Sketch in type-level changes to track insertion splits

crates/text/src/locator.rs

@@ -0,0 +1,74 @@
+use smallvec::{smallvec, SmallVec};
+use std::iter;
+
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Locator(SmallVec<[u32; 4]>);
+
+impl Locator {
+    pub fn min() -> Self {
+        Self(smallvec![u32::MIN])
+    }
+
+    pub fn max() -> Self {
+        Self(smallvec![u32::MAX])
+    }
+
+    pub fn between(lhs: &Self, rhs: &Self) -> Self {
+        let lhs = lhs.0.iter().copied().chain(iter::repeat(u32::MIN));
+        let rhs = rhs.0.iter().copied().chain(iter::repeat(u32::MAX));
+        let mut location = SmallVec::new();
+        for (lhs, rhs) in lhs.zip(rhs) {
+            let mid = lhs + (rhs.saturating_sub(lhs)) / 2;
+            location.push(mid);
+            if mid > lhs {
+                break;
+            }
+        }
+        Self(location)
+    }
+}
+
+impl Default for Locator {
+    fn default() -> Self {
+        Self::min()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use rand::prelude::*;
+    use std::mem;
+
+    #[gpui::test(iterations = 100)]
+    fn test_locators(mut rng: StdRng) {
+        let mut lhs = Default::default();
+        let mut rhs = Default::default();
+        while lhs == rhs {
+            lhs = Locator(
+                (0..rng.gen_range(1..=5))
+                    .map(|_| rng.gen_range(0..=100))
+                    .collect(),
+            );
+            rhs = Locator(
+                (0..rng.gen_range(1..=5))
+                    .map(|_| rng.gen_range(0..=100))
+                    .collect(),
+            );
+        }
+
+        if lhs > rhs {
+            mem::swap(&mut lhs, &mut rhs);
+        }
+
+        let middle = Locator::between(&lhs, &rhs);
+        assert!(middle > lhs);
+        assert!(middle < rhs);
+        for ix in 0..middle.0.len() - 1 {
+            assert!(
+                middle.0[ix] == *lhs.0.get(ix).unwrap_or(&0)
+                    || middle.0[ix] == *rhs.0.get(ix).unwrap_or(&0)
+            );
+        }
+    }
+}