c# - Is there any scenario where the Rope data structure is more efficient than a string builder

Question

Related to this question, based on a comment of user Eric Lippert.

Is there any scenario where the Rope data structure is more efficient than a string builder? It is some people's opinion that rope data structures are almost never better in terms of speed than the native string or string builder operations in typical cases, so I am curious to see realistic scenarios where indeed ropes are better.

Answer 1

The documentation for the SGI C++ implementation goes into some detail on the big O behaviours verses the constant factors which is instructive.

Their documentation assumes very long strings being involved, the examples posited for reference talk about 10 MB strings. Very few programs will be written which deal with such things and, for many classes of problems with such requirements reworking them to be stream based rather than requiring the full string to be available where possible will lead to significantly superior results. As such ropes are for non streaming manipulation of multi megabyte character sequences when you are able to appropriately treat the rope as sections (themselves ropes) rather than just a sequence of characters.

Significant Pros:

• Concatenation/Insertion become nearly constant time operations
• Certain operations may reuse the previous rope sections to allow sharing in memory.
  • Note that .Net strings, unlike java strings do not share the character buffer on substrings - a choice with pros and cons in terms of memory footprint. Ropes tend to avoid this sort of issue.
• Ropes allow deferred loading of substrings until required
  • Note that this is hard to get right, very easy to render pointless due to excessive eagerness of access and requires consuming code to treat it as a rope, not as a sequence of characters.

Significant Cons:

• Random read access becomes O(log n)
• The constant factors on sequential read access seem to be between 5 and 10
• efficient use of the API requires treating it as a rope, not just dropping in a rope as a backing implementation on the 'normal' string api.

This leads to a few 'obvious' uses (the first mentioned explicitly by SGI).

• Edit buffers on large files allowing easy undo/redo
  • Note that, at some point you may need to write the changes to disk, involving streaming through the entire string, so this is only useful if most edits will primarily reside in memory rather than requiring frequent persistence (say through an autosave function)
• Manipulation of DNA segments where significant manipulation occurs, but very little output actually happens
• Multi threaded Algorithms which mutate local subsections of string. In theory such cases can be parcelled off to separate threads and cores without needing to take local copies of the subsections and then recombine them, saving considerable memory as well as avoiding a costly serial combining operation at the end.

There are cases where domain specific behaviour in the string can be coupled with relatively simple augmentations to the Rope implementation to allow:

• Read only strings with significant numbers of common substrings are amenable to simple interning for significant memory savings.
• Strings with sparse structures, or significant local repetition are amenable to run length encoding while still allowing reasonable levels of random access.
• Where the sub string boundaries are themselves 'nodes' where information may be stored, though such structures are quite possible better done as a Radix Trie if they are rarely modified but often read.

As you can see from the examples listed, all fall well into the 'niche' category. Further, several may well have superior alternatives if you are willing/able to rewrite the algorithm as a stream processing operation instead.