From 920a06ebc26ae925ecd2b975c07a56db63109b43 Mon Sep 17 00:00:00 2001
From: Marshall Lochbaum <mwlochbaum@gmail.com>
Date: Thu, 6 Aug 2020 15:43:04 -0400
Subject: Faster structural Under: only grade the inserted indices

---
 impl.bqn | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/impl.bqn b/impl.bqn
index fafbad43..90d5f6fb 100644
--- a/impl.bqn
+++ b/impl.bqn
@@ -106,6 +106,16 @@ Indices←{
   }⍟(0<l)𝕩
 }
 
+_under_←{
+  i←↕l←1×´s←≢𝕩
+  v←⥊𝕨𝔽○𝔾𝕩
+  g←Cmp0 _grade gi←⥊𝔾s⥊i
+  P←(≠g)⊸≤◶⟨(⊑⟜g)⊑gi˜,l⟩
+  e←P j←0
+  s⥊{e=𝕩}◶⟨⊑⟜(⥊𝕩),{𝕩⋄r←(j⊑g)⊑v⋄e↩P j↩1+j⋄r}⟩¨i
+}
+⌾ ← _under_
+
 + ↩ + _perv
 - ↩ - _perv
 × ↩ (0⊸(<->) ⊘ ×) _perv
@@ -441,10 +451,3 @@ _repeat_←{
   (|⊑<⟜0⊑pos‿neg˜)⚇0 n
 }
 ⍟ ↩ _repeat_
-
-_under_←{
-  i←↕×´s←≢𝕩
-  v←⥊𝕨𝔽○𝔾𝕩⋄gi←⥊𝔾s⥊i⋄k←¬i∊gi
-  s⥊(⍋(/k)∾gi)⊏(k/⥊𝕩)∾v
-}
-⌾ ← _under_
-- 
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