2 files changed, 138 insertions, 0 deletions

diff --git a/crates/smol_str/Cargo.toml b/crates/smol_str/Cargo.toml
new file mode 100644
index 000000000..83ca12f62
--- /dev/null
+++ b/crates/smol_str/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "smol_str"
+version = "0.1.0"
+authors = ["Aleksey Kladov <[email protected]>"]
+
+[dependencies]
diff --git a/crates/smol_str/src/lib.rs b/crates/smol_str/src/lib.rs
new file mode 100644
index 000000000..4d5fef593
--- /dev/null
+++ b/crates/smol_str/src/lib.rs
@@ -0,0 +1,132 @@
+use std::{sync::Arc, ops::Deref};
+
+#[derive(Clone, Debug)]
+pub struct SmolStr(Repr);
+
+impl SmolStr {
+    pub fn new(text: &str) -> SmolStr {
+        SmolStr(Repr::new(text))
+    }
+
+    pub fn as_str(&self) -> &str {
+        self.0.as_str()
+    }
+
+    pub fn to_string(&self) -> String {
+        self.as_str().to_string()
+    }
+}
+
+impl Deref for SmolStr {
+    type Target = str;
+
+    fn deref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl PartialEq<str> for SmolStr {
+    fn eq(&self, other: &str) -> bool {
+        self.as_str() == other
+    }
+}
+
+impl PartialEq<SmolStr> for str {
+    fn eq(&self, other: &SmolStr) -> bool {
+        other == self
+    }
+}
+
+impl<'a> PartialEq<&'a str> for SmolStr {
+    fn eq(&self, other: &&'a str) -> bool {
+        self == *other
+    }
+}
+
+impl<'a> PartialEq<SmolStr> for &'a str {
+    fn eq(&self, other: &SmolStr) -> bool {
+        *self == other
+    }
+}
+
+const INLINE_CAP: usize = 22;
+const WS_TAG: u8 = (INLINE_CAP + 1) as u8;
+
+#[derive(Clone, Debug)]
+enum Repr {
+    Heap(Arc<str>),
+    Inline {
+        len: u8,
+        buf: [u8; INLINE_CAP],
+    },
+}
+
+impl Repr {
+    fn new(text: &str) -> Repr {
+        let len = text.len();
+        if len <= INLINE_CAP {
+            let mut buf = [0; INLINE_CAP];
+            buf[..len].copy_from_slice(text.as_bytes());
+            return Repr::Inline { len: len as u8, buf };
+        }
+
+        let newlines = text.bytes().take_while(|&b| b == b'\n').count();
+        let spaces = text[newlines..].bytes().take_while(|&b| b == b' ').count();
+        if newlines + spaces == len && newlines <= N_NEWLINES && spaces <= N_SPACES {
+            let mut buf = [0; INLINE_CAP];
+            buf[0] = newlines as u8;
+            buf[1] = spaces as u8;
+            return Repr::Inline { len: WS_TAG, buf };
+        }
+
+        Repr::Heap(
+            text.to_string().into_boxed_str().into()
+        )
+    }
+
+    fn as_str(&self) -> &str {
+        match self {
+            Repr::Heap(data) => &*data,
+            Repr::Inline { len, buf } => {
+                if *len == WS_TAG {
+                    let newlines = buf[0] as usize;
+                    let spaces = buf[1] as usize;
+                    assert!(newlines <= N_NEWLINES && spaces <= N_SPACES);
+                    return &WS[N_NEWLINES - newlines..N_NEWLINES + spaces];
+                }
+
+                let len = *len as usize;
+                let buf = &buf[..len];
+                unsafe { ::std::str::from_utf8_unchecked(buf) }
+            }
+        }
+    }
+}
+
+const N_NEWLINES: usize = 32;
+const N_SPACES: usize = 128;
+const WS: &str =
+    "\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n                                                                                                                                ";
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[cfg(target_pointer_width = "64")]
+    fn smol_str_is_smol() {
+        assert_eq!(::std::mem::size_of::<SmolStr>(), 8 + 8 + 8)
+    }
+
+    #[test]
+    fn test_round_trip() {
+        let mut text = String::new();
+        for n in 0..256 {
+            let smol = SmolStr::new(&text);
+            assert_eq!(smol.as_str(), text.as_str());
+            text.push_str(&n.to_string());
+        }
+    }
+}
+