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bash/grains/README.md

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+# Grains
+
+Calculate the number of grains of wheat on a chessboard given that the number
+on each square doubles.
+
+There once was a wise servant who saved the life of a prince. The king
+promised to pay whatever the servant could dream up. Knowing that the
+king loved chess, the servant told the king he would like to have grains
+of wheat. One grain on the first square of a chess board, with the number
+of grains doubling on each successive square.
+
+There are 64 squares on a chessboard (where square 1 has one grain, square 2 has two grains, and so on).
+
+Write code that shows:
+- how many grains were on a given square, and
+- the total number of grains on the chessboard
+
+## For bonus points
+
+Did you get the tests passing and the code clean? If you want to, these
+are some additional things you could try:
+
+- Optimize for speed.
+- Optimize for readability.
+
+Then please share your thoughts in a comment on the submission. Did this
+experiment make the code better? Worse? Did you learn anything from it?
+
+
+Run the tests with:
+
+```bash
+bats grains_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 grains_test.sh
+```
+
+## Source
+
+JavaRanch Cattle Drive, exercise 6 [http://www.javaranch.com/grains.jsp](http://www.javaranch.com/grains.jsp)
+
+
+## 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/grains/grains.sh

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+#!/usr/bin/env bash
+#
+# V1: initial version
+#
+# Note: untested on 32 bits and big-endian architectures
+
+shopt -s extglob
+
+# set to mask to enable logs. 0: none, 255: all
+(( debug=2#00000000 ))
+#(( debug=2#00001111 ))
+# $1: log level (mask), then strings to display.
+debug () {
+    (( debug & $1 )) && shift && echo Line ${BASH_LINENO[0]}: "${@}" >&2
+}
+
+die () {
+    echo "${@}" >&2
+    exit 1
+}
+usage() {
+    die "Error: invalid input"
+}
+
+# we want 1 arg only, "total" or digits only.
+(($# !=1 )) || \
+    [[ "$1" != +([[:digit:]]) ]] && \
+    [[ "$1" != "total" ]] && usage
+
+arg="${1##+(0)}"                        # we strip leading zeroes if any
+debug 1 arg="$arg"
+
+case "$arg" in
+    total)
+        # formula to calculate a geometric series of common ratio r and first
+        # term f, that is: S=f + fr² + fr³ ... + frⁿ
+        # is: S = f * [ (1 - rⁿ⁺¹) / (1 - r) ]
+        #
+        # for r=2 and f=1, it becomes:
+        # S = 1 * (1 - 2ⁿ⁺¹) / -1 = 2ⁿ⁺¹ - 1
+        # So here, as total does not accept an argument, such as "last square",
+        # the value is 2⁶⁴-1.
+        # Notes: (2**64) is 0 on 64 bits, but better not to use this property.
+        #        We could also directly output the value, as it is a constant.
+        printf -v result "%llu" $(( 2**64 - 1 ))
+        ;;
+    *)                                  # always digits here
+        debug 2 "digits=args"
+        (( arg < 1 || arg > 64 )) && usage
+        printf -v result "%llu" $(( 2**(arg-1) ))
+        ;;
+esac
+
+echo "$result"
+
+exit 0
+
+# emacs/vim settings.
+# Local Variables:
+# sh-basic-offset: 4
+# indent-tabs-mode: nil
+# comment-column: 40
+# fill-column: 80
+# End:
+# vim: set tabstop=4 expandtab: