Invariants: /// /// * `unmapped_head` must be in bounds /// * `unmapped_head..` always in bounds for [`MapInPlace.buf`] unmapped_head: usize, } /// Checks that `byte` is the first byte in a UTF-8 code point /// sequence /// /// Based of std library [`str::is_char_boundary`] #[inline] // we intentionally wrap here #[allow(clippy::cast_possible_wrap)] const fn is_char_start(byte: u8) -> bool { // This is bit magic equivalent to: b < 128 || b >= 192 (byte as i8) >= -0x40 } /// An error indicating that there was no capacity remaining when a push was attempted. /// /// You should [`MapInPlace::pop`] more characters in order to make room for a push. /// /// Keep in mind that not every UTF-8 character is the same size, so you may get this error even if /// you always have more pops than pushes, if you are pushing larger characters. /// /// # Examples /// /// ```rust /// use in_place_string_map::{MapInPlace, NoCapacityError}; /// /// let mut string = String::from("$"); /// let mut map = MapInPlace::new(&mut string); /// /// map.pop(); /// /// let error: NoCapacityError = map.push('£').unwrap_err(); /// ``` #[derive(Debug)] pub struct NoCapacityError(()); // When zeroing sections of the string to ensure valid UTF-8, anything smaller than this will just // be zeroed with a .fill(), rather than trying to stop early. // // Anything larger will try to stop as early as it can and still ensure valid UTF-8 const PARTIAL_ZERO_SIZE: usize = 32; impl<'a> MapInPlace<'a> { /// Creates a new `MapInPlace`, used to do in-place string conversions without allocating a new /// buffer. /// /// ```rust /// let mut string = String::from("Hello, World!"); /// let mut map = in_place_string_map::MapInPlace::new(&mut string); /// ``` pub fn new(s: &'a mut str) -> Self { // Safety: // // When this borrow ends (MapInPlace is dropped/forgotten), the string must be valid UTF-8. // We also need to never expose invalid UTF-8 to the user. let buf = unsafe { s.as_bytes_mut() }; MapInPlace { buf, mapped_head: 0, unmapped_head: 0, } } /// Returns the mapped portion of the string. /// /// ```rust /// let mut string = String::from("Hello, World!"); /// let mut map = in_place_string_map::MapInPlace::new(&mut string); /// /// assert_eq!(map.mapped(), ""); /// /// map.pop_chars(6); /// /// map.push_str("Yellow"); /// /// assert_eq!(map.mapped(), "Yellow"); /// ``` #[must_use] pub fn mapped(&self) -> &str { debug_assert!(self.buf.get(0..self.mapped_head).is_some()); // Safety: self.mapped_head has the invariant that it is always in bounds of `buf`. let bytes = unsafe { self.buf.get_unchecked(0..self.mapped_head) }; unsafe { from_utf8_unchecked(bytes) } } /// Consumes this [`MapInPlace`] and returns the mapped slice of the original string with the /// original lifetime. /// /// This is useful for when you want the lifetime of the returned string to outlive the /// instance of [`MapInPlace`]. /// /// ```rust /// fn push_yellow(s: &mut str) -> &mut str { /// let mut map = in_place_string_map::MapInPlace::new(s); /// map.pop_chars(6); /// map.push_str("Yellow"); /// map.into_mapped() /// } /// /// let mut string = String::from("Hello, World!"); /// let result = push_yellow(&mut string); /// assert_eq!(result, "Yellow"); /// ``` /// /// You cannot simply use [`MapInPlace::mapped`] because that will return a reference that /// can't outlive the original [`MapInPlace`] /// /// ```compile_fail /// fn push_yellow(s: &mut str) -> &str { /// let mut map = in_place_string_map::MapInPlace::new(s); /// map.pop_chars(6); /// map.push_str("Yellow"); /// /// // cannot return value referencing local variable `map` /// map.mapped() /// } /// /// let mut string = String::from("Hello, World!"); /// let result = push_yellow(&mut string); /// assert_eq!(result, "Yellow"); /// ``` #[must_use] pub fn into_mapped(self) -> &'a mut str { let mapped_head = self.mapped_head; &mut self.into_all()[0..mapped_head] } /// Returns the not yet mapped portion of the string. /// /// ```rust /// let mut string = String::from("Hello, World!"); /// let mut map = in_place_string_map::MapInPlace::new(&mut string); /// /// assert_eq!(map.unmapped(), "Hello, World!"); /// /// map.pop_chars(5); /// /// assert_eq!(map.unmapped(), ", World!"); /// ``` #[must_use] pub fn unmapped(&self) -> &str { &self.all()[self.unmapped_head..] } /// Consumes this [`MapInPlace`] and returns the unmapped slice of the original string with the /// original lifetime. /// /// This is useful for when you want the lifetime of the returned string to outlive the /// instance of [`MapInPlace`]. /// /// ```rust /// fn pop_five(s: &mut str) -> &mut str { /// let mut map = in_place_string_map::MapInPlace::new(s); /// map.pop_chars(5); /// map.into_unmapped() /// } /// /// let mut string = String::from("Hello, World!"); /// let result = pop_five(&mut string); /// assert_eq!(result, ", World!"); /// ``` /// /// You cannot simply use [`MapInPlace::mapped`] because that will return a reference that /// can't outlive the original [`MapInPlace`] /// /// ```compile_fail /// fn pop_five(s: &mut str) -> &str { /// let mut map = in_place_string_map::MapInPlace::new(s); /// map.pop_chars(5); /// /// // cannot return value referencing local variable `map` /// map.unmapped() /// } /// /// let mut string = String::from("Hello, World!"); /// let result = pop_five(&mut string); /// assert_eq!(result, ", World!"); /// ``` #[must_use] pub fn into_unmapped(self) -> &'a mut str { let unmapped_head = self.unmapped_head; &mut self.into_all()[unmapped_head..] } #[must_use] fn all(&self) -> &str { // Safety: self.buf is always valid UTF-8 if the user has access to it, so this is safe. unsafe { from_utf8_unchecked(&self.buf[..]) } } #[must_use] fn into_all(self) -> &'a mut str { // Safety: self.buf is always valid UTF-8 if the user has access to it, so this is safe. unsafe { from_utf8_unchecked_mut(self.buf) } } /// Pushes a character onto the end of the mapped portion. /// /// ```rust /// let mut string = String::from("Hello!"); /// let mut map = in_place_string_map::MapInPlace::new(&mut string); /// map.pop_chars(6); /// /// assert_eq!(map.mapped(), ""); /// map.push('£').unwrap(); /// map.push('1').unwrap(); /// map.push('.').unwrap(); /// map.push('2').unwrap(); /// map.push('5').unwrap(); /// /// assert_eq!(map.mapped(), "£1.25"); /// /// map.push('5').unwrap_err(); /// /// assert_eq!(map.mapped(), "£1.25"); /// ``` /// /// # Errors /// /// * [`NoCapacityError`]: If there is not enough room to fit `ch` being pushed. pub fn push(&mut self, ch: char) -> Result<(), NoCapacityError> { let mut tempbuf = [0_u8; 4_usize]; let sbytes = ch.encode_utf8(&mut tempbuf); self.push_str(sbytes)?; Ok(()) } /// Pushes a string onto the end of the mapped portion. If the string is too long, an error is /// returned, and no changes will be made to the input. /// /// ```rust /// let mut string = String::from("Hello!"); /// let mut map = in_place_string_map::MapInPlace::new(&mut string); /// map.pop_chars(6); /// /// assert_eq!(map.mapped(), ""); /// /// map.push_str("This string is *far* too long!").unwrap_err(); /// /// assert_eq!(map.mapped(), ""); /// /// map.push_str("Short").unwrap(); /// /// assert_eq!(map.mapped(), "Short"); /// /// map.push_str(".").unwrap(); /// /// assert_eq!(map.mapped(), "Short."); /// ``` /// /// # Errors /// /// * [`NoCapacityError`]: If there is not enough room to fit `s` being pushed. pub fn push_str(&mut self, s: &str) -> Result<(), NoCapacityError> { let bytes = s.as_bytes(); if self.buf.len() < self.mapped_head + bytes.len() { return Err(NoCapacityError(())); } if self.unmapped_head < self.mapped_head + bytes.len() { return Err(NoCapacityError(())); } // Safety: self.buf must be valid UTF-8 once this ends. // // It consists of: // ..mapped_head, which is a `str` and we only push valid strs onto it // mapped_head..unmapped_head, which consists of the previous contents of the str // where an unspecified amount is zeroed // unmapped_head.., which is a `str` and we only pop chars from it self.buf[self.mapped_head..self.mapped_head + bytes.len()].copy_from_slice(bytes); self.mapped_head += bytes.len(); debug_assert!(self.mapped_head <= self.unmapped_head); let area_to_zero = &mut self.buf[self.mapped_head..self.unmapped_head]; if area_to_zero.len() > PARTIAL_ZERO_SIZE { for byte in area_to_zero { if is_char_start(*byte) { break; } *byte = 0; } } else { area_to_zero.fill(0); } Ok(()) } /// Pops a character from the start of the unmapped portion /// /// Will return [`None`] if there are no more characters left to pop. /// /// ```rust /// let mut string = String::from("Hi!"); /// let mut map = in_place_string_map::MapInPlace::new(&mut string); /// /// assert_eq!(map.pop(), Some('H')); /// assert_eq!(map.unmapped(), "i!"); /// /// assert_eq!(map.pop(), Some('i')); /// assert_eq!(map.unmapped(), "!"); /// /// assert_eq!(map.pop(), Some('!')); /// assert_eq!(map.unmapped(), ""); /// /// assert_eq!(map.pop(), None); /// assert_eq!(map.unmapped(), ""); /// ``` pub fn pop(&mut self) -> Option<char> { self.pop_chars(1) .map(|x| x.chars().next().expect("pop_chars did not pop a char")) } /// Pops `n` characters from the start of the unmapped portion. /// /// Note how this pops in terms of *characters*, not bytes. /// /// If `n` is 0 then will always return [`None`] /// /// If this fails because there are not enough characters then will return [`None`], and no /// changes will have been made to `self`. /// /// ```rust /// let mut string = String::from("A £3.00 sandwich"); /// let mut map = in_place_string_map::MapInPlace::new(&mut string); /// /// assert_eq!(map.pop_chars(0), None); /// assert_eq!(map.pop_chars(2), Some("A ")); /// assert_eq!(map.pop_chars(5), Some("£3.00")); /// /// // Nothing is done if you try to pop too many characters /// assert_eq!(map.pop_chars(10), None); /// /// assert_eq!(map.pop_chars(9), Some(" sandwich")); /// ``` pub fn pop_chars(&mut self, n: usize) -> Option<&str> { if n == 0 { return None; } let (idx, c) = self.unmapped().char_indices().nth(n - 1)?; let to_take = idx + c.len_utf8(); let s = &self.buf[self.unmapped_head..self.unmapped_head + to_take]; self.unmapped_head += to_take; // Safety of from_utf8_unchecked: // // We slice the buffer starting at the original value for self.unmapped_head, which must be // on a char boundary, this is an invariant of the type for safety (otherwise, // self.unmapped() would create invalid UTF-8). // // self.unmapped_head is incremented by to_take, which also leaves it on a char boundary, // because `idx` is on a char boundary when looked at relative to self.unmapped_head (since // we got it from char_indices on self.unmapped()) // // We add c.len_utf8() on top, which makes to_take on a char boundary relative to // self.unmapped_head. // // Therefore, `s` is always valid UTF-8. // We also need to keep the invariant that self.unmapped_head is always on a char boundary. // But we already know it must be, since we're adding `to_take` to it, which the argument // above proves is on a char boundary. unsafe { Some(from_utf8_unchecked(s)) } } } // Source: https://github.com/rust-lang/cargo/issues/383#issuecomment-720873790 #[cfg(doctest)] mod test_readme { macro_rules! external_doc_test { ($x:expr) => { #[doc = $x] extern "C" {} }; } external_doc_test!(include_str!("../README.md")); } #[cfg(test)] mod tests { use super::*; #[test] fn cannot_remove_from_end() { let mut initial = "㉉".to_string(); let mut mapper = MapInPlace::new(&mut initial); mapper.pop_chars(3); } }