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diff --git a/challenge-103/abigail/lua/ch-1.lua b/challenge-103/abigail/lua/ch-1.lua
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+#!/opt/local/bin/lua
+
+--
+-- See ../README.md
+--
+
+--
+-- Run as: lua ch-1.lua < 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.
+--
+
+local yin_yang         = {"Yang",   "Yin"};
+local heavenly_stems   = {"Metal",  "Metal",   "Water", "Water",
+                          "Wood",   "Wood",    "Fire",  "Fire",
+                          "Earth",  "Earth"};
+local earthly_branches = {"Monkey", "Rooster", "Dog",   "Pig",
+                          "Rat",    "Ox",      "Tiger", "Rabbit",
+                          "Dragon", "Snake",   "Horse", "Goat"};
+
+for line in io . lines () do
+    local year = tonumber (line)
+    io . write (yin_yang         [1 + (year % #yin_yang)],         " ",
+                heavenly_stems   [1 + (year % #heavenly_stems)],   " ",
+                earthly_branches [1 + (year % #earthly_branches)], "\n")
+end