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//! A circular buffer for strings and traits for in-place string transforms. //! //! This crate provides two key types: the [`CharCircle`] struct and the //! [`StringTransform`] trait. The `CharCircle` is a circular buffer //! specialized for UTF-8 strings, and the `StringTransform` trait builds upon //! it to provide a character-oriented API for in-place string transformations. //! In short, `StringTransform` allows you to implement transformations as //! iterator adaptors, with copy-on-write optimizations in mind. //! //! The `CharCircle` uses internal mutability. This enables its contents to be //! consumed by an external iterator, [`Chars`]. As a consequence, the //! `CharCircle` is not `Sync`. It is implemented as a `RefCell` around the //! [`RawCircle`], which has a nearly identical API, uses external mutability, //! is thread-safe, and does not provide a consuming iterator. //! //! The `StringTransform` trait is implemented by factories of iterator //! adaptors. For simple cases, the [`SimpleTransform`] trait provides an //! alternative that is implemented directly by the adaptor. //! //! //! Example: To Uppercase //! --------------------------------------------------------------------------- //! //! Transforms which don't require configuration are most easily implemented //! with `SimpleTransform`. //! //! Here we implement an uppercase transform: //! //! ``` //! use char_circle::{SimpleTransform, Chars}; //! //! // Step 1: Define the transform as an iterator adaptor. //! struct ToUpper<I>(I); //! //! impl<I> Iterator for ToUpper<I> where I: Iterator<Item=char> { //! type Item = char; //! fn next(&mut self) -> Option<char> { //! self.0.next().map(|ch| ch.to_ascii_uppercase()) //! } //! } //! //! // Step 2: Define a constructor for the adaptor with `SimpleTransform`. //! impl<'a> SimpleTransform<'a> for ToUpper<Chars<'a>> { //! fn transform_chars(chars: Chars<'a>) -> Self { //! ToUpper(chars) //! } //! } //! //! // Step 3: Profit! //! let s = "can you hear me in the back?"; //! let s = ToUpper::transform(s); //! assert_eq!(&s, "CAN YOU HEAR ME IN THE BACK?"); //! ``` //! //! //! Example: Caesar Cipher //! --------------------------------------------------------------------------- //! //! Transforms that need to be configured should define a factory which //! implements `StringTransform`. //! //! Here we implement a Caesar cipher configured with its key: //! //! ``` //! use char_circle::{StringTransform, Chars}; //! //! // Step 1: Define the transform as an iterator adaptor. //! struct CaesarCipherIter<I> { //! inner: I, //! key: i32, //! } //! //! impl<I> Iterator for CaesarCipherIter<I> where I: Iterator<Item=char> { //! type Item = char; //! fn next(&mut self) -> Option<char> { //! let plaintext = self.inner.next()?; //! let ciphertext = plaintext as i32 + self.key; //! let ciphertext = std::char::from_u32(ciphertext as u32).unwrap(); //! Some(ciphertext) //! } //! } //! //! // Step 2: Define a factory for the adaptor with `StringTransform`. //! struct CaesarCipher(i32); //! //! impl<'a> StringTransform<'a> for CaesarCipher { //! type Iter = CaesarCipherIter<Chars<'a>>; //! fn transform_chars(&self, chars: Chars<'a>) -> Self::Iter { //! CaesarCipherIter { inner: chars, key: self.0 } //! } //! } //! //! // Step 3: Profit! //! let encoder = CaesarCipher(8); //! let decoder = CaesarCipher(-8); //! let plaintext = "Veni, vidi, vici"; //! let ciphertext = encoder.transform(plaintext); //! assert_eq!(&ciphertext, "^mvq4(~qlq4(~qkq"); //! let plaintext = decoder.transform(ciphertext); //! assert_eq!(&plaintext, "Veni, vidi, vici"); //! ``` use std::borrow::Cow; use std::cell::RefCell; use std::char; use std::cmp; use std::io::{self, Read, Write}; use std::marker::PhantomData; use std::mem; use std::ptr; // RawCircle // --------------------------------------------------------------------------- /// A thread-safe version of [`CharCircle`]. /// /// The API of this buffer is almost identical to `CharCircle` except that it /// uses external mutability, and it does not provide a means to consume its /// characters from an external iterator. #[derive(Debug, Default, Clone)] pub struct RawCircle { buf: Vec<u8>, // The backing storage. len: usize, // The number of used bytes in the buffer. n_chars: usize, // The number of characters in the buffer. read: usize, // Index of the read head. May equal the capacity. write: usize, // Index of the write head. May equal the capacity. } impl RawCircle { /// Construct a new `RawCircle` using a string as the initial buffer. pub fn new(s: String) -> RawCircle { let n_chars = s.chars().count(); let buf = s.into_bytes(); let len = buf.len(); RawCircle { buf, len, n_chars, read: 0, write: 0 } } /// Construct a new, empty `RawCircle`. pub fn empty() -> RawCircle { RawCircle::default() } /// The number of UTF-8 bytes in the buffer. pub fn len(&self) -> usize { self.len } /// The number of characters in the buffer. pub fn n_chars(&self) -> usize { self.n_chars } /// The number of bytes the buffer can hold before reallocating. /// /// This refers to the length of the backing vector. That vector may have /// additional capacity allocated to it that is not reported by this method. pub fn capacity(&self) -> usize { self.buf.len() } /// Reallocate the buffer if it is full. fn grow_if_full(&mut self) { // SAFTEY: There are several safety concerns here. // // 1. The read and write heads may equal the capacity when this method is // called (but may not otherwise be out of bounds). We move the read head // to 0 if it is at the capacity, and we prove the write head is in bounds // before we use it. // // 2. We move data around using `ptr::copy`. We must maintain that the affected // regions are in bounds and that the destination is trash bytes. // // 3. We must properly maintain the read and write heads. When we return, the // [BACK] and [FRONT] areas must be valid UTF-8, and the read head may equal // the capacity. The length should never change. // // It is safe to directly set the length of the buffer to the capacity. It is // in-bounds and there are no initialization or drop-safety concerns with // plain-old bytes. unsafe { let old_cap = self.capacity(); if self.read == old_cap { self.read = 0 }; if self.read == 0 { self.write = self.len }; if old_cap == 0 { // We have no capacity, so add some. // Only hit the allocator if the backing vector has no extra capacity. debug_assert!(self.read == 0); debug_assert!(self.write == 0); if self.buf.capacity() == 0 { self.buf.reserve(1); } let new_cap = self.buf.capacity(); self.buf.set_len(new_cap); } else if self.len == old_cap { // If the backing vector has excess capacity, we just use that. // Otherwise we ask the allocator to double our capacity. if old_cap == self.buf.capacity() { self.buf.reserve(old_cap); } let new_cap = self.buf.capacity(); self.buf.set_len(new_cap); if self.write == self.read { // The memory areas are [BACK, FRONT, TMP]. // The read and write heads are between [BACK] and [FRONT]. // Rearange: [BACK, FRONT, TMP] -> [BACK, TMP, FRONT]. let front_size = old_cap - self.read; let new_read = new_cap - front_size; let src = self.buf.get_unchecked_mut(self.read) as *mut u8; let dest = self.buf.get_unchecked_mut(new_read) as *mut u8; ptr::copy(src, dest, front_size); self.read = new_read; } else { debug_assert!(self.read == 0); debug_assert!(self.write == self.len); } } } } /// Return the number of UTF-8 bytes of the next character. fn peek_char_len(&self) -> Option<usize> { // SAFETY: It is safe to use an unchecked get at the read head when the // length is non-zero. if self.len == 0 { return None }; let byte = unsafe { self.buf.get_unchecked(self.read) }; let reverse = byte ^ 0b11111111; let leading_ones = reverse.leading_zeros(); match leading_ones { 0 => Some(1), 1 => unreachable!("invalid utf-8"), 2 => Some(2), 3 => Some(3), 4 => Some(4), _ => unreachable!("invalid utf-8"), } } /// Read a single byte from the buffer. /// /// It is unsafe to partially read a multibyte UTF-8 character. /// /// This method DOES update `len` but DOES NOT update `n_chars`. /// Callers MUST update `n_chars`. unsafe fn read_byte(&mut self) -> Option<u8> { // SAFTEY: The read head may equal the capacity, // but may not otherwise be out of bounds. if self.len == 0 { return None }; if self.read == self.capacity() { self.read = 0 }; let byte = self.buf.get_unchecked(self.read); self.read += 1; self.len -= 1; Some(*byte) } /// Read the next character in the buffer. pub fn read_char(&mut self) -> Option<char> { // SAFETY: We MUST read an entire character, and we MUST update `n_chars`. unsafe { let byte = self.read_byte()?; let reverse = byte ^ 0b11111111; let leading_ones = reverse.leading_zeros(); let mask = 0b11111111 >> leading_ones; let mut ch = (byte & mask) as u32; let n_additional_bytes = leading_ones.saturating_sub(1); for _ in 0..n_additional_bytes { let byte = self.read_byte().unwrap(); let byte = byte & 0b00111111; ch = (ch << 6) | byte as u32; } self.n_chars -= 1; Some(char::from_u32_unchecked(ch)) } } /// Read bytes from this circle into a buffer. /// /// This method will only ever read complete UTF-8 characters. It returns the /// number of bytes read; it never returns an error. /// /// This is the implementation of [`std::io::Read`] for `RawCircle`. pub fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> { let max_len = cmp::min(buf.len(), self.len()); let mut len = 0; loop { match self.peek_char_len() { None => return Ok(len), Some(n) => { if len + n <= max_len { // SAFETY: We MUST read an entire character, and we MUST update `n_chars`. for i in 0..n { buf[len+i] = unsafe { self.read_byte().unwrap() }; } self.n_chars -= 1; len += n; } else { return Ok(len); } } } } } /// Read bytes from this circle into a buffer. /// /// This method is equivalent to [`RawCircle::read`] except the return value /// is the buffer cast to a `&str`. pub fn read_str<'a>(&mut self, buf: &'a mut [u8]) -> &'a str { // SAFETY: It is safe to cast to a string because `self.read` only ever reads // valid UTF-8. let n = self.read(buf).unwrap(); let str = &buf[..n] as *const [u8] as *const str; return unsafe { &*str } } /// Write a single byte into the buffer. /// /// It is unsafe to write invalid UTF-8. /// /// This method DOES update `len` but DOES NOT update `n_chars`. /// Callers MUST update `n_chars`. unsafe fn write_byte(&mut self, byte: u8) { // SAFTEY: The write head may equal the capacity, // but may not otherwise be out of bounds. self.grow_if_full(); if self.write == self.capacity() { self.write = 0 }; let byte_ref = self.buf.get_unchecked_mut(self.write); *byte_ref = byte; self.write += 1; self.len += 1; } /// Write a character into the buffer. pub fn write_char(&mut self, ch: char) { // SAFETY: We MUST write an entire character, and we MUST update `n_chars`. unsafe { let mut utf8 = [0u8; 4]; for byte in ch.encode_utf8(&mut utf8).as_bytes() { self.write_byte(*byte) } self.n_chars += 1; } } /// Read bytes from a string into this buffer; /// /// This method will only ever write complete UTF-8 characters. It returns the /// number of bytes written. This method returns an error if the input is not /// valid UTF-8. /// /// This is the implementation of [`std::io::Write`] for `RawCircle`. pub fn write(&mut self, buf: &[u8]) -> io::Result<usize> { let mut len = 0; let mut tmp = [0u8; 4]; loop { // SAFETY: We must make sure the read is in-bounds to use `get_unchecked`. if len == buf.len() { return Ok(len) }; let first_byte = unsafe { *buf.get_unchecked(len) }; let reverse = first_byte ^ 0b11111111; let leading_ones = reverse.leading_zeros(); let n = match leading_ones { 0 => 1, 1 => return Err(io::Error::new(io::ErrorKind::InvalidData, "invalid UTF-8")), 2 => 2, 3 => 3, 4 => 4, _ => return Err(io::Error::new(io::ErrorKind::InvalidData, "invalid UTF-8")), }; tmp[0] = first_byte; for i in 1..n { // SAFETY: We must make sure the read is in-bounds to use `get_unchecked`. if len + i == buf.len() { return Ok(len) }; let byte = unsafe { *buf.get_unchecked(len + i) }; let reverse = byte ^ 0b11111111; let leading_ones = reverse.leading_zeros(); if leading_ones != 1 { return Err(io::Error::new(io::ErrorKind::InvalidData, "invalid UTF-8")); } tmp[i] = byte; } // SAFETY: We MUST write an entire character, and we MUST update `n_chars`. for i in 0..n { unsafe { self.write_byte(tmp[i]) }; } self.n_chars += 1; len += n; } } /// Read bytes from a string into this buffer; /// /// This method is equivalent to [`RawCircle::write`] except that it cannot return /// an error because the input is valid UTF-8. pub fn write_str(&mut self, buf: &str) -> usize { self.write(buf.as_bytes()).unwrap() } /// Rearange the contents so that the read head is at 0. fn realign(&mut self) { // SAFTEY: There are several safety concerns here. // // 1. The read and write heads may equal the capacity when this method is // called (but may not otherwise be out of bounds). We move the read head // to 0 if it is at the capacity, and we prove the write head is in bounds // before we use it. // // 2. We move data around using `ptr::copy`, `ptr::copy_nonoverlapping`, and // `ptr::swap_nonoverlapping`. We must maintain that all affected regions // are in bounds and non-overlapping when required. We must also maintain // that the destination of copys is trash bytes. // // 3. We must properly maintain the read and write heads. When we return, the // read head should be at 0, and the write head should be at `self.len`. // The range `read..write` must be valid UTF-8, and the write head may // equal the capacity. The length should never change. unsafe { if self.read == self.capacity() { self.read = 0 }; if self.read == 0 { self.write = self.len }; if self.len == 0 || self.read == 0 { // Nothing to do. } else if self.read < self.write { // The areas in memory are [LEFT, STR, RIGHT]. // [LEFT, STR, RIGHT] -> [STR, LEFT, RIGHT]. let src = self.buf.get_unchecked_mut(self.read) as *mut u8; let dest = self.buf.get_unchecked_mut(0) as *mut u8; ptr::copy(src, dest, self.len); } else { // SAFTEY: Note that the write head is in bounds in this case. debug_assert!(self.write < self.read); // The areas in memory are [BACK, TMP, FRONT]. [TMP] may be empty. // We are trying to get to [FRONT, BACK, TMP] where [FRONT, BACK] is our string. let mut back = (0, self.write); let mut tmp = (self.write, self.read); let mut front = (self.read, self.capacity()); let len = |bounds: (usize, usize)| bounds.1 - bounds.0; // [BACK, TMP, FRONT] -> [BACK, FRONT, TMP] let src = self.buf.get_unchecked_mut(front.0) as *mut u8; let dest = self.buf.get_unchecked_mut(tmp.0) as *mut u8; let count = len(front); ptr::copy(src, dest, count); front = (tmp.0, tmp.0 + count); tmp = (front.1, self.capacity()); // Iterativly move parts of [FRONT] to the correct position. // When we're done, we still have three areas [BACK, FRONT, TMP], // but they do not include the bytes that are already in place. while len(tmp) < len(back) { if len(front) < len(back) { // Say [BACK] = [B0, B1] where [B0] has the same size as [FRONT]. // [B0, B1, FRONT, TMP] -> [FRONT, B1, B0, TMP] let count = len(front); let b0 = (back.0, back.0 + count); let b1 = (back.0 + count, back.1); let x = self.buf.get_unchecked_mut(b0.0) as *mut u8; let y = self.buf.get_unchecked_mut(front.0) as *mut u8; ptr::swap_nonoverlapping(x, y, count); back = b1; } else { // Say [FRONT] = [F0, F1] where [F0] has the same size as [BACK]. // [BACK, F0, F1, TMP] -> [F0, BACK, F1, TMP] let count = len(back); let f0 = (front.0, front.0 + count); let f1 = (front.0 + count, front.1); let x = self.buf.get_unchecked_mut(f0.0) as *mut u8; let y = self.buf.get_unchecked_mut(back.0) as *mut u8; ptr::swap_nonoverlapping(x, y, count); back = f0; front = f1; } } if len(tmp) != 0 { // Say [TMP] = [T0, T1] where [T0] has the same size as [BACK]. // [BACK, FRONT, T0, T1] -> [T0, FRONT, BACK, T1] let src = self.buf.get_unchecked_mut(back.0) as *mut u8; let dest = self.buf.get_unchecked_mut(tmp.0) as *mut u8; let count = len(back); ptr::copy_nonoverlapping(src, dest, count); let t0 = (back.0, back.0 + count); back = (tmp.0, tmp.0 + count); // [T0, FRONT, BACK, T1] -> [FRONT, BACK, T0, T1] let src = self.buf.get_unchecked_mut(front.0) as *mut u8; let dest = self.buf.get_unchecked_mut(t0.0) as *mut u8; let count = len(front) + len(back); ptr::copy(src, dest, count); } else { // [BACK, FRONT, TMP] == [FRONT] debug_assert!(len(back) == 0); } } } // Fix the read and write heads. self.read = 0; self.write = self.len; } /// Unpack this circular buffer into a byte vector. pub fn into_vec(mut self) -> Vec<u8> { // SAFTEY: It is safe to directly set the length of the buffer to the length // of the content. It is in-bounds and there are no drop-safety concerns with // plain-old bytes. When the buffer has been realigned, the content will be // on the range `0..len`. self.realign(); let mut vec = self.buf; unsafe { vec.set_len(self.len) }; vec } /// Unpack this circular buffer into a string. pub fn into_string(self) -> String { // SAFTEY: The public API only allows valid UTF-8 to be read or written from // the buffer. Thus it is safe to cast to a string. let vec = self.into_vec(); unsafe { String::from_utf8_unchecked(vec) } } } impl From<String> for RawCircle { fn from(s: String) -> RawCircle { RawCircle::new(s) } } impl From<RawCircle> for String { fn from(c: RawCircle) -> String { c.into_string() } } impl From<RawCircle> for Vec<u8> { fn from(c: RawCircle) -> Vec<u8> { c.into_vec() } } impl Read for RawCircle { fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> { RawCircle::read(self, buf) } } impl Write for RawCircle { fn write(&mut self, buf: &[u8]) -> io::Result<usize> { RawCircle::write(self, buf) } fn flush(&mut self) -> io::Result<()> { Ok(()) } } // CharCircle // --------------------------------------------------------------------------- /// A circular buffer of characters. /// /// The reallocation policy of [`std::collections::VecDeque`] makes it a poor /// choice as a circular buffer for modifying strings in place. It reallocates /// early (!) and when it does, it places its contents in such a way that /// requires shuffling to convert it back into a string in the common case. /// This circular buffer provides a better alternative. /// /// This circular buffer will not allocate early and is optimized for the case /// when you read exactly the contents that were in the initial buffer. In that /// case, the read head is guaranteed to be at index 0 of the underlying /// buffer, making the conversion into a string trivial. /// /// Additionally, this circular buffer provides a `char` oriented interface, but /// uses UTF-8 internally, allowing it to operate directly on [`String`]s. #[derive(Debug, Default, Clone)] pub struct CharCircle { raw: RefCell<RawCircle>, } impl CharCircle { /// Construct a new `CharCircle` using a string as the initial buffer. pub fn new(s: String) -> CharCircle { CharCircle { raw: RefCell::new(RawCircle::new(s)) } } /// Construct a new, empty `CharCircle`. pub fn empty() -> CharCircle { CharCircle::default() } /// The number of UTF-8 bytes in the buffer. pub fn len(&self) -> usize { self.raw.borrow().len() } /// The number of characters in the buffer. pub fn n_chars(&self) -> usize { self.raw.borrow().n_chars() } /// The number of bytes the buffer can hold before reallocating. /// /// This refers to the length of the backing vector. That vector may have /// additional capacity allocated to it that is not reported by this method. pub fn capacity(&self) -> usize { self.raw.borrow().capacity() } /// Read the next character in the buffer. pub fn read_char(&self) -> Option<char> { self.raw.borrow_mut().read_char() } /// Read bytes from this circle into a buffer. /// /// This method will only ever read complete UTF-8 characters. It returns the /// number of bytes read; it never returns an error. /// /// This is the implementation of [`std::io::Read`] for `CharCircle`. pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> { self.raw.borrow_mut().read(buf) } /// Read bytes from this circle into a buffer. /// /// This method is equivalent to [`RawCircle::read`] except the return value /// is the buffer cast to a `&str`. pub fn read_str<'a>(&self, buf: &'a mut [u8]) -> &'a str { self.raw.borrow_mut().read_str(buf) } /// Write a character into the buffer. pub fn write_char(&self, ch: char) { self.raw.borrow_mut().write_char(ch) } /// Read bytes from a string into this buffer; /// /// This method will only ever write complete UTF-8 characters. It returns the /// number of bytes written. This method returns an error if the input is not /// valid UTF-8. /// /// This is the implementation of [`std::io::Write`] for `CharCircle`. pub fn write(&mut self, buf: &[u8]) -> io::Result<usize> { self.raw.borrow_mut().write(buf) } /// Read bytes from a string into this buffer; /// /// This method is equivalent to [`CharCircle::write`] except that it cannot return /// an error because the input is valid UTF-8. pub fn write_str(&self, buf: &str) -> usize { self.raw.borrow_mut().write_str(buf) } /// Unpack this circular buffer into a byte vector. pub fn into_vec(self) -> Vec<u8> { self.raw.into_inner().into_vec() } /// Unpack this circular buffer into a string. pub fn into_string(self) -> String { self.raw.into_inner().into_string() } /// Read characters from the buffer with an iterator. /// /// The returned iterator will read at most `n` characters and ensures that it /// has been exhausted upon drop. /// /// Calling `next` on the iterator is equivalent to calling `read_char` on /// this buffer. pub fn take_chars(&self, n: usize) -> Chars { Chars::new(&self, n) } /// Read the current characters from the buffer with an iterator. /// /// The returned iterator will read at most `n` characters, where `n` is the /// number of characters currently in the buffer, and ensures that it has been /// exhausted upon drop. /// /// Calling `next` on the iterator is equivalent to calling `read_char` on /// this buffer. /// /// This is equivalent to calling [`CharCircle::take_chars`] with the current /// number of characters in the buffer. In particular, interleaving calls to /// `read_char` and `write_char` on the buffer with calls to `next` on the /// iterator may cause the iterator to consume characters that were not in the /// buffer at the time it was created. pub fn take_current_chars(&self) -> Chars { self.take_chars(self.n_chars()) } } impl From<String> for CharCircle { fn from(s: String) -> CharCircle { CharCircle::new(s) } } impl From<CharCircle> for String { fn from(c: CharCircle) -> String { c.into_string() } } impl From<CharCircle> for Vec<u8> { fn from(c: CharCircle) -> Vec<u8> { c.into_vec() } } impl Read for CharCircle { fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> { CharCircle::read(self, buf) } } impl Write for CharCircle { fn write(&mut self, buf: &[u8]) -> io::Result<usize> { CharCircle::write(self, buf) } fn flush(&mut self) -> io::Result<()> { Ok(()) } } // Chars // --------------------------------------------------------------------------- /// An iterator that consumes characters in a [`CharCircle`]. /// /// Calling `next` on this iterator is equivalent to calling /// [`CharCircle::read_char`] on the corresponding `CharCircle`. /// /// This iterator ensures that it has been exhausted upon drop. /// /// In the [`StringTransform`] API, this struct is an iterator over the /// characters of the string being transformed. #[derive(Debug)] pub struct Chars<'a> { circle: &'a CharCircle, n_chars: usize, } impl<'a> Chars<'a> { fn new(circle: &'a CharCircle, n_chars: usize) -> Chars<'a> { Chars{ circle, n_chars } } } impl<'a> Iterator for Chars<'a> { type Item = char; fn next(&mut self) -> Option<char> { if self.n_chars == 0 { None } else { let ch = self.circle.read_char()?; self.n_chars -= 1; Some(ch) } } } impl<'a> Drop for Chars<'a> { /// Ensure the iterator is exhausted. fn drop(&mut self) { for _ in self { } } } // StringTransform // --------------------------------------------------------------------------- /// A factory trait for in-place string transformations. /// /// This trait allows character-iterator adaptors to modify strings in-place. /// It is implemented by adaptor factories with a single required method, /// `transform_chars`, for constructing the adaptors. In return, this trait /// provides a method, `transform`, which can apply the transformation to /// both owned strings and string slices, without allocating when possible. /// /// The in-place operation works by treating the underlying string as a /// circular buffer. The transform reads characters from the front of the /// buffer and writes characters to the back of the buffer. Once the transform /// returns `None`, the unread characters are deleted and the circular buffer /// is cast back into a string. Note that the transformation cannot always be /// applied in place; if the transform ever returns more bytes than it has /// read, an allocation is required to grow the buffer. /// /// This trait supports copy-on-write optimizations. Implementors can override /// the [`StringTransform::will_modify`] method to short-circuit the transform. /// /// See [`SimpleTransform`] for a variant of this trait that is implemented /// directly by iterator adaptors. /// /// The lifetime `'a` refers to the lifetime of the `transform` method. pub trait StringTransform<'a> { /// The type after applying this transform to a [`Chars`] iterator. type Iter: Iterator<Item=char>; /// Transform the characters of a string. fn transform_chars(&self, chars: Chars<'a>) -> Self::Iter; /// A hint to short-circuit string transformations. /// /// If true, the string might be modified by the transform. If false, the /// string will definitely not be modified by the transform. /// /// Implementors may override this function to facilitate copy-on-write /// optimizations. The default implementation always returns true, which /// disables copy-on-write. #[allow(unused_variables)] fn will_modify(&self, val: &str) -> bool { return true } /// Transform a string in-place /// /// This method can operate on both `String` and `&str`. /// /// A new string may be allocated if the input is not owned or if the /// transformation needs to buffer more characters than the string has capacity. fn transform<'b, T: Into<Cow<'b, str>>>(&self, s: T) -> Cow<'b, str> { // SAFTEY: The lifetime `'a` refers to the circular buffer, but the borrow // checker isn't smart enough to know that, because `'a` is a type parameter // of the trait. Transumting the iterator to a new lifetime solves the issue. // The safety condition is that we must not let the iterator outlive the // buffer. This means that `old_chars` must be dropped before `circle`. // // Note that it is not safe for the buffer and iterator to be used from // different threads. `CharCircle` is implemented with a `RefCell`, ensuring // that it is not `Send` and that `Chars` is neither `Send` nor `Sync`. let s = s.into(); if self.will_modify(&s) { let s = s.into_owned(); let circle = CharCircle::new(s); let old_chars = circle.take_current_chars(); let old_chars: Chars<'_> = unsafe { mem::transmute(old_chars) }; let new_chars = self.transform_chars(old_chars); for ch in new_chars { circle.write_char(ch) }; // `old_chars` is dropped after loop. let t = circle.into_string(); // `circle` is dropped here. Cow::Owned(t) } else { s } } } // SimpleTransform // --------------------------------------------------------------------------- /// A simple trait for in-place string transformations. /// /// This trait allows character-iterator adaptors to modify strings in-place. /// It is implemented by adaptors with a single required associated function, /// `transform_chars`, for constructing the adaptor. In return, this trait /// provides an associated function, `transform`, which can apply the /// transformation to both owned strings and string slices, without allocating /// when possible. /// /// This trait is a simplified version of [`StringTransform`]. See the /// documentation of that trait for more details. pub trait SimpleTransform<'a>: Iterator<Item=char> + Sized { /// Transform the characters of a string. fn transform_chars(chars: Chars<'a>) -> Self; /// A hint to short-circuit string transformations. /// /// If true, the string might be modified by the transform. If false, the /// string will definitely not be modified by the transform. /// /// Implementors may override this function to facilitate copy-on-write /// optimizations. The default implementation always returns true, which /// disables copy-on-write. #[allow(unused_variables)] fn will_modify(val: &str) -> bool { return true } /// Transform a string in-place /// /// This function can operate on both `String` and `&str`. /// /// A new string may be allocated if the input is not owned or if the /// transformation needs to buffer more characters than the string has capacity. fn transform<'b, T: Into<Cow<'b, str>>>(s: T) -> Cow<'b, str> { SimpleStringTransform::<Self>::new().transform(s) } } // SimpleStringTransform // --------------------------------------------------------------------------- /// Used when you have a [`SimpleTransform`] but need a [`StringTransform`]. /// /// ### Example /// /// ``` /// # use char_circle::*; /// // `Identity` is a `SimpleTransform`. /// struct Identity<I>(I); /// /// impl<I: Iterator<Item=char>> Iterator for Identity<I> { /// type Item = char; /// fn next(&mut self) -> Option<char> { /// self.0.next() /// } /// } /// /// impl<'a> SimpleTransform<'a> for Identity<Chars<'a>> { /// fn transform_chars(chars: Chars<'a>) -> Self { /// Identity(chars) /// } /// } /// /// // This function takes a `StringTransform`. /// fn apply_transform<'a, T: StringTransform<'a>>(t: T, s: String) -> String { /// t.transform(s).into_owned() /// } /// /// // Use `SimpleStringTransform` to bridge the gap. /// let t = SimpleStringTransform::<Identity<Chars>>::new(); /// let s = apply_transform(t, "Hello World".to_string()); /// assert_eq!(&s, "Hello World"); /// ``` pub struct SimpleStringTransform<T>(pub PhantomData<T>); impl<'a, T: SimpleTransform<'a>> SimpleStringTransform<T> { /// Create a new `SimpleStringTransform`. pub fn new() -> Self { SimpleStringTransform(PhantomData) } } impl<'a, T: SimpleTransform<'a>> StringTransform<'a> for SimpleStringTransform<T> { type Iter = T; fn transform_chars(&self, chars: Chars<'a>) -> Self::Iter { Self::Iter::transform_chars(chars) } fn will_modify(&self, val: &str) -> bool { Self::Iter::will_modify(val) } } // Unit Tests // --------------------------------------------------------------------------- #[cfg(test)] mod tests { use super::*; // Identity transform // ------------------------- /// An identity function as a string transform. struct Identity<I>(I); impl<I: Iterator<Item=char>> Iterator for Identity<I> { type Item = char; fn next(&mut self) -> Option<char> { self.0.next() } } impl<'a> SimpleTransform<'a> for Identity<Chars<'a>> { fn transform_chars(chars: Chars<'a>) -> Self { Identity(chars) } } // Half transform // ------------------------- /// Deletes every other character. struct Half<I>(I); impl<I: Iterator<Item=char>> Iterator for Half<I> { type Item = char; fn next(&mut self) -> Option<char> { self.0.next()?; self.0.next() } } impl<'a> SimpleTransform<'a> for Half<Chars<'a>> { fn transform_chars(chars: Chars<'a>) -> Self { Half(chars) } } // Double transform // ------------------------- /// Duplicates every character. struct Double<I> { inner: I, state: Option<char>, } impl<I: Iterator<Item=char>> Iterator for Double<I> { type Item = char; fn next(&mut self) -> Option<char> { match self.state { Some(_) => self.state.take(), None => { let state = self.inner.next(); self.state = state; state } } } } impl<'a> SimpleTransform<'a> for Double<Chars<'a>> { fn transform_chars(chars: Chars<'a>) -> Self { Double { inner: chars, state: None } } } // Tests // ------------------------- #[test] fn test_transform() { let s: String = "Hello World".to_string(); // An identity transform does not reallocate. let ptr = s.as_ptr(); let cap = s.capacity(); let s = Identity::transform(s).into_owned(); assert_eq!(&s, "Hello World"); assert_eq!(s.as_ptr(), ptr); assert_eq!(s.capacity(), cap); // A half transform does not reallocate. let ptr = s.as_ptr(); let cap = s.capacity(); let s = Half::transform(s).into_owned(); assert_eq!(&s, "el ol"); assert_eq!(s.as_ptr(), ptr); assert_eq!(s.capacity(), cap); // A first double transform does not reallocate because there is excess capacity. let ptr = s.as_ptr(); let cap = s.capacity(); let s = Double::transform(s).into_owned(); assert_eq!(&s, "eell ooll"); assert_eq!(s.as_ptr(), ptr); assert_eq!(s.capacity(), cap); // A second double transform _does_ reallocate because it needs additional capacity. let cap = s.capacity(); let s = Double::transform(s).into_owned(); assert_eq!(&s, "eeeellll oooollll"); assert_ne!(s.capacity(), cap); } #[test] fn test_chars() { let s: String = "Hello World".to_string(); let circle = CharCircle::new(s); // The iterator consumes characters from the buffer. let mut chars = circle.take_chars(6); assert_eq!(chars.next(), Some('H')); assert_eq!(chars.next(), Some('e')); assert_eq!(chars.next(), Some('l')); // The iterator consumes all `n` characters upon drop. std::mem::drop(chars); let s = circle.into_string(); assert_eq!(s.as_str(), "World"); // The iterator does not consume more than `n` characters. let s: String = "Hello World".to_string(); let circle = CharCircle::new(s); let mut chars = circle.take_chars(6); assert_eq!(chars.next(), Some('H')); assert_eq!(chars.next(), Some('e')); assert_eq!(chars.next(), Some('l')); assert_eq!(chars.next(), Some('l')); assert_eq!(chars.next(), Some('o')); assert_eq!(chars.next(), Some(' ')); assert_eq!(chars.next(), None); } #[test] fn test_circle() { // `read` and `write` work as expected. // The `read_str` and `write_str` versions hit that code path. let circle = CharCircle::empty(); circle.write_str("Hello World!"); assert_eq!(circle.len(), 11); assert_eq!(circle.n_chars(), 11); let mut buf = [0u8; 6]; let buf_str = circle.read_str(&mut buf); assert_eq!(buf_str, "Hello "); assert_eq!(circle.len(), 5); assert_eq!(circle.n_chars(), 5); assert_eq!(&circle.into_string(), "World!"); // A more complicated test that alternates between reading and writing. let circle = CharCircle::empty(); circle.write_char('F'); circle.write_char('o'); circle.write_char('o'); circle.write_char('B'); circle.write_char('a'); circle.write_char('r'); assert_eq!(circle.read_char(), Some('F')); assert_eq!(circle.read_char(), Some('o')); assert_eq!(circle.read_char(), Some('o')); assert_eq!(circle.read_char(), Some('B')); assert_eq!(circle.read_char(), Some('a')); assert_eq!(circle.read_char(), Some('r')); assert_eq!(circle.read_char(), None); circle.write_char('H'); circle.write_char('e'); circle.write_char('l'); circle.write_char('l'); circle.write_char('o'); circle.write_char(' '); circle.write_char('W'); circle.write_char('o'); circle.write_char('r'); circle.write_char('l'); circle.write_char('d'); circle.write_char('!'); assert_eq!(circle.read_char(), Some('H')); assert_eq!(circle.read_char(), Some('e')); assert_eq!(circle.read_char(), Some('l')); assert_eq!(circle.read_char(), Some('l')); assert_eq!(circle.read_char(), Some('o')); assert_eq!(circle.read_char(), Some(' ')); circle.write_char('F'); circle.write_char('o'); circle.write_char('o'); circle.write_char('F'); circle.write_char('o'); circle.write_char('o'); assert_eq!(circle.read_char(), Some('W')); assert_eq!(circle.read_char(), Some('o')); assert_eq!(circle.read_char(), Some('r')); assert_eq!(circle.read_char(), Some('l')); assert_eq!(circle.read_char(), Some('d')); assert_eq!(circle.read_char(), Some('!')); assert_eq!(circle.read_char(), Some('F')); assert_eq!(circle.read_char(), Some('o')); assert_eq!(circle.read_char(), Some('o')); circle.write_char('B'); circle.write_char('a'); circle.write_char('r'); assert_eq!(&circle.into_string(), "FooBar"); } }