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bash/resistor-color-duo/README.md

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+# Resistor Color Duo
+
+If you want to build something using a Raspberry Pi, you'll probably use _resistors_. For this exercise, you need to know two things about them:
+
+* Each resistor has a resistance value.
+* Resistors are small - so small in fact that if you printed the resistance value on them, it would be hard to read.
+To get around this problem, manufacturers print color-coded bands onto the resistors to denote their resistance values. Each band acts as a digit of a number. For example, if they printed a brown band (value 1) followed by a green band (value 5), it would translate to the number 15.
+
+In this exercise, you are going to create a helpful program so that you don't have to remember the values of the bands. The program will take two colors as input, and output the correct number.
+
+The band colors are encoded as follows:
+
+- Black: 0
+- Brown: 1
+- Red: 2
+- Orange: 3
+- Yellow: 4
+- Green: 5
+- Blue: 6
+- Violet: 7
+- Grey: 8
+- White: 9
+
+
+Run the tests with:
+
+```bash
+bats resistor_color_duo_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 resistor_color_duo_test.sh
+```
+
+## Source
+
+Maud de Vries, Erik Schierboom [https://github.com/exercism/problem-specifications/issues/1464](https://github.com/exercism/problem-specifications/issues/1464)
+
+
+## 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/resistor-color-duo/resistor_color_duo.sh

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+#!/usr/bin/env bash
+
+usage () {
+    if [[ $# == 1 ]]; then
+        echo "$1: invalid color."
+    else
+        echo "usage: resistor_color.sh color1 color2"
+    fi
+    exit 1
+}
+
+declare -A colors
+colors=(
+    [black]=0
+    [brown]=1
+    [red]=2
+    [orange]=3
+    [yellow]=4
+    [green]=5
+    [blue]=6
+    [violet]=7
+    [grey]=8
+    [white]=9
+)
+result=""
+
+# not sure here if 1 color only should be accepted (case is not in test). I assume yes:
+# "resistor_color.sh Black" will return 0
+# also (not in test cases), no args will return an error.
+[[ $# == 0 ]] && usage
+
+# we will accept capitalized colors: converting to lower case
+for i in ${1,,} ${2,,}
+do
+    [[ ${colors[$i]} == "" ]] && usage "$i"
+    result+=${colors[$i]}
+done
+echo $result
+exit 0