Repeating strings and iterating over chars in portable shell

chars.sh

set -euf
PATH=
IFS=' '

# Shells like dash and posh don't have the substring parameter expansion
# commonly found in bash and zsh.
#
# To make iterating over each char fast, instead of breaking each
# char individually (which forces the shell to deal with the entire
# string twice, due to the nature of parameter substitution), we
# do consecutive breaks in half.
#
# The str_repeat function here is used to generate the specific
# pattern that splits an arbitrary string into its half.
#
# You should use this only if you want to support these kind
# of operations on these lofi shells.
#

! command -v emulate >/dev/null 2>&1 || emulate ksh >/dev/null 2>&1
command -v local >/dev/null 2>&1 || alias local=typeset

_str_repeat () {
	local str="$1" length_plus1=$(($2 + 1)) pow_of_two=1 next_pow= repeated=

    # these cases (0-4) are not worth the cost of the while below
	case $length_plus1 in
		1) REPLY=""; return;;
		2) REPLY="$1"; return;;
		3) REPLY="$1$1"; return;;
		4) REPLY="$1$1$1"; return;;
		5) REPLY="$1$1$1$1"; return;;
	esac

    # doubles $repeated by powers of two
	while next_pow=$((pow_of_two * 2))
	do
		if test $next_pow -lt $length_plus1
		then
			pow_of_two=$next_pow
			str="$str$str"
		else
			length_plus1=$((length_plus1 - pow_of_two))
			
			if test $length_plus1 -lt 1
			then break
			fi
			
			repeated="$repeated$str"
			str="$1"
			pow_of_two=1
		fi
	done
	
	REPLY="$repeated"
}

_str_scan_chars () {
	local result= left= right= result= any_char=
	
    # changes the orientation of string cutting
    if test "${1:-}" = 1
    then begin='#' end='%'
    else begin='%' end='#'
    fi
	
    if test $# -gt 1
    then shift
    fi

    # recursively splits string in half, left side first
	while test "$#" -gt 0
	do
        if test -z "$1"
        then shift && continue
        fi

		_str_repeat '?' $((${#1} / 2))
        any_char="$REPLY"
        
		eval "right=\"\${1$begin$any_char}\""   
		eval "left=\"\${1$end$right}\""
		shift
        
        if test 1 = ${#left} 
        then 
            result="${result} ${left}"
            left=''
        fi

        if test 1 = ${#right}
        then 
            result="${result} ${right}"
            right=''
        fi

		set -- "$left" "$right" "$@"
	done

	REPLY="$result"
}

_str_repeat '_' 1000; echo "${#REPLY}"
_str_scan_chars 1 "$REPLY";