feat: add custom CowStr type

Related issue: #20

src/lib.rs

@@ -58,12 +58,14 @@ mod block;
 mod inline;
 mod lex;
 mod span;
+mod string;
 mod tree;
 
 use span::DiscontinuousString;
 use span::Span;
 
 pub use attr::{AttributeValue, AttributeValueParts, Attributes};
+pub use string::CowStr as JotdownCowStr;
 
 type CowStr<'s> = std::borrow::Cow<'s, str>;
 

src/string.rs

@@ -0,0 +1,129 @@
+use std::{
+    fmt::Display,
+    ops::{Deref, DerefMut},
+    str::{from_utf8_unchecked, from_utf8_unchecked_mut},
+};
+
+#[derive(Debug)]
+pub enum CowStr<'s> {
+    Owned(String),
+    Borrowed(&'s str),
+    Inlined(InlineStr),
+}
+
+impl<'s> Deref for CowStr<'s> {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        match self {
+            Self::Owned(ref s) => s,
+            Self::Borrowed(s) => s,
+            Self::Inlined(s) => s.deref(),
+        }
+    }
+}
+
+impl<'s> AsRef<str> for CowStr<'s> {
+    fn as_ref(&self) -> &str {
+        self.deref()
+    }
+}
+
+impl<'s> From<&'s str> for CowStr<'s> {
+    fn from(value: &'s str) -> Self {
+        CowStr::Borrowed(value)
+    }
+}
+
+impl<'s> From<String> for CowStr<'s> {
+    fn from(value: String) -> Self {
+        CowStr::Owned(value)
+    }
+}
+
+impl<'s> Clone for CowStr<'s> {
+    fn clone(&self) -> Self {
+        match self {
+            CowStr::Owned(s) => match InlineStr::try_from(&**s) {
+                Ok(inline) => CowStr::Inlined(inline),
+                Err(_) => CowStr::Owned(s.clone()),
+            },
+            CowStr::Borrowed(s) => CowStr::Borrowed(s),
+            CowStr::Inlined(s) => CowStr::Inlined(*s),
+        }
+    }
+}
+
+impl<'s> PartialEq for CowStr<'s> {
+    fn eq(&self, other: &Self) -> bool {
+        self.deref() == other.deref()
+    }
+}
+
+impl<'s> Eq for CowStr<'s> {}
+
+impl<'s> Display for CowStr<'s> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.deref())
+    }
+}
+
+impl<'s, 'a> FromIterator<&'a str> for CowStr<'s> {
+    fn from_iter<T: IntoIterator<Item = &'a str>>(iter: T) -> Self {
+        CowStr::Owned(FromIterator::from_iter(iter))
+    }
+}
+
+const MAX_INLINE_STR_LEN: usize = 3 * std::mem::size_of::<isize>() - 2;
+
+pub struct Error;
+
+#[derive(Clone, Copy, Debug)]
+pub struct InlineStr {
+    inner: [u8; MAX_INLINE_STR_LEN],
+    len: usize,
+}
+
+impl Deref for InlineStr {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: `InlineStr` can only be constructed from strings or chars, which means they are
+        // guaranteed to be valid UTF-8.
+        unsafe { from_utf8_unchecked(&self.inner[..self.len]) }
+    }
+}
+
+impl DerefMut for InlineStr {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // SAFETY: `InlineStr` can only be constructed from strings or chars, which means they are
+        // guaranteed to be valid UTF-8.
+        unsafe { from_utf8_unchecked_mut(&mut self.inner[..self.len]) }
+    }
+}
+
+impl From<char> for InlineStr {
+    fn from(value: char) -> Self {
+        let mut inner = [0u8; MAX_INLINE_STR_LEN];
+        value.encode_utf8(&mut inner);
+        Self {
+            inner,
+            len: value.len_utf8(),
+        }
+    }
+}
+
+impl TryFrom<&str> for InlineStr {
+    type Error = Error;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        let len = value.len();
+        if len > MAX_INLINE_STR_LEN {
+            Err(Error)
+        } else {
+            let mut inner = [0u8; MAX_INLINE_STR_LEN];
+            inner.copy_from_slice(value.as_bytes());
+            Ok(Self { inner, len })
+        }
+    }
+}