macos_version: define a method instead

Library/Homebrew/macos_version.rb

@@ -33,20 +33,15 @@ def initialize(version)
     el_capitan:  "10.11",
   }.freeze
 
-  # Map of macOS version strings to kernel major versions.
-  # https://en.wikipedia.org/wiki/MacOS_version_history#Releases
-  KERNEL_MAJOR_VERSIONS = {
-    "15"    => "24",
-    "14"    => "23",
-    "13"    => "22",
-    "12"    => "21",
-    "11"    => "20",
-    "10.15" => "19",
-    "10.14" => "18",
-    "10.13" => "17",
-    "10.12" => "16",
-    "10.11" => "15",
-  }.freeze
+  def self.kernel_major_version(macos_version)
+    version_major = macos_version.major.to_i
+    if version_major > 10
+      T.must(superclass).new((version_major + 9).to_s)
+    else
+      version_minor = macos_version.minor.to_i
+      T.must(superclass).new((version_minor + 4).to_s)
+    end
+  end
 
   sig { params(version: Symbol).returns(T.attached_class) }
   def self.from_symbol(version)

Library/Homebrew/test/macos_version_spec.rb

@@ -7,6 +7,18 @@
   let(:big_sur_major) { described_class.new("11.0") }
   let(:big_sur_update) { described_class.new("11.1") }
 
+  describe ".kernel_major_version" do
+    it "returns the kernel major version" do
+      expect(described_class.kernel_major_version(version)).to eq "18"
+      expect(described_class.kernel_major_version(big_sur_major)).to eq "20"
+      expect(described_class.kernel_major_version(big_sur_update)).to eq "20"
+    end
+
+    it "matches the major version returned by OS.kernel_version", :needs_macos do
+      expect(described_class.kernel_major_version(OS::Mac.version)).to eq OS.kernel_version.major
+    end
+  end
+
   specify "comparison with Symbol" do
     expect(version).to be > :high_sierra
     expect(version).to eq :mojave