indexmap

An insertion-order-preserving hash map for MoonBit.

Features

• O(1) average lookup by key (hash map)
• O(1) access by insertion-order index
• Iteration always in insertion order
• Swap-remove for O(1) removal
• Compatible with MoonBit’s Eq and Debug traits

Installation

moon add moonbit-community/indexmap

Then import in your .mbt files:

let m = @indexmap.IndexMap::new()

Quick Start

test {
  // Create and populate
  let m = @indexmap.IndexMap::from([("b", 2), ("a", 1), ("c", 3)])

  // Lookup by key — O(1)
  assert_eq(m.get("a"), Some(1))
  assert_eq(m["b"], 2)  // panics if missing

  // Iteration preserves insertion order
  let keys = m.keys_iter().to_array()  // ["b", "a", "c"]

  // Access by index
  assert_eq(m.get_index_key(0), Some("b"))
  assert_eq(m.get_index(1), Some(("a", 1)))

  // Update (keeps original position)
  m.set("a", 99)
  // still: ["b", "a", "c"], but m["a"] == 99

  // Remove
  assert_eq(m.remove("a"), Some(99))
  // now: ["b", "c"]
}

API Reference

Construction

Function	Description
IndexMap::new()	Create an empty map
IndexMap::with_capacity(n)	Create with at least n slots
IndexMap::from(arr)	Create from ArrayView[(K, V)]

Core Operations

Method	Description
m.set(key, value)	Insert or update
m.get(key) -> V?	Lookup, returns Option
m.at(key) -> V	Lookup, panics if missing
m.contains_key(key) -> Bool	Key existence check
m.remove(key) -> V?	Remove and return value

Index Access

Method	Description
m.get_index(i) -> (K, V)?	Pair at position i
m.get_index_key(i) -> K?	Key at position i
m.get_index_value(i) -> V?	Value at position i

Iteration

Method	Description
m.iter() -> Iter[(K, V)]	Key-value pairs in order
m.keys_iter() -> Iter[K]	Keys in order
m.values_iter() -> Iter[V]	Values in order

Collection Info

Method	Description
m.length() -> Int	Number of entries
m.is_empty() -> Bool	Whether empty

Operators

• m[key] — bracket read (panics if missing)
• m[key] = value — bracket write

Design

Internally uses:

• Robin Hood hashing (open addressing with linear probing) for the hash table
• A separate order array storing keys in insertion order
• Swap-remove semantics: removing an entry swaps it with the last, keeping the order array contiguous

License

Apache-2.0