Re: efficient way to use matched string in variable substitution

[Koichi Murase]

Fun to see different techniques from different people. I have also
played with this interesting problem. In my environment, the following
implementations seem to be the fastest.

f21b() { local -a "arr=('\${1:'{0..$((${#1}-1))}':1}')";
arr=("${arr[@]@P}"); } # Any strings
f31() { local arr i=${#1} v=$1 x='arr[--i]=v%10,v/=10,i&&x'; : $((x));
} # Only numbers ([0-9]+)

I compared the following implementations:

# L A Walsh
f1() { local x n="$1" arr; for x in 0 1 2 3 4 5 6 7 8 9; do
n=${n//$x/$x }; done; arr=($n); }
f2() { local i n="${#1}" arr; for ((i=0; i<n; i++)); do
arr+=("${1:i:1}"); done; }

# Greg Wooledge
f3() { local n="$1" tmp arr i; while ((n > 0)); do tmp+=("$((n%10))");
((n /= 10)); done; for ((i=${#tmp[@]}-1; i >= 0; i--)); do
arr+=("${tmp[i]}"); done; }
f4() { local n="$1" i=${#1} arr; while ((n > 0)); do
arr[--i]=$((n%10)); ((n /= 10)); done; }
f5() { local i n=${#1} arr; while ((i < n)); do arr[i]="${1:i:1}";
((i++)); done; }
f6() { local i n=${#1} arr; for ((i=0; i<n; i++)); do arr[i]="${1:i:1}"; done; }

# Lea Gris
shopt -s extglob
f7() { local arr; IFS=' ' read -ra arr <<< "${1//?()/ }"; }
f8() { local arr; IFS= mapfile -s1 -t -d $'\37' arr
<<<"${1//?()/$'\37'}"; arr[-1]="${arr[-1]%?}"; }

# Mike Jonkmans
f12() { local arr; [[ "$1" =~ ${1//?/(.)} ]]; arr=( "${BASH_REMATCH[@]:1}" ); }

# my experiments with brace expansions and eval
f20() { local arr; eval "for i in {0..$((${#1}-1))}; do
arr[i]=\${1:i:1}; done"; }
f21() { local arr; eval "arr=('\${1:'{0..$((${#1}-1))}':1}')";
arr=("${arr[@]@P}"); }
f21b() { local -a "arr=('\${1:'{0..$((${#1}-1))}':1}')"; arr=("${arr[@]@P}"); }
f22() { local arr; eval "arr=('\"\${1:'{0..$((${#1}-1))}':1}\"')";
local -a "arr=(${arr[*]})"; }
f23() { local arr; eval "arr=('\"\${1:'{0..$((${#1}-1))}':1}\"')";
eval "arr=(${arr[*]})"; }
f24() { local -a "arr=('\"\${1:'{0..$((${#1}-1))}':1}\"')"; local -a
"arr=(${arr[*]})"; }

# my experiments with arithmetic evaluations
f30() { local arr; eval "let i={1..${#1}}-1,'arr[i]=$1/10**i%10'"; }
f31() { local arr i=${#1} v=$1 x='arr[--i]=v%10,v/=10,i&&x'; : $((x)); }

The result is summarized in the following table:

NAME ARG  bash-5.0          bash-5.1          bash-dev
---- ---  ----------------  ----------------  ----------------
f1   *N   37.633 usec/eval  39.822 usec/eval  39.791 usec/eval
f2   *A   44.171 usec/eval  42.643 usec/eval  43.752 usec/eval
f3   *N   79.760 usec/eval  77.892 usec/eval  79.842 usec/eval
f4   *N   39.870 usec/eval  40.510 usec/eval  40.929 usec/eval
f5   *A   42.635 usec/eval  43.349 usec/eval  43.617 usec/eval
f6   *A   39.060 usec/eval  40.191 usec/eval  40.704 usec/eval
f7   *B   35.110 usec/eval  30.255 usec/eval  31.550 usec/eval
f8   *R   46.324 usec/eval  36.974 usec/eval  37.672 usec/eval
f12  *A   30.016 usec/eval  30.566 usec/eval  31.865 usec/eval
f20  *A   37.930 usec/eval  38.076 usec/eval  38.143 usec/eval
f21  *A   36.189 usec/eval  34.517 usec/eval  34.967 usec/eval
f21b *A   29.954 usec/eval  28.655 usec/eval  29.729 usec/eval
f22  *A   39.251 usec/eval  37.458 usec/eval  38.099 usec/eval
f23  *A   45.939 usec/eval  42.630 usec/eval  43.187 usec/eval
f24  *A   33.615 usec/eval  31.600 usec/eval  32.905 usec/eval
f30  *N   28.908 usec/eval  28.750 usec/eval  28.871 usec/eval
f31  *N   23.723 usec/eval  23.659 usec/eval  24.366 usec/eval

The column ARG denotes the accepted type of the argument: *N = only
numbers are accepted, *A = any characters (except for NUL), *B = any
non-space characters, *R = any characters except for RS. bash-dev is
built with "define(relstatus, release)" (configure.ac:27).  The
times are measured by EPOCHREALTIME.  I have run "fN 682390" for
5000 times and obtained the average time of each call.  The whole
measurements are repeated five times, and the minimal time from the
five results is picked up for each function and bash version.