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bash/armstrong-numbers/README.md

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+# Armstrong Numbers
+
+An [Armstrong number](https://en.wikipedia.org/wiki/Narcissistic_number) is a number that is the sum of its own digits each raised to the power of the number of digits.
+
+For example:
+
+- 9 is an Armstrong number, because `9 = 9^1 = 9`
+- 10 is *not* an Armstrong number, because `10 != 1^2 + 0^2 = 1`
+- 153 is an Armstrong number, because: `153 = 1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153`
+- 154 is *not* an Armstrong number, because: `154 != 1^3 + 5^3 + 4^3 = 1 + 125 + 64 = 190`
+
+Write some code to determine whether a number is an Armstrong number.
+
+
+Run the tests with:
+
+```bash
+bats armstrong_numbers_test.sh
+```
+
+After the first test(s) pass, continue by commenting out or removing the
+`[[ $BATS_RUN_SKIPPED == true ]] || skip` 
+annotations prepending other tests.
+
+To run all tests, including the ones with `skip` annotations, run:
+
+```bash
+BATS_RUN_SKIPPED=true bats armstrong_numbers_test.sh
+```
+
+## Source
+
+Wikipedia [https://en.wikipedia.org/wiki/Narcissistic_number](https://en.wikipedia.org/wiki/Narcissistic_number)
+
+
+## External utilities
+`Bash` is a language to write "scripts" -- programs that can call
+external tools, such as
+[`sed`](https://www.gnu.org/software/sed/),
+[`awk`](https://www.gnu.org/software/gawk/),
+[`date`](https://www.gnu.org/software/coreutils/manual/html_node/date-invocation.html)
+and even programs written in other programming languages, 
+like [`Python`](https://www.python.org/).
+This track does not restrict the usage of these utilities, and as long
+as your solution is portable between systems and does not require
+installation of third party applications, feel free to use them to solve
+the exercise.
+
+For an extra challenge, if you would like to have a better understanding
+of the language, try to re-implement the solution in pure `Bash`,
+without using any external tools. Note that there are some types of
+problems that bash cannot solve, such as performing floating point
+arithmetic and manipulating dates: for those, you must call out to an
+external tool.
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others
+have completed the exercise.

bash/armstrong-numbers/armstrong_numbers.sh

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+#!/usr/bin/env bash
+
+# set to mask to enable logs. 0: none, 255: all
+(( DEBUG=2#00000000 ))
+
+# $1: log level (mask), then strings to display.
+debug () {
+    (( DEBUG & $1 )) && shift && echo "${@}" >&2
+}
+
+usage () {
+    echo "usage: ./armstrong_numbers.sh <number>" >&2
+    exit 1
+}
+
+# basic args checks: 1 arg, digits  (at least 1) only
+(( $# != 1)) || [[ ! "$1" =~ ^[[:digit:]]+$ ]] && usage
+
+number="$1"
+digits=${#number}
+armstrong=0
+result="false"
+
+debug 1 d=$digits n=$number
+
+# armstrong number calculation
+for (( i=0; i<digits; ++i )); do
+    ((armstrong += ${number:$i:1} ** digits ))
+done
+
+debug 2 "armstrong=$armstrong"
+
+((number == armstrong)) && result="true"
+
+echo "$result"
+
+exit 0
+
+# Indent style for emacs
+# Local Variables:
+# sh-basic-offset: 4
+# indent-tabs-mode: nil
+# comment-column: 60
+# End: