Util.filter_halves.go: def of unified-nsga-iii@0.2.2.0
Import: import unified-nsga-iii@0.2.2.0/util.bend as Util
Lines 305-328 of unified-nsga-iii@0.2.2.0/util.bend: https://jend.leeeet.dev/read.txt?path=unified-nsga-iii@0.2.2.0/util.bend&start_line=305&end_line=328

import Base

# Independent keep/drop. Mid-split; `take ++ drop` matches the old
# tail-first cons_if walk (original relative order).
def filter_halves.go(
  ~A: Data,
  ~C: Data,
  ~p: @+c:C -> @+x:A -> Bool,
  +ctx: C,
  +fuel: Nat,
  xs: List<&2, A>
) -> List<&2, A>:
  match fuel xs:
    case 0n Nil{}:
      Nil{}
    case 0n +h <> t:
      keep_if(A, h, filter_halves.go(~A, ~C, ~p, ctx, 0n, t), p(ctx, h))
    case 1n+p0 Nil{}:
      Nil{}
    case 1n+p0 +h <> Nil{}:
      keep_if(A, h, Nil{}, p(ctx, h))
    case 1n+p0 h <> t:
      +all = {h <> t : List<&2, A>}
      +mid = Nat.div(list_len(A, all), 2n)
      a b = filter_halves.go(~A, ~C, ~p, ctx, p0, List.take(&2, A, all, mid)) filter_halves.go(~A, ~C, ~p, ctx, p0, List.drop(&2, A, all, mid))
      List.append(&2, A, a, b)

Uses (9):
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Bool
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List.append
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List.drop
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List.take
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Nat
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Nat.div
  https://jend.leeeet.dev/definition.txt?path=unified-nsga-iii@0.2.2.0/util.bend&name=keep_if
  https://jend.leeeet.dev/definition.txt?path=unified-nsga-iii@0.2.2.0/util.bend&name=list_len