macos_version: add KERNEL_MAJOR_VERSIONS map

Library/Homebrew/macos_version.rb

@@ -31,6 +31,17 @@ def initialize(version)
     el_capitan:  "10.11",
   }.freeze
 
+  sig { params(macos_version: MacOSVersion).returns(Version) }
+  def self.kernel_major_version(macos_version)
+    version_major = macos_version.major.to_i
+    if version_major > 10
+      Version.new((version_major + 9).to_s)
+    else
+      version_minor = macos_version.minor.to_i
+      Version.new((version_minor + 4).to_s)
+    end
+  end
+
   sig { params(version: Symbol).returns(T.attached_class) }
   def self.from_symbol(version)
     str = SYMBOLS.fetch(version) { raise MacOSVersion::Error, 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