Limit the number of symbol completion candidates and make it faster

lib/repl_type_completor/result.rb

@@ -64,7 +64,7 @@ def completion_candidates
       in [:gvar, name, scope]
         scope.global_variables
       in [:symbol, name]
-        Symbol.all_symbols.map { _1.inspect[1..] }
+        filter_symbol_candidates(Symbol.all_symbols, name, limit: 100)
       in [:call, name, type, self_call]
         (self_call ? type.all_methods : type.methods).map(&:to_s) - HIDDEN_METHODS
       in [:lvar_or_method, name, scope]
@@ -116,6 +116,25 @@ def doc_namespace(matched)
 
     private
 
+    def filter_symbol_candidates(symbols, prefix, limit:)
+      sym_prefix = ":#{prefix}"
+      candidates = symbols.filter_map do |s|
+        next unless s.start_with?(prefix) # Fast and inaccurate check before calling inspect
+
+        inspect = s.inspect
+        inspect[1..] if inspect.start_with?(sym_prefix) # Reject `:"a b"` when completing `:a`
+      rescue EncodingError
+        # ignore
+      end
+
+      if candidates.size > limit
+        # min(n) + max(n) is faster than sort and slice
+        candidates.min(limit - limit / 2) + candidates.max(limit / 2).reverse
+      else
+        candidates.sort
+      end
+    end
+
     def method_doc(type, name)
       type = type.types.find { _1.all_methods.include? name.to_sym }
       case type

test/repl_type_completor/test_repl_type_completor.rb

@@ -63,6 +63,16 @@ def test_symbol
       assert_completion(prefix, include: sym.inspect.delete_prefix(prefix))
     end
 
+    def test_symbol_limit
+      result = ReplTypeCompletor::Result.new([:symbol, 'sym'], binding, 'file')
+      symbols = [:ae, :ad, :ab1, :ab2, :ac, :aa, :b, :"a a", 75.chr('utf-7').to_sym]
+      assert_equal(%w[aa ab1 ab2 ac ad ae], result.send(:filter_symbol_candidates, symbols, 'a', limit: 100))
+      assert_equal(%w[aa ab1 ab2 ad ae], result.send(:filter_symbol_candidates, symbols, 'a', limit: 5))
+      assert_equal(%w[aa ab1 ad ae], result.send(:filter_symbol_candidates, symbols, 'a', limit: 4))
+      assert_equal(%w[ab1 ab2], result.send(:filter_symbol_candidates, symbols, 'ab', limit: 4))
+      assert_equal([], result.send(:filter_symbol_candidates, symbols, 'c', limit: 4))
+    end
+
     def test_call
       assert_completion('1.', include: 'abs')
       assert_completion('1.a', include: 'bs')