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Explore potential for optimization in array::insert(Iter, InputIt, InputIt)

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
30/100
Issue type
Refactor
Clarity
Mostly clear
Activity status
Stale
Tech stack
cpp
Domain
performance

Research direction

Start at array::insert(Iter, InputIt, InputIt) and trace its buffer-growth and exception-guarantee paths. Evaluate the proposed end-fill, temporary-buffer, relocation, and rotation cases; done means avoiding even temporary allocation when existing capacity fits while preserving the strong guarantee.

Written by the indexing model from the issue text.

Description

Performance

The idea is this:

  1. Fill the buffer up to capacity at the end (needs a revert_insert protection).
  2. Create a temporary buffer with the remaining elements (can throw).
  3. If temporary buffer is not empty
    1. Allocate new buffer with necessary size (can throw).
    2. Relocate elements
      • first original elements, up to insertion point,
      • then new elements (at the end of the original buffer),
      • then new elements from the temporary buffer,
      • then remaining old elements.
  4. If the temporary buffer is empty, rotate elements in the original buffer instead.

In the end result:

  1. If the original capacity could accommodate the input range, then no new buffer (even a temporary one) is allocated. This is particularly useful when the caller uses input iterators, but does know the input range size, and thus can call reserve.
  2. We should be able to keep the strong guarantee.
Dominant language
C++
Stars
479
Forks
110
Avg merge
1d 23h
Merged PRs (30d)
3

Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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