Lazy evaluation chain

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
20/100
Issue type
Feature
Clarity
Needs clarification
Activity status
Stale
Tech stack
rust
Domain
data, performance

Research direction

Start with the linked pull-request discussion, then inspect the existing ArrayBase methods that could move into ArrayOps. Review the proposed ArrayBase.chain(), LazyChain, and LazyChain.eval() API along with the ownership and copy-versus-in-place decision points. Done requires those design questions to be resolved and the lazy operation chain specified.

Written by the indexing model from the issue text.

Description

Motivation

Following from discussion starting here: https://github.com/rust-ndarray/ndarray/pull/1042#issuecomment-957134294

With contemporary systems, moving the data from memory into caches and registers often dominates the runtime of numerical algorithms depending on how many arithmetic operations they perform relative to each memory access. Fusing operations from multiple passes into a single pass increases this ratio and thereby the likelihood of efficient CPU utilisation. [1]

In theory this problem is solved using loops and/or mapv, but this doesn't provide a very user-friendly interface. Users who want to work at a high level still want to be able to use arr.pow(), arr.log() etc, but also retain the speed of this per-element processing (rather than applying each operation to each element and then the next operation to each element).

Proposed Changes

  • We move a subset of the ArrayBase methods into an ArrayOps trait. Note: this probably can't be all methods because it can only work for methods that return an array, so axes, as_slice, get etc don't necessarily make sense
  • This trait gets implemented by ArrayBase, but also a new struct called LazyChain (or something idk)
  • LazyChain gets created by ArrayBase.chain()
  • LazyChain will own (?) an ArrayBase, as well as a list of operations that will be sequentially applied to it
  • Then, LazyChain.eval() runs these operations sequentially on each element of the owned array, and then returns the modified array

Decision Points

  • Names for structs and traits
  • Should LazyChain return a copy or a modified in-place array?
  • Should LazyChain own the array or just borrow it? Should we have two different types do each?
Dominant language
Rust
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