Issue #12373 has been updated by hsbt (Hiroshi SHIBATA).

File csv-shift-hsbt.path added
Target version set to 2.5

@ksss

I confirm your patch and benchmark results.

```
Warming up --------------------------------------
           csv_shift     1.000  i/100ms
       new_csv_shift     1.000  i/100ms
Calculating -------------------------------------
           csv_shift      1.192  (¡Þ 0.0%) i/s -      6.000  in   5.034250s
       new_csv_shift      1.527  (¡Þ 0.0%) i/s -      8.000  in   5.243446s

Comparison:
       new_csv_shift:        1.5 i/s
           csv_shift:        1.2 i/s - 1.28x  slower
```

Should we use `@double_quote_char` instead of `@quote_char` ?
your patch named csv-shift-3.patch is only assign `@double_quote_char`. It's not used in csv.rb

I update replacement it in csv.rb and attched csv-shift-hsbt.patch.

Can you confirm it? If it's ok, I will merge it.

----------------------------------------
Bug #12373: Optimize CSV#shift
https://bugs.ruby-lang.org/issues/12373#change-64852

* Author: ksss (Yuki Kurihara)
* Status: Assigned
* Priority: Normal
* Assignee: hsbt (Hiroshi SHIBATA)
* Target version: 2.5
* ruby -v: ruby 2.4.0dev (2016-05-11 trunk 54976) [x86_64-darwin15]
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
I think that **`str1.start_with?(str2)`** is faster than **`str1[0] == str2`**.
Because **`str1.start_with?(str2)`** just call **`String#start_with?`**, But **`str1[0] == str2`** call **`String#[]`**, make new `String` object and call **`String#==`**.

(The patch is csv-shift.patch)

Benchmark results.

csv-benchmark.rb make temp CSV file and call **`CSV#each`** method(inner call **`CSV#shift`**)

~~~
$ ruby csv-benchmark.rb
Warming up --------------------------------------
       old_csv_shift     1.000  i/100ms
       new_csv_shift     1.000  i/100ms
Calculating -------------------------------------
       old_csv_shift      0.444  (¡Þ 0.0%) i/s -      3.000  in   6.759200s
       new_csv_shift      0.479  (¡Þ 0.0%) i/s -      3.000  in   6.264069s

Comparison:
       new_csv_shift:        0.5 i/s
       old_csv_shift:        0.4 i/s - 1.08x slower
~~~

string-start_with.rb is a micro benchmark for **`str1[0] == str2`** and **`str1.start_with?(str2)`**

~~~
$ ruby string-start_with.rb
Warming up --------------------------------------
           a[0] == b    90.881k i/100ms
    a.start_with?(b)   115.557k i/100ms
Calculating -------------------------------------
           a[0] == b      1.836M (¡Þ 3.8%) i/s -      9.179M in   5.006568s
    a.start_with?(b)      3.183M (¡Þ 4.2%) i/s -     15.947M in   5.018654s

Comparison:
    a.start_with?(b):  3183386.0 i/s
           a[0] == b:  1836263.5 i/s - 1.73x slower
~~~

Of course $ make test-all TESTS="test/csv/*" passed


---Files--------------------------------
csv-benchmark.rb (561 Bytes)
old_csv_shift.rb (5.1 KB)
new_csv_shift.rb (5.11 KB)
string-start_with.rb (215 Bytes)
csv-shift.patch (434 Bytes)
csv-shift-1.patch (1.15 KB)
csv-shift-2.patch (1.2 KB)
csv-shift-3.patch (2.13 KB)
csv-shift-hsbt.path (2.05 KB)


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