commit ea3131ed1aaf2e102ffc2d1af3ed3ba1da92111f Author: Thomas Karpiniec Date: Fri Jun 16 13:31:33 2023 +1000 Initial commit diff --git a/.gitignore b/.gitignore new file mode 100644 index 0000000..4fffb2f --- /dev/null +++ b/.gitignore @@ -0,0 +1,2 @@ +/target +/Cargo.lock diff --git a/Cargo.toml b/Cargo.toml new file mode 100644 index 0000000..1280504 --- /dev/null +++ b/Cargo.toml @@ -0,0 +1,9 @@ +[package] +name = "with-checked-bytes" +description = "Extension trait for safely editing strings as byte slices" +version = "0.1.0" +edition = "2021" +authors = ["Thomas Karpiniec "] +license = "Apache-2.0" + +[dependencies] diff --git a/LICENSE b/LICENSE new file mode 100644 index 0000000..261eeb9 --- /dev/null +++ b/LICENSE @@ -0,0 +1,201 @@ + Apache License + Version 2.0, January 2004 + http://www.apache.org/licenses/ + + TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION + + 1. 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The result is only applied to the original string if the end result of +the modification is valid UTF-8. + +## Examples + +```rust +use with_checked_bytes::WithCheckedBytes; + +let mut my_string = String::from("hello"); +my_string.with_checked_bytes_mut(|s| { + s[1] += 1; +}).unwrap(); +assert_eq!(my_string, "hfllo"); +``` + +```rust +let mut my_string = String::from("hello"); +let old_value = my_string.with_checked_bytes_mut(|s| { + std::mem::replace(&mut s[3], b'z') +}).unwrap(); +assert_eq!(old_value, b'l'); +assert_eq!(my_string, "helzo"); +``` + +```rust +let mut my_string = String::from("hello"); +my_string.with_checked_bytes_mut(|s| { + s[1] = 0xff; // not valid UTF-8 +}).unwrap(); // will panic - original string remains unmodified +``` + +## Licence + +Apache 2.0. See `LICENSE`. diff --git a/src/lib.rs b/src/lib.rs new file mode 100644 index 0000000..abfc7d0 --- /dev/null +++ b/src/lib.rs @@ -0,0 +1,156 @@ +//! An extension trait for `&mut str` to simplify manipulating UTF-8 strings as if they were +//! plain ASCII bytes. The result is only applied to the original string if the end result of +//! the modification is valid UTF-8. +//! +//! # Examples: +//! +//! ``` +//! use with_checked_bytes::WithCheckedBytes; +//! +//! let mut my_string = String::from("hello"); +//! my_string.with_checked_bytes_mut(|s| { +//! s[1] += 1; +//! }).unwrap(); +//! assert_eq!(my_string, "hfllo"); +//! ``` +//! +//! ``` +//! # use with_checked_bytes::WithCheckedBytes; +//! let mut my_string = String::from("hello"); +//! let old_value = my_string.with_checked_bytes_mut(|s| { +//! std::mem::replace(&mut s[3], b'z') +//! }).unwrap(); +//! assert_eq!(old_value, b'l'); +//! assert_eq!(my_string, "helzo"); +//! ``` +//! +//! ```should_panic +//! # use with_checked_bytes::WithCheckedBytes; +//! let mut my_string = String::from("hello"); +//! my_string.with_checked_bytes_mut(|s| { +//! s[1] = 0xff; // not valid UTF-8 +//! }).unwrap(); // will panic - original string remains unmodified +//! ``` + +use std::ops::{Deref, DerefMut}; + +/// Extension trait for safely editing mutable UTF-8 strings as bytes +pub trait WithCheckedBytes { + /// Edit a mutable `String` or `&mut str` as if it were a byte array. + /// + /// The provided closure will be executed with a mutable view of the String. + /// If the mutable buffer doesn't contain valid UTF-8 when the closure returns, + /// the original string will not be modified and an error will be returned. + /// + /// If the buffer contains valid UTF-8, the original string will be overwritten + /// with the buffer's contents. Any value returned from the closure will be + /// passed back to the caller. + fn with_checked_bytes_mut<'a, R, F>(&'a mut self, f: F) -> Result + where + F: for<'b> FnOnce(&'b mut MutableStringBytes) -> R; +} + +impl WithCheckedBytes for str { + fn with_checked_bytes_mut<'a, R, F>(&'a mut self, f: F) -> Result + where + F: for<'b> FnOnce(&'b mut MutableStringBytes) -> R, + { + let mut target = MutableStringBytes::Borrowed(self.as_bytes()); + let res = f(&mut target); + match target { + MutableStringBytes::Borrowed(_) => (), + MutableStringBytes::Owned(v) => match std::str::from_utf8(&v) { + // SAFETY: We just proved that the new slice content is valid UTF-8 + Ok(s) => unsafe { self.as_bytes_mut().copy_from_slice(s.as_bytes()) }, + Err(_) => return Err(Error::InvalidUtf8), + }, + } + Ok(res) + } +} + +/// Mutable view into a string's content expressed as bytes +pub enum MutableStringBytes<'a> { + Borrowed(&'a [u8]), + Owned(Vec), +} + +impl<'a> Deref for MutableStringBytes<'a> { + type Target = [u8]; + + fn deref(&self) -> &Self::Target { + match self { + Self::Borrowed(slice) => *slice, + Self::Owned(vec) => vec.as_slice(), + } + } +} + +impl<'a> DerefMut for MutableStringBytes<'a> { + fn deref_mut(&mut self) -> &mut Self::Target { + if let Self::Borrowed(slice) = self { + let v = slice.to_vec(); + let _ = std::mem::replace(self, Self::Owned(v)); + } + match self { + Self::Borrowed(_) => unreachable!(), + Self::Owned(vec) => vec.as_mut_slice(), + } + } +} + +/// Errors that can occur while mutating strings +#[derive(Debug)] +pub enum Error { + InvalidUtf8, +} + +impl std::error::Error for Error {} + +impl std::fmt::Display for Error { + fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { + write!(f, "MutableStringBytes contains invalid UTF-8 after modifications") + } +} + +#[cfg(test)] +mod tests { + use super::*; + + #[test] + fn twiddle_a_byte() { + let mut my_string = "Hello".to_owned(); + my_string.with_checked_bytes_mut(|s| { + s[3] += 1; + }).unwrap(); + assert_eq!(my_string, "Helmo"); + } + + #[test] + fn twiddle_a_byte_bad() { + let mut my_string = "Hello".to_owned(); + my_string.with_checked_bytes_mut(|s| { + s[3] = 0xff; + }).unwrap_err(); + assert_eq!(my_string, "Hello"); + } + + #[test] + fn return_a_byte() { + let mut my_string = "Hello".to_owned(); + let ascii = my_string.with_checked_bytes_mut(|s| { + s[3] + }).unwrap(); + assert_eq!(ascii, 108u8); + } + + #[test] + fn slice_edit() { + let mut my_string = "Hello".to_owned(); + let mut_slice = my_string.as_mut(); + mut_slice.with_checked_bytes_mut(|s| { + s[0] += 1; + }).unwrap(); + assert_eq!(my_string, "Iello"); + } +}