ignore_keys ignores only common keys

[rellampec]

require 'hashdiff'
Hashdiff::VERSION
#=> "1.1.0"

The following snippet fails to ignore top level key :a

a = { a: 4, b: {a: 5, c: 6} }
b = {       b: {a: 7, c: 3} }

diff = Hashdiff.diff(a, b, ignore_keys: :a)
#=> [["-", "a", 4], ["~", "b.c", 6, 3]]

Shouldn't diff be expected to be the below?

[["~", "b.c", 6, 3]]

The documentation on ignore_keys states that:

The :ignore_keys option allows you to specify one or more keys to ignore, which defaults to [] (none). Ignored keys are ignored at all levels.

Here line where :ignore_keys happens (in Hashdiff::ComparableHashes::call):

opts[:ignore_keys].each { |k| common_keys.delete k }

added_keys and deleted_keys don't seem to have the exclusion applied.

[rellampec]

a = { a: 4, b: {a: 5, c: 6}         }
b = {       b: {a: 7, c: 3},  d: 8 }

(1) ignore_keys: keep backwards compatibility

diff = Hashdiff.diff(a, b, ignore_keys: [:a, :d])
#=> [["-", "a", 4], ["~", "b.a", 5, 7], ["~", "b.c", 6, 3], ["+", "d", 8]]

• ignores change of b.a but includes :a removal and :d addition.

(2) new ignore_added_keys: only affects added keys

diff = Hashdiff.diff(a, b, ignore_keys: [:a, :d])
#=> [["-", "a", 4], ["~", "b.a", 5, 7], ["~", "b.c", 6, 3]]

• like ignore_keys but also ignoring added key :d