Bash solution for week 103, part 1

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diff --git a/challenge-103/abigail/bash/ch-1.sh b/challenge-103/abigail/bash/ch-1.sh
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+#!/bin/sh
+
+#
+# See ../README.md
+#
+
+#
+# Run as: bash ch-1.sh < input-file
+#
+
+#
+# We're reading years from standard input, one year per line, outputting
+# years from the sexagenary cycle [1]. This is slightly more than what
+# the challenge ask; the challenge asks to output the heavenly stem [2],
+# and the earthly branch [3]. But we also output its Yin/Yang.
+#
+# [1] https://en.wikipedia.org/wiki/Sexagenary_cycle
+# [2] https://en.wikipedia.org/wiki/Heavenly_Stems
+# [3] https://en.wikipedia.org/wiki/Earthly_Branches
+#
+
+#
+# Each of the cycles have been rotated so the first entry corresponds to
+# the year 0 in the Proleptic Gregorian calendar. (We're using the
+# convention of having a year 0, as per ISO 8601).
+# That way, we can just mod the year with the number of entries, without
+# first having to subtract something from the year.
+#
+# The heavenly stems last for 2 years, so we just duplicate the entries.
+#
+
+declare -a yin_yang
+declare -a heavenly_stems
+declare -a earthly_branches
+
+yin_yang=(Yang Yin)
+yin_yang_size=2
+heavenly_stems=(Metal Metal Water Water Wood Wood Fire Fire Earth Earth)
+heavenly_stems_size=10
+earthly_branches=(Monkey Rooster Dog    Pig   Rat   Ox
+                  Tiger  Rabbit  Dragon Snake Horse Goat)
+earthly_branches_size=12
+
+
+
+while read year
+do    echo ${yin_yang[$((year % yin_yang_size))]}                 \
+           ${heavenly_stems[$((year % heavenly_stems_size))]}     \
+           ${earthly_branches[$((year % earthly_branches_size))]}
+done