&.downcase(Unicode::CaseOptions::Fold) options for casefolded compari…

…son of strings and characters.

spec/std/char_spec.cr

@@ -1,4 +1,5 @@
 require "spec"
+require "unicode"
 
 describe "Char" do
   describe "upcase" do
@@ -9,6 +10,7 @@ describe "Char" do
   describe "downcase" do
     it { 'A'.downcase.should eq('a') }
     it { '1'.downcase.should eq('1') }
+    it { '\u00df'.downcase(Unicode::CaseOptions::Fold) { |equiv| equiv.should eq('s') } }
   end
 
   describe "succ" do

spec/std/string_spec.cr

@@ -469,6 +469,10 @@ describe "String" do
     it { "AEIİOU".downcase(Unicode::CaseOptions::Turkic).should eq("aeıiou") }
     it { "ÁEÍOÚ".downcase(Unicode::CaseOptions::ASCII).should eq("ÁeÍoÚ") }
     it { "İ".downcase.should eq("i̇") }
+    it { "Baffle".downcase(Unicode::CaseOptions::Fold).should eq("baffle") }
+    it { "ff".downcase(Unicode::CaseOptions::Fold).should eq("ff") }
+    it { "tschüß".downcase(Unicode::CaseOptions::Fold).should eq("tschüss") }
+    it { "ΣίσυφοςfiÆ".downcase(Unicode::CaseOptions::Fold).should eq("σίσυφοσfiæ") }
   end
 
   describe "upcase" do

src/unicode/unicode.cr

@@ -22,6 +22,9 @@ module Unicode
     # 'ı'.upcase(Unicode::CaseOptions::Turkic)   # => 'I'
     # ```
     Turkic
+
+    # Unicode case folding, which is more far-reaching than Unicode case mapping.
+    Fold
   end
 
   def self.upcase(char : Char, options : CaseOptions)
@@ -94,6 +97,9 @@ module Unicode
     result = check_downcase_turkic(char, options)
     return result if result
 
+    results = check_downcase_fold(char, options)
+    return results[0].unsafe_chr if results && results.size == 1
+
     check_downcase_ranges(char)
   end
 
@@ -110,6 +116,12 @@ module Unicode
       return
     end
 
+    result = check_downcase_fold(char, options)
+    if result
+      result.each { |c| yield c.unsafe_chr if c != 0 }
+      return
+    end
+
     result = special_cases_downcase[char.ord]?
     if result
       result.each { |c| yield c.unsafe_chr if c != 0 }
@@ -141,6 +153,13 @@ module Unicode
     nil
   end
 
+  private def self.check_downcase_fold(char, options)
+    if options.fold?
+      return fold_cases[char.ord]?
+    end
+    nil
+  end
+
   private def self.check_downcase_ranges(char)
     result = search_ranges(downcase_ranges, char.ord)
     return char + result if result