Created: 2012-08-21 05:09
Updated: 2019-02-19 12:35
License: other


This is a little C library for heavyweight utf-8 strings (rope). Unlike regular C strings, ropes can do substring insertion and deletion in O(log n) time.

librope is implemented using skip lists, which have the same big-O time complexity as trees but don't require rebalancing.

librope is fast. It will happily perform ~1-5 million edit operations per second, depending on the size of your strings. Inserts and deletes in librope outperform straight C strings for any document longer than a few hundred bytes.


Just add rope.c and rope.h to your project.

// Make a new empty rope
rope *r = rope_new();

// Put some content in it (at position 0)
rope_insert(r, 0, "Hi there!");

// Delete 6 characters at position 2
rope_delete(r, 2, 6);

// Get the whole string back out of the rope
uint8_t *str = rope_createcstr(r, NULL);

// str now contains "Hi!"!

// Done with the rope

Wide Character String Compatibility

String insertion / deletion positions in Javascript, Objective-C (NSString), Java, C# and others are wrong sometimes!!!

These languages store strings as wchar arrays (arrays of two byte characters). Some characters in the unicode character set require more than two bytes. These languages encode such characters using multiple wchars as per UTF-16. This works most of the time. However, insertion and deletion positions in these strings still refer to offsets in the underlying array. So unicode characters which take up 4 bytes in UTF-16 count as two characters for the purpose of deletion ranges, insertion positions and string length.

Even though these characters are exceptionally rare, I don't want my editor to go all funky if people start getting creative. About a quarter of librope's code is dedicated to fixing this mismatch. However, bookkeeping isn't free - librope performance drops by 35% when wchar conversion support is enabled.

For more information, read my blog post about it.

Long story short, if you need to interoperate with strings from any of these dodgy languages, here's what you do:

  • Compile with -DROPE_WCHAR=1. This macro enables the expensive wchar bookkeeping.
  • Use the alternate insert & delete functions rope_insert_at_wchar(...) and rope_del_at_wchar(...) when your index / size is specified in UTF-16 offsets.

Take a look at the header file for documentation.


  • When using rope_insert_at_wchar you still need to convert the string you're inserting into UTF-8 before you pass it into librope.
  • The API lets you try to delete or insert halfway through a large character. You probably don't want to do that.
  • librope is 100% faithful when it comes to the characters you're inserting. If your string has byte order marks, you might want to remove them before passing the string into librope.
Cookies help us deliver our services. By using our services, you agree to our use of cookies Learn more