Move calculation into Fingerprint model

** Why are these changes being introduced:

It makes sense that the Fingerprint model is where the logic to
calculate a fingerprint value is maintained, and not in the Term model.

** Relevant ticket(s):

code review

** How does this address that need:

This moves the fingerprint logic from the Term to Fingerprint model.
Because of this, the Term model changes to use its new location (from
inside the lifecycle hook where the Fingerprint record gets created).

We use an instance method for this, because at the time of use there is
not yet a Fingerprint record that could call the method internally.

We also copy-paste the tests for this method from the SuggestedResource
implementation (which should be removed in a future PR).

** Document any side effects to this change:

I can think of two possible side effects:
1. the SuggestedResource implementation of the fingerprint is now very
much duplicative, and should be removed (coming in a future ticket)

2. It is a bit awkward to rely on an instance method for calculating
the fingerprint value. In an ideal case, register_fingerprint would
use something like:

self.fingerprint = Fingerprint.find_or_create_by(phrase)

... and then the Fingerprint model would:
a. receive the phrase argument
b. calculate the fingerprint for this phrase
c. look up to see if such a record exists already, and return it
d. create the record if none exists, and return _that_

A custom initialize method feels like it would work for this, but I
think that's considered an antipattern in Rails?

For now, I think this works.

app/models/fingerprint.rb

@@ -15,4 +15,21 @@ class Fingerprint < ApplicationRecord
   validates :fingerprint, uniqueness: true
 
   alias_attribute :fingerprint_value, :fingerprint
+
+  # This is similar to the SuggestedResource fingerprint method, with the exception that it also replaces &quot; with "
+  # during its operation. This switch may also need to be added to the SuggestedResource method, at which point they can
+  # be abstracted to a helper method.
+  def self.calculate(phrase)
+    modified = phrase
+    modified = modified.strip
+    modified = modified.downcase
+    modified = modified.gsub('&quot;', '"') # This line does not exist in SuggestedResource implementation.
+    modified = modified.gsub(/\p{P}|\p{S}/, '')
+    modified = modified.to_ascii
+    modified = modified.gsub(/\p{P}|\p{S}/, '')
+    tokens = modified.split
+    tokens = tokens.uniq
+    tokens = tokens.sort
+    tokens.join(' ')
+  end
 end

app/models/term.rb

@@ -21,7 +21,7 @@ class Term < ApplicationRecord
   has_many :confirmations, dependent: :destroy
   belongs_to :fingerprint, optional: true
 
-  before_save :store_fingerprint
+  before_save :register_fingerprint
   after_destroy :check_fingerprint_count
 
   scope :user_confirmed, -> { where.associated(:confirmations).distinct }
@@ -87,27 +87,13 @@ def calculate_categorizations
 
   private
 
-  # The store_fingerprint method gets called before a Term record is saved, ensuring that Terms should always have a
+  # register_fingerprint method gets called before a Term record is saved, ensuring that Terms should always have a
   # related Fingerprint method.
-  def store_fingerprint
-    self.fingerprint = Fingerprint.find_or_create_by({ fingerprint: calculate_fingerprint })
-  end
-
-  # This is similar to the SuggestedResource fingerprint method, with the exception that it also replaces &quot; with "
-  # during its operation. This switch may also need to be added to the SuggestedResource method, at which point they can
-  # be abstracted to a helper method.
-  def calculate_fingerprint
-    modified = phrase
-    modified = modified.strip
-    modified = modified.downcase
-    modified = modified.gsub('&quot;', '"') # This line does not exist in SuggestedResource implementation.
-    modified = modified.gsub(/\p{P}|\p{S}/, '')
-    modified = modified.to_ascii
-    modified = modified.gsub(/\p{P}|\p{S}/, '')
-    tokens = modified.split
-    tokens = tokens.uniq
-    tokens = tokens.sort
-    tokens.join(' ')
+  def register_fingerprint
+    new_record = {
+      fingerprint: Fingerprint.calculate(phrase)
+    }
+    self.fingerprint = Fingerprint.find_or_create_by(new_record)
   end
 
   # This is called during the after_destroy hook. If removing that term means that its fingerprint is now abandoned,

test/models/fingerprint_test.rb

@@ -37,4 +37,49 @@ class FingerprintTest < ActiveSupport::TestCase
     assert_nil target_term.fingerprint_value
     assert_predicate target_term, :valid?
   end
+
+  # These tests appear in order of operation within the calculate method.
+  test 'fingerprints strip excess spaces' do
+    example = '  i  need  space   '
+
+    assert_equal 'i need space', Fingerprint.calculate(example)
+  end
+
+  test 'fingerprints are coerced to lower case' do
+    example = 'InterCapping FTW'
+
+    assert_equal 'ftw intercapping', Fingerprint.calculate(example)
+  end
+
+  test 'fingerprints strip out &quot;' do
+    example = '&quot;in quotes&quot;'
+
+    assert_equal 'in quotes', Fingerprint.calculate(example)
+  end
+
+  test 'fingerprints remove punctuation and symbols' do
+    example = 'symbols™ + punctuation: * bullets! - "quoted phrase" (perfect) ¥€$'
+
+    assert_equal 'bullets perfect phrase punctuation quoted symbols', Fingerprint.calculate(example)
+  end
+
+  test 'fingerprints coerce characters to ASCII' do
+    example = 'а а̀ а̂ а̄ ӓ б в г ґ д ђ ѓ е ѐ е̄ е̂ ё є ж з з́ ѕ и і ї ꙇ ѝ и̂ ӣ й ј к л љ м н њ о о̀ о̂ ō ӧ п р с с́ ' \
+                'т ћ ќ у у̀ у̂ ӯ ў ӱ ф х ц ч џ ш щ ꙏ ъ ъ̀ ы ь ѣ э ю ю̀ я'
+
+    assert_equal 'a b ch d dj dz dzh e f g gh gj i ia ie io iu j k kh kj l lj m n nj o p r s ' \
+                   'sh shch t ts tsh u v y yi z zh', Fingerprint.calculate(example)
+  end
+
+  test 'fingerprints remove repeated words' do
+    example = 'double double'
+
+    assert_equal 'double', Fingerprint.calculate(example)
+  end
+
+  test 'fingerprints list words in alphabetical order' do
+    example = 'delta beta gamma alpha'
+
+    assert_equal 'alpha beta delta gamma', Fingerprint.calculate(example)
+  end
 end