Added lua solution for day 1.

README.md

@@ -2,6 +2,7 @@
 
 This repository has my solutions for this year's Advent of Code challenges. Each day's challenge will be placed into a directory named `day_N`, where `N` is the day of the challenge. Each day will have a solution program named `solution.*` (with the filetype depending on which programming language I used).
 
-For D programs, you can simply run them with `rdmd solution.d`.
+- For D programs, you can simply run them with `rdmd solution.d`.
+- For Lua programs, you can run them with `lua solution.lua`.
 
 Note that I try and solve the problems as fast as possible, but then I go back and clean up my solution, so what you see isn't really my ugly first attempt.

day_1/solution.lua

@@ -0,0 +1,56 @@
+local function parseDigits(f, s)
+    local a = -1
+    local b = -1
+    for i = 1, #s do
+        local an = f(s, i)
+        local bn = f(s, #s - i + 1)
+        if a == -1 and an ~= -1 then a = an end
+        if b == -1 and bn ~= -1 then b = bn end
+        if a ~= -1 and b ~= -1 then break end
+    end
+    return a * 10 + b
+end
+
+local function part1()
+    local f = io.open("part_1_input.txt")
+    local sum = 0
+    local line = f:read("*line")
+    while line ~= nil do
+        sum = sum + parseDigits(
+            function (s, i)
+                local num = tonumber(string.sub(s, i, i))
+                if num ~= nil then return num else return -1 end
+            end,
+            line
+        )
+        line = f:read("*line")
+    end
+    f:close()
+    print(sum)
+end
+
+local function part2()
+    local function findDigit(s, idx)
+        local num = tonumber(string.sub(s, idx, idx))
+        if num ~= nil then return num end
+        local words = {"one", "two", "three", "four", "five", "six", "seven", "eight", "nine"}
+        for i = 1, #words do
+            local word = words[i]
+            if #s - idx + 1 >= #word and string.sub(s, idx, idx + #word - 1) == word then
+                return i
+            end
+        end
+        return -1
+    end
+    local f = io.open("part_2_input.txt")
+    local sum = 0
+    local line = f:read("*line")
+    while line ~= nil do
+        sum = sum + parseDigits(findDigit, line)
+        line = f:read("*line")
+    end
+    print(sum)
+end
+
+part1()
+part2()