1 files changed, 63 insertions, 0 deletions
diff --git a/src/builder.rs b/src/builder.rs
new file mode 100644
index 0000000..8e50a20
--- /dev/null
+++ b/src/builder.rs
@@ -0,0 +1,63 @@
+use crate::{Entry, Slab};
+
+// Building `Slab` from pairs (usize, T).
+pub(crate) struct Builder<T> {
+    slab: Slab<T>,
+    vacant_list_broken: bool,
+    first_vacant_index: Option<usize>,
+}
+
+impl<T> Builder<T> {
+    pub(crate) fn with_capacity(capacity: usize) -> Self {
+        Self {
+            slab: Slab::with_capacity(capacity),
+            vacant_list_broken: false,
+            first_vacant_index: None,
+        }
+    }
+    pub(crate) fn pair(&mut self, key: usize, value: T) {
+        let slab = &mut self.slab;
+        if key < slab.entries.len() {
+            // iterator is not sorted, might need to recreate vacant list
+            if let Entry::Vacant(_) = slab.entries[key] {
+                self.vacant_list_broken = true;
+                slab.len += 1;
+            }
+            // if an element with this key already exists, replace it.
+            // This is consistent with HashMap and BtreeMap
+            slab.entries[key] = Entry::Occupied(value);
+        } else {
+            if self.first_vacant_index.is_none() && slab.entries.len() < key {
+                self.first_vacant_index = Some(slab.entries.len());
+            }
+            // insert holes as necessary
+            while slab.entries.len() < key {
+                // add the entry to the start of the vacant list
+                let next = slab.next;
+                slab.next = slab.entries.len();
+                slab.entries.push(Entry::Vacant(next));
+            }
+            slab.entries.push(Entry::Occupied(value));
+            slab.len += 1;
+        }
+    }
+
+    pub(crate) fn build(self) -> Slab<T> {
+        let mut slab = self.slab;
+        if slab.len == slab.entries.len() {
+            // no vacant entries, so next might not have been updated
+            slab.next = slab.entries.len();
+        } else if self.vacant_list_broken {
+            slab.recreate_vacant_list();
+        } else if let Some(first_vacant_index) = self.first_vacant_index {
+            let next = slab.entries.len();
+            match &mut slab.entries[first_vacant_index] {
+                Entry::Vacant(n) => *n = next,
+                _ => unreachable!(),
+            }
+        } else {
+            unreachable!()
+        }
+        slab
+    }
+}