smol strings

1 files changed, 60 insertions, 0 deletions

diff --git a/src/smol_str.rs b/src/smol_str.rs
new file mode 100644
index 000000000..2a330c343
--- /dev/null
+++ b/src/smol_str.rs
@@ -0,0 +1,60 @@
+use std::{sync::Arc};
+
+const INLINE_CAP: usize = 22;
+
+#[derive(Clone, Debug)]
+pub(crate) enum SmolStr {
+    Heap(Arc<str>),
+    Inline {
+        len: u8,
+        buf: [u8; INLINE_CAP],
+    },
+}
+
+impl SmolStr {
+    pub fn new(text: &str) -> SmolStr {
+        let len = text.len();
+        if len <= INLINE_CAP {
+            let mut buf = [0; INLINE_CAP];
+            buf[..len].copy_from_slice(text.as_bytes());
+            SmolStr::Inline { len: len as u8, buf }
+        } else {
+            SmolStr::Heap(
+                text.to_string().into_boxed_str().into()
+            )
+        }
+    }
+
+    pub fn as_str(&self) -> &str {
+        match self {
+            SmolStr::Heap(data) => &*data,
+            SmolStr::Inline { len, buf } => {
+                let len = *len as usize;
+                let buf = &buf[..len];
+                unsafe { ::std::str::from_utf8_unchecked(buf) }
+            }
+        }
+    }
+}
+
+#[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());
+        }
+    }
+}
+