If all you need the hash for is membership, consider using a Set:

Set

Set implements a collection of unordered values with no
duplicates. This is a hybrid of Array’s intuitive inter-operation
facilities and Hash’s fast lookup.

Set is easy to use with Enumerable objects (implementing
each). Most of the initializer methods and binary operators accept
generic Enumerable objects besides sets and arrays. An
Enumerable object can be converted to Set using the
to_set method.

Set uses Hash as storage, so you must note the following points:

• Equality of elements is determined according to Object#eql? and Object#hash.
• Set assumes that the identity of each element does not change while it is stored. Modifying an element of a set will render the set to an
  unreliable state.
• When a string is to be stored, a frozen copy of the string is stored instead unless the original string is already frozen.

Comparison

The comparison operators <, >, <= and >= are implemented as
shorthand for the {proper_,}{subset?,superset?} methods. However, the
<=> operator is intentionally left out because not every pair of
sets is comparable. ({x,y} vs. {x,z} for example)

Example

require 'set'
s1 = Set.new [1, 2]                   # -> #<Set: {1, 2}>
s2 = [1, 2].to_set                    # -> #<Set: {1, 2}>
s1 == s2                              # -> true
s1.add("foo")                         # -> #<Set: {1, 2, "foo"}>
s1.merge([2, 6])                      # -> #<Set: {1, 2, "foo", 6}>
s1.subset? s2                         # -> false
s2.subset? s1                         # -> true

[…]

Public Class Methods

new(enum = nil)

Creates a new set containing the elements of the given enumerable
object.

If a block is given, the elements of enum are preprocessed by the
given block.