Implement GPA score

I'm trying to follow the [GPA (grade point average)](https://bigfuture.collegeboard.org/plan-for-college/college-basics/how-to-convert-gpa-4.0-scale)
but the example there used the 4.0 scale, and I'm not pretty sure if we
want that.

Closes #95

lib/skunk/cli/grade_point_average.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+module Skunk
+ module Cli
+ class GradePointAverage
+ def initialize(skunk_score)
+ @skunk_score = skunk_score
+ end
+
+ def score
+ case @skunk_score
+ when (0..64)
+ "A+"
+ when (65..66)
+ "A"
+ when (67..69)
+ "A-"
+ when (70..72)
+ "B+"
+ when (73..76)
+ "B"
+ when (77..79)
+ "B-"
+ when (80..82)
+ "C+"
+ when (83..86)
+ "C"
+ when (87..89)
+ "C-"
+ when (90..92)
+ "D+"
+ when (93..96)
+ "D"
+ when (93..Float::INFINITY)
+ "E"
+ end
+ end
+ end
+ end
+end

test/lib/skunk/cli/grade_point_average_test.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+=begin
+|Letter Grade | Percent Grade | Scale |
+|:------------|:-------------:|------:|
+| A+ | Below 65 | 4.0 |
+| A | 65-66 | 4.0 |
+| A- | 67-69 | 3.7 |
+| B+ | 70-72 | 3.3 |
+| B | 73-76 | 3.0 |
+| B- | 77-79 | 2.7 |
+| C+ | 80-82 | 2.3 |
+| C | 83-86 | 2.0 |
+| C- | 87-89 | 1.7 |
+| D+ | 90-92 | 1.3 |
+| D | 93-96 | 1.0 |
+| E/F | 97-100 | 0.0 |
+=end
+
+require "test_helper"
+
+require "skunk/cli/grade_point_average"
+
+describe Skunk::Cli::GradePointAverage do
+ subject { Skunk::Cli::GradePointAverage }
+
+ describe "#score" do
+ context "with A score" do
+ it { expect(subject.new(0).score).must_equal "A+" }
+ it { expect(subject.new(65).score).must_equal "A" }
+ it { expect(subject.new(66).score).must_equal "A" }
+ it { expect(subject.new(67).score).must_equal "A-" }
+ it { expect(subject.new(69).score).must_equal "A-" }
+ end
+
+ context "with B score" do
+ it { expect(subject.new(70).score).must_equal "B+" }
+ it { expect(subject.new(72).score).must_equal "B+" }
+ it { expect(subject.new(73).score).must_equal "B" }
+ it { expect(subject.new(76).score).must_equal "B" }
+ it { expect(subject.new(77).score).must_equal "B-" }
+ it { expect(subject.new(79).score).must_equal "B-" }
+ end
+
+ context "with C score" do
+ it { expect(subject.new(80).score).must_equal "C+" }
+ it { expect(subject.new(82).score).must_equal "C+" }
+ it { expect(subject.new(83).score).must_equal "C" }
+ it { expect(subject.new(86).score).must_equal "C" }
+ it { expect(subject.new(87).score).must_equal "C-" }
+ it { expect(subject.new(89).score).must_equal "C-" }
+ end
+
+ context "with D score" do
+ it { expect(subject.new(90).score).must_equal "D+" }
+ it { expect(subject.new(92).score).must_equal "D+" }
+ it { expect(subject.new(93).score).must_equal "D" }
+ it { expect(subject.new(96).score).must_equal "D" }
+ end
+
+ context "with E score" do
+ it { expect(subject.new(97).score).must_equal "E" }
+ it { expect(subject.new(1000).score).must_equal "E" }
+ it { expect(subject.new(10000).score).must_equal "E" }
+ end
+ end
+end