From 7025dfed3ef57840286d81a19cac517bcd62f321 Mon Sep 17 00:00:00 2001
From: Marshall Lochbaum <mwlochbaum@gmail.com>
Date: Mon, 3 Aug 2020 13:37:35 -0400
Subject: Non-recursive iterative Indices by scanning backwards for nonzeros

---
 impl.bqn | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/impl.bqn b/impl.bqn
index 4a1c82bb..79c64fa3 100644
--- a/impl.bqn
+++ b/impl.bqn
@@ -71,9 +71,11 @@ Indices←{
   l←≠𝕩
   {
     ! 1×´Nat¨𝕩
-    es ← +`𝕩 ⋄ e←0 ⋄ Ei←{e↩𝕩⊑es}
-    Ei i←0
-    {j←𝕩⋄U←{U⍟(j=Ei)1+𝕩}⋄{i↩U𝕩}⍟(𝕩=e)i}¨↕(l-1)⊑es
+    k←l-1
+    N ← ((⊢+-×0=𝕩⊑˜⊢)`k⊸-⌜↕l)⊑˜k-⊢  # Next nonzero
+    E ← ⊑⟜(+`𝕩)
+    ei←E i←N 0
+    {{ei↩E i↩N𝕩+1⋄i}⍟(𝕩=ei)i}¨↕E k
   }⍟(0<l)𝕩
 }
 
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