1 files changed, 95 insertions, 0 deletions
diff --git a/20/a.lua b/20/a.lua
new file mode 100644
index 0000000..ee469d1
--- /dev/null
+++ b/20/a.lua
@@ -0,0 +1,95 @@
+tiles = {}
+
+current_tile_number = nil
+current_tile = {}
+
+function border(tile, horizontal, index)
+    first_column = 0
+    first_row = 0
+    last_column = 0
+    last_row = 0
+    if horizontal then
+        first_column = 1
+        last_column = #tile[1]
+        first_row = index
+        last_row = index
+    else
+        first_column = index
+        last_column = index
+        first_row = 1
+        last_row = #tile
+    end
+
+    b = ''
+    for row = first_row, last_row do
+        for column = first_column, last_column do
+            b = b .. tile[row][column]
+        end
+    end
+
+    if (horizontal and index > 1) or (not horizontal and index == 1) then
+        b = b:reverse()
+    end
+
+    return b
+end
+
+function borders(tile)
+    return {
+        border(tile, true, 1),
+        border(tile, true, 10),
+        border(tile, false, 1),
+        border(tile, false, 10)
+    }
+end
+
+for line in io.lines('input.txt') do
+    if line == "" and current_tile_number then
+        tiles[current_tile_number] = current_tile
+        current_tile = {}
+    end
+
+    space_index = line:find(' ')
+
+    if space_index then
+        current_tile_number = line:sub(space_index+1, -2)
+    elseif #line > 0 then
+        row = {}
+        for i = 1, line:len() do
+            table.insert(row, line:sub(i, i))
+        end
+        table.insert(current_tile, row)
+    end
+end
+
+border_counts = {}
+for _, tile in pairs(tiles) do
+   for _, border in pairs(borders(tile)) do
+       if not border_counts[border] then
+            border_counts[border] = 0
+        end
+        border_counts[border] = border_counts[border] + 1
+    end
+end
+
+corners = {}
+for number,tile in pairs(tiles) do
+    count_neighboorless = 0
+    for _, border in pairs(borders(tile)) do
+        has_flipped_neighbor = border_counts[border] > 1
+        has_non_flipped_neighbor = border_counts[border:reverse()]
+        if not (has_flipped_neighbor or has_non_flipped_neighbor) then
+            count_neighboorless = count_neighboorless + 1
+        end
+    end
+    if count_neighboorless == 2 then
+        table.insert(corners, number)
+    end
+end
+
+product = 1
+for i,c in pairs(corners) do
+    product = product * c
+end
+
+print(product)