AppendInLoop.Append.message: def of bend-lint-rules@0.2.0.0
Import: import bend-lint-rules@0.2.0.0/performance/append_in_loop.bend as AppendInLoop
Lines 44-86 of bend-lint-rules@0.2.0.0/performance/append_in_loop.bend: https://jend.leeeet.dev/read.txt?path=bend-lint-rules@0.2.0.0/performance/append_in_loop.bend&start_line=44&end_line=86

import Base
import bend-lint-api@0.2.0.0/linter/lint.bend as Lint

# What a finding says about an append of `acc` in the loop `loop`. `args` are the append's arguments.
def Append.message(
  a: Append,
  loop: String,
  +acc: String,
  +source: String,
  +args: List<&2, Lint.Node>
) -> String:
  match a:
    case ListAppend{}:
      +q = Lint.Span.text(source, Lint.Node.span(List.get(&2, Lint.Node, args, 0n)))
      +t = Lint.Span.text(source, Lint.Node.span(List.get(&2, Lint.Node, args, 1n)))
      "List.append copies "
        ++ acc
        ++ " on each call of "
        ++ loop
        ++ ", so a loop of n steps copies O(n²) items. All lanes. Confidence: high, as the copies grow with the square of the steps. Add each item at the head instead, with `item <> "
        ++ acc
        ++ "`, and reverse "
        ++ acc
        ++ " once when the loop ends: `List.reverse("
        ++ q
        ++ ", "
        ++ t
        ++ ", "
        ++ acc
        ++ ")`; to add a list each step, collect the lists, with `piece <> pieces`, and join them once: `List.concat("
        ++ q
        ++ ", "
        ++ t
        ++ ", List.reverse("
        ++ q
        ++ ", List<"
        ++ q
        ++ ", "
        ++ t
        ++ ">, pieces))`. If the loop runs only a few steps, expect this finding."
    case StringAppend{}:
      "++ copies "
        ++ acc
        ++ " on each call of "
        ++ loop
        ++ ", so a loop of n steps copies O(n²) characters. CPU and GPU: measured 12.6 s for 20,000 lines, against 0.07 s after the fix. JS: strings join natively, at no cost. Confidence: high, as the copies grow with the square of the steps. Collect the pieces in a list instead, with `piece <> pieces`, and join them once when the loop ends: `String.concat(List.reverse(&2, String, pieces))`. If the loop runs only a few steps, expect this finding."

Uses (8):
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List.get
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=String
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=String.append
  https://jend.leeeet.dev/definition.txt?path=bend-lint-api@0.2.0.0/linter/lint.bend&name=Node
  https://jend.leeeet.dev/definition.txt?path=bend-lint-api@0.2.0.0/linter/lint.bend&name=Node.span
  https://jend.leeeet.dev/definition.txt?path=bend-lint-api@0.2.0.0/linter/lint.bend&name=Span.text
  https://jend.leeeet.dev/definition.txt?path=bend-lint-rules@0.2.0.0/performance/append_in_loop.bend&name=Append