From c34d01ff6a18dac04a90b2e0f820cdb1d5c7e1b2 Mon Sep 17 00:00:00 2001
From: Watson <watson1978@gmail.com>
Date: Sat, 3 Mar 2018 01:16:34 +0900
Subject: Convert string encoding to UTF-8 only when needed
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## Before
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json   129.000  i/100ms
Calculating -------------------------------------
                json      1.300k (± 2.3%) i/s -      6.579k in   5.064656s
```

## After
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json   189.000  i/100ms
Calculating -------------------------------------
                json      1.964k (± 3.3%) i/s -      9.828k in   5.011237s
```

## Code
```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id" => i,
    :age => 42,
  }
end

Benchmark.ips do |x|
  x.report "json" do |iter|
    count = 0
    while count < iter
      JSON.generate(obj)
      count += 1
    end
  end
end
```
---
 ext/json/ext/generator/generator.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
index a971a06..cebe7e9 100644
--- a/ext/json/ext/generator/generator.c
+++ b/ext/json/ext/generator/generator.c
@@ -846,7 +846,9 @@ static void generate_json_string(FBuffer *buffer, VALUE Vstate, JSON_Generator_S
 {
     fbuffer_append_char(buffer, '"');
 #ifdef HAVE_RUBY_ENCODING_H
-    obj = rb_str_encode(obj, CEncoding_UTF_8, 0, Qnil);
+    if (!rb_enc_str_asciicompat_p(obj)) {
+        obj = rb_str_encode(obj, CEncoding_UTF_8, 0, Qnil);
+    }
 #endif
     if (state->ascii_only) {
         convert_UTF8_to_JSON_ASCII(buffer, obj);
-- 
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