day 4 part 2: doesn't work

day4/input

@@ -0,0 +1,140 @@
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day4/part1.lua

@@ -0,0 +1,94 @@
+-- see if the file exists
+local function file_exists(file)
+	local f = io.open(file, "rb")
+	if f then
+		f:close()
+	end
+	return f ~= nil
+end
+
+-- get all lines from a file, returns an empty
+-- list/table if the file does not exist
+local function lines_from(file)
+	if not file_exists(file) then
+		return {}
+	end
+	local lines = {}
+	for line in io.lines(file) do
+		lines[#lines + 1] = line
+	end
+	return lines
+end
+
+-- tests the functions above
+local file = "input"
+local lines = lines_from(file)
+
+-- Solution Starts here
+
+local target = "XMAS"
+
+---@diagnostic disable-next-line: redefined-local
+local function count_xmas_at_index(lines, line_idx, char_idx)
+	local count = 0
+
+	local line = lines[line_idx]
+
+	local test_words = {}
+	-- Forward
+	test_words[#test_words + 1] = line:sub(char_idx, char_idx + 3)
+	-- Backward
+	test_words[#test_words + 1] = line:sub(char_idx - 3, char_idx):reverse()
+
+	local up_right = ""
+	local down_right = ""
+	local up_left = ""
+	local down_left = ""
+	local up = ""
+	local down = ""
+	for offset = 0, 3 do
+		local up_idx = line_idx - offset
+		local down_idx = line_idx + offset
+		local right_idx = char_idx + offset
+		local left_idx = char_idx - offset
+
+		if up_idx >= 1 then
+			up = up .. lines[up_idx]:sub(char_idx, char_idx)
+
+			up_right = up_right .. lines[up_idx]:sub(right_idx, right_idx)
+			up_left = up_left .. lines[up_idx]:sub(left_idx, left_idx)
+		end
+
+		if down_idx <= #lines then
+			down = down .. lines[down_idx]:sub(char_idx, char_idx)
+
+			down_right = down_right .. lines[down_idx]:sub(right_idx, right_idx)
+			down_left = down_left .. lines[down_idx]:sub(left_idx, left_idx)
+		end
+	end
+	test_words[#test_words + 1] = up_right
+	test_words[#test_words + 1] = down_right
+	test_words[#test_words + 1] = up_left
+	test_words[#test_words + 1] = down_left
+	test_words[#test_words + 1] = up
+	test_words[#test_words + 1] = down
+
+	for _, v in ipairs(test_words) do
+		if v == target then
+			count = count + 1
+		end
+	end
+
+	return count
+end
+
+local xmas_count = 0
+-- print all line numbers and their contents
+---@diagnostic disable-next-line: unused-local
+for line_idx, line in ipairs(lines) do
+	for char_idx in line:gmatch("()X") do
+		xmas_count = xmas_count + count_xmas_at_index(lines, line_idx, char_idx)
+	end
+end
+
+io.output():write(xmas_count)

day4/part2.lua

@@ -0,0 +1,91 @@
+-- see if the file exists
+local function file_exists(file)
+	local f = io.open(file, "rb")
+	if f then
+		f:close()
+	end
+	return f ~= nil
+end
+
+-- get all lines from a file, returns an empty
+-- list/table if the file does not exist
+local function lines_from(file)
+	if not file_exists(file) then
+		return {}
+	end
+	local lines = {}
+	for line in io.lines(file) do
+		lines[#lines + 1] = line
+	end
+	return lines
+end
+
+-- tests the functions above
+local file = "input"
+local lines = lines_from(file)
+
+-- Solution Starts here
+
+---@diagnostic disable-next-line: redefined-local
+local function count_xmas_at_index(lines, line_idx, char_idx)
+	local line = lines[line_idx]
+
+	local up_idx = line_idx - 1
+	local down_idx = line_idx + 1
+	local right_idx = char_idx + 1
+	local left_idx = char_idx - 1
+
+	local char_count = {}
+	char_count["M"] = 0
+	char_count["S"] = 0
+	if up_idx >= 1 then
+		if right_idx <= #line then
+			local key = lines[up_idx]:sub(right_idx, right_idx)
+			if char_count[key] ~= nil then
+				char_count[key] = char_count[key] + 1
+			end
+		end
+
+		if left_idx >= 1 then
+			local key = lines[up_idx]:sub(left_idx, left_idx)
+			if char_count[key] ~= nil then
+				char_count[key] = char_count[key] + 1
+			end
+		end
+	end
+
+	if down_idx <= #lines then
+		if right_idx <= #line then
+			local key = lines[down_idx]:sub(right_idx, right_idx)
+			if char_count[key] ~= nil then
+				char_count[key] = char_count[key] + 1
+			end
+		end
+
+		if left_idx >= 1 then
+			local key = lines[down_idx]:sub(left_idx, left_idx)
+			if char_count[key] ~= nil then
+				char_count[key] = char_count[key] + 1
+			end
+		end
+	end
+
+	if char_count["M"] == 2 and char_count["S"] == 2 then
+		return true
+	end
+
+	return false
+end
+
+local xmas_count = 0
+-- print all line numbers and their contents
+---@diagnostic disable-next-line: unused-local
+for line_idx, line in ipairs(lines) do
+	for char_idx in line:gmatch("()A") do
+		if count_xmas_at_index(lines, line_idx, char_idx) then
+			xmas_count = xmas_count + 1
+		end
+	end
+end
+
+io.output():write(xmas_count)

day4/test-input

@@ -0,0 +1,10 @@
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX