fix small string

Co-authored-by: Aapo Alasuutari <[email protected]>

nova_vm/src/small_string.rs

@@ -1,6 +1,6 @@
 #[derive(Clone, Copy)]
 pub struct SmallString {
-    data: [u8; 7],
+    bytes: [u8; 7],
 }
 
 impl std::fmt::Debug for SmallString {
@@ -11,33 +11,84 @@ impl std::fmt::Debug for SmallString {
 
 impl SmallString {
     pub fn len(&self) -> usize {
-        self.data.iter().position(|byte| *byte == 0).unwrap_or(7)
+        // Find the last non-null character and add one to its index to get length.
+        self.bytes
+            .as_slice()
+            .iter()
+            .rev()
+            .position(|&x| x != 0)
+            .map_or(0, |i| 7 - i)
     }
 
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        // SAFETY: Guaranteed to be ASCII, which is a subset of UTF-8.
+        unsafe { &std::str::from_utf8_unchecked(self.as_bytes()) }
+    }
+
+    #[inline]
     pub fn as_bytes(&self) -> &[u8] {
-        &self.data[0..self.len()]
+        &self.bytes.as_slice().split_at(self.len()).0
     }
 
-    pub fn as_str(&self) -> &str {
-        // SAFETY: Guaranteed to be valid UTF-8.
-        unsafe { std::str::from_utf8_unchecked(self.as_bytes()) }
+    #[inline]
+    pub fn data(&self) -> &[u8; 7] {
+        return &self.bytes;
     }
 
-    pub(crate) fn from_str_unchecked(value: &str) -> Self {
-        debug_assert!(value.len() < 8 && !value.as_bytes().contains(&0));
-        let mut data: [u8; 7] = [0, 0, 0, 0, 0, 0, 0];
-        data.copy_from_slice(value.as_bytes());
-        Self { data }
+    pub(crate) fn from_str_unchecked(string: &str) -> Self {
+        let string_bytes = string.as_bytes();
+
+        // We have only 7 bytes to work with, and we cannot tell apart
+        // UTF-8 strings that end with a null byte from our null
+        // terminator so we must fail to convert on those.
+        debug_assert!(string_bytes.len() < 8 && string_bytes.last() != Some(&0));
+
+        let mut bytes = [0, 0, 0, 0, 0, 0, 0];
+        bytes
+            .as_mut_slice()
+            .split_at_mut(string_bytes.len())
+            .0
+            .copy_from_slice(string_bytes);
+
+        Self { bytes }
     }
 }
 
 impl TryFrom<&str> for SmallString {
     type Error = ();
     fn try_from(value: &str) -> Result<Self, Self::Error> {
-        if value.len() < 8 && !value.as_bytes().contains(&0) {
+        // We have only 7 bytes to work with, and we cannot tell apart
+        // UTF-8 strings that end with a null byte from our null
+        // terminator so we must fail to convert on those.
+        if value.len() < 8 && value.as_bytes().last() != Some(&0) {
             Ok(Self::from_str_unchecked(value))
         } else {
             Err(())
         }
     }
 }
+
+#[test]
+fn valid_stack_strings() {
+    assert!(SmallString::try_from("").is_ok());
+    assert_eq!(SmallString::try_from("").unwrap().len(), 0);
+    assert!(SmallString::try_from("asd").is_ok());
+    assert_eq!(SmallString::try_from("asd").unwrap().len(), 3);
+    assert!(SmallString::try_from("asdasd").is_ok());
+    assert_eq!(SmallString::try_from("asdasd").unwrap().len(), 6);
+    assert!(SmallString::try_from("asdasda").is_ok());
+    assert_eq!(SmallString::try_from("asdasda").unwrap().len(), 7);
+    assert!(SmallString::try_from("asd76fd").is_ok());
+    assert_eq!(SmallString::try_from("asd76fd").unwrap().len(), 7);
+    assert!(SmallString::try_from("💩").is_ok());
+    assert_eq!(SmallString::try_from("💩 ").unwrap().len(), 5);
+    assert!(SmallString::try_from("asd\0foo").is_ok());
+    assert_eq!(SmallString::try_from("asd\0foo").unwrap().len(), 7);
+}
+
+#[test]
+fn not_valid_stack_strings() {
+    assert!(SmallString::try_from("asd asd r 547 gdfg").is_err());
+    assert!(SmallString::try_from("asdfoo\0").is_err());
+}