update significant-digits to not use string conversions (#862)

Thanks to kfarmer (a
[conversation](https://discord.com/channels/601130461678272522/615253963645911060/1247031648965492766)
on discord) for proposing his version and motivating me for this update

stdlib-candidate/std-rfc/math/mod.nu

@@ -1,6 +1,3 @@
-
-use std iter scan
-
 # replace all insignificant digits with 0
 #
 # | Significant Digits | Maximum Relative Error |
@@ -18,29 +15,50 @@ use std iter scan
 # > 1.2346789 | significant-digits 3
 # 1.23
 #
+# > 123456789.89 | significant-digits 5
+# 123450000
+#
 # > 1sec / 3 | math significant-digits
 # 333ms
 export def 'significant-digits' [
- n: int = 4 # a number of first non-zero digits to keep
+ n: int = 3 # a number of significant digits
 ]: [int -> int, float -> float, duration -> duration] {
  let $input = $in
- let $chars = $input | into string | split chars
+ let $type = $input | describe
+
+ let $num = match $type {
+ 'duration' => {$input | into int}
+ _ => {$input}
+ }
 
- let $rounded_str = $chars
- | scan --noinit 0 {|ind i|
- if $i =~ '\d' {
- if $ind == 0 and $i == '0' {
- $ind
- } else { $ind + 1 }
- } else {$ind}
+ let insignif_position = $n - 1 - ($num | math abs | math log 10 | math floor)
+
+ # See the note below the code for an explanation of the construct used.
+ let scaling_factor = 10 ** ($insignif_position | math abs)
+
+ let res = $num
+ | if $insignif_position > 0 {
+ $in * $scaling_factor
+ } else {
+ $in / $scaling_factor
+ }
+ | math floor
+ | if $insignif_position <= 0 {
+ $in * $scaling_factor
+ } else {
+ $in / $scaling_factor
  }
- | zip $chars
- | each {|i| if $i.1 =~ '\d' and $i.0 > $n {'0'} else {$i.1}}
- | str join
 
- match ($input | describe) {
- 'duration' => {$rounded_str | into duration}
- 'int' => {$rounded_str | into int}
- 'float' => {$rounded_str | into float}
+ match $type {
+ 'duration' => {$res | into duration}
+ 'int' => {$res | into int}
+ _ => {$res}
  }
 }
+
+# I started with `10.0 ** $insignif_position`, but it was sometimes producing
+# not rounded digits in `$num / $scaling_factor` if `$insignif_position` was negative
+# like with
+# > 3456789 | math round --precision -5
+# 3499999.9999999995
+# so I use what I have now.