Bug #8129

String#index has drastically different performance when a single unicode character is included

Added by zmoazeni (Zach Moazeni) over 7 years ago. Updated over 7 years ago.

Target version:
ruby -v:


I created a simple ruby script:

#! /usr/bin/env ruby

raise "need a file name" unless ARGV[0]
contents =[0])

326_000.times do |i|
contents[(i + 23) % contents.size]

And I uploaded two files below. One is all ASCII characters and the other has a single Unicode character in the first line (an "em dash").

String#index has dramatically different performance for the two strings. Locally, I'm seeing ~1.5 seconds with all_ascii.css and ~30 seconds with one_unicode.css on 1.9.3-p385. It gets worse with ruby 2.0, all_ascii.css still takes ~1 sec, but one_unicode.css takes ~2.5 minutes!

Any idea why the performance is so dramatically different between the two?


all_ascii.css (193 KB) all_ascii.css zmoazeni (Zach Moazeni), 03/20/2013 08:23 AM
one_unicode.css (193 KB) one_unicode.css The first line contains a unicode "em dash", otherwise all ascii zmoazeni (Zach Moazeni), 03/20/2013 08:23 AM

Updated by charliesome (Charlie Somerville) over 7 years ago

  • Status changed from Open to Rejected

When all the characters in a string are ASCII characters (single bytes), the byte index for any given character can be calculated in constant time.

When the string contains multibyte characters, finding the byte index given a character index becomes O(n).

If you need fast character indexing, try splitting the string into an array or characters.

Updated by nobu (Nobuyoshi Nakada) over 7 years ago

  • Description updated (diff)

Updated by nobu (Nobuyoshi Nakada) over 7 years ago

You may want to:

  • use regexp, e.g. (({scan})).
  • convert to fix width wide char encoding, i.e., ((|UTF-32LE|)) or ((|UTF-32BE|)). =end

Updated by zmoazeni (Zach Moazeni) over 7 years ago

Thanks for the feedback Charlie and Nobuyoshi. This came up from which heavily uses String#index ( by passing a position to search from as the source content was consumed.

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