From 4b9976f7b7973c0dc98294c4761a0e48b0cb6184 Mon Sep 17 00:00:00 2001
From: Emmanuel T Odeke <emmanuel@orijtech.com>
Date: Mon, 17 Oct 2022 12:21:40 -0700
Subject: [PATCH] prune: speed up using 1<<N instead of math.Pow(2, N)
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This change fixes the use of expensive code that used
math.Pow(float(2), N) but really that's expensive code
that could be simplified by using left bitshifts.

The improvements in speed show in benchmarks:

```shell
$ benchstat before.txt after.txt
name    old time/op    new time/op    delta
Pow2-8     322ns ± 2%     114ns ± 1%  -64.69%  (p=0.000 n=9+8)

name    old alloc/op   new alloc/op   delta
Pow2-8     40.0B ± 0%     40.0B ± 0%     ~     (all equal)

name    old allocs/op  new allocs/op  delta
Pow2-8      5.00 ± 0%      5.00 ± 0%     ~     (all equal)
```

Fixes #72
---
 subrootpaths.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/subrootpaths.go b/subrootpaths.go
index d9f5f208..0e333b08 100644
--- a/subrootpaths.go
+++ b/subrootpaths.go
@@ -71,7 +71,9 @@ func prune(idxStart uint, idxEnd uint, maxWidth uint) [][]int {
 	rightTraversed := false
 
 	for i := treeDepth - 1; i >= 0 && capturedSpan < idxEnd; i-- {
-		nodeSpan := uint(math.Pow(float64(2), float64(treeDepth-i)))
+		// nodeSpan is 2**(treeDepth-i) == 1<<(treeDepth-i)
+		// Please see: https://github.com/celestiaorg/nmt/issues/72
+		nodeSpan := uint(1 << (treeDepth - i))
 		if pathStart[i] == 0 {
 			// if nodespan is less than end index, continue traversing upwards
 			lastNode := nodeSpan + idxStart - 1