InvMap

A bidirectional Map for Elixir.

InvMap mirrors the public API of the standard Map module (WIP), but its lookups are symmetric: get(m, key) and get(m, value) both return the counterpart. This is useful when the key-value relationship is 1-to-1 and you want lookups from both directions.

Involution

Looking up any key in an InvMap and then looking up the result gets you back the original key. In other words, get/2 is a self-inverting function, or an involution. This means no keys can share a value, and no key can collide with a value. InvMap enforces this for you: violating inputs are rejected in new/{1,2}, and put/3 drops conflicting entries to keep the involution intact, similar to how Map.put overwrites a conflicting key.

Examples

iex> country_codes = InvMap.new(%{"BR" => "Brazil", "US" => "United States"})
InvMap.new(%{"BR" => "Brazil", "US" => "United States"})
iex> InvMap.get(country_codes, "BR")
"Brazil"
iex> InvMap.get(country_codes, "Brazil")
"BR"

# Inserting {:a, 2} into InvMap.new(%{a: 1, b: 2}) would collide with both
# {:a, 1} and {:b, 2}, so put/3 drops both entries to preserve the involution.
iex> InvMap.put(InvMap.new(%{a: 1, b: 2}), :a, 2)
InvMap.new(%{a: 2})

Installation

Once published to Hex, add inv_map to your dependencies in mix.exs:

def deps do
  [
    {:inv_map, "~> 0.1.0"}
  ]
end

Documentation will be available at https://hexdocs.pm/inv_map.