Implementation of a split function.

Been busy at work so this is an isolated commit the rest of the change is still intermingled

src/engine/io/index_formats/split_branch.rs

@@ -0,0 +1,90 @@
+//! Utility function to correctly split a branch and return a new sub set
+// Split out to make testing easier
+use thiserror::Error;
+
+pub fn split_branch<T, U>(
+    keys: &mut Vec<T>,
+    pointers: &mut Vec<U>,
+) -> Result<(T, Vec<T>, Vec<U>), SplitBranchError>
+where
+    T: Clone,
+    U: Clone,
+{
+    if keys.len() + 1 != pointers.len() {
+        return Err(SplitBranchError::KeysPointLen(keys.len(), pointers.len()));
+    }
+
+    if keys.len() < 2 {
+        return Err(SplitBranchError::KeysTooShort(keys.len()));
+    }
+
+    let middle_index = keys.len() / 2;
+    let middle = keys[middle_index].clone();
+
+    let mut right_keys = vec![middle.clone(); keys.len() - middle_index - 1];
+    right_keys.clone_from_slice(&keys[(middle_index + 1)..]);
+    keys.truncate(middle_index);
+
+    let mut right_pointers = vec![pointers[0].clone(); pointers.len() - middle_index - 1];
+    right_pointers.clone_from_slice(&pointers[middle_index + 1..]);
+    pointers.truncate(middle_index + 1);
+
+    Ok((middle, right_keys, right_pointers))
+}
+
+#[derive(Debug, Error)]
+pub enum SplitBranchError {
+    #[error("Keys len {0}, must be one less than pointers len {1}")]
+    KeysPointLen(usize, usize),
+    #[error("Cannot split without at least 2 keys, have {0}")]
+    KeysTooShort(usize),
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_simple_split_even() -> Result<(), Box<dyn std::error::Error>> {
+        let mut keys = vec![0, 1, 2, 3, 4, 5];
+        let mut pointers = vec!["a", "b", "c", "d", "e", "f", "g"];
+
+        let left_keys = vec![0, 1, 2];
+        let left_pointers = ["a", "b", "c", "d"];
+
+        let middle = 3;
+        let right_keys = vec![4, 5];
+        let right_pointers = vec!["e", "f", "g"];
+
+        let (out_middle, out_keys, out_pointers) = split_branch(&mut keys, &mut pointers)?;
+        assert_eq!(keys, left_keys);
+        assert_eq!(pointers, left_pointers);
+        assert_eq!(out_middle, middle);
+        assert_eq!(out_keys, right_keys);
+        assert_eq!(out_pointers, right_pointers);
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple_split_odd() -> Result<(), Box<dyn std::error::Error>> {
+        let mut keys = vec![0, 1, 2, 3, 4];
+        let mut pointers = vec!["a", "b", "c", "d", "e", "f"];
+
+        let left_keys = vec![0, 1];
+        let left_pointers = ["a", "b", "c"];
+
+        let middle = 2;
+        let right_keys = vec![3, 4];
+        let right_pointers = vec!["d", "e", "f"];
+
+        let (out_middle, out_keys, out_pointers) = split_branch(&mut keys, &mut pointers)?;
+        assert_eq!(keys, left_keys);
+        assert_eq!(pointers, left_pointers);
+        assert_eq!(out_middle, middle);
+        assert_eq!(out_keys, right_keys);
+        assert_eq!(out_pointers, right_pointers);
+
+        Ok(())
+    }
+}