From eeaffa5fa6e5f9f8c42c931eadb9787368bbeac5 Mon Sep 17 00:00:00 2001
From: Marcin Chrzanowski <marcin.j.chrzanowski@gmail.com>
Date: Fri, 25 Dec 2020 15:52:04 +0100
Subject: Add day 24

---
 24/a.lua | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 24/a.lua

(limited to '24/a.lua')

diff --git a/24/a.lua b/24/a.lua
new file mode 100644
index 0000000..dbf6868
--- /dev/null
+++ b/24/a.lua
@@ -0,0 +1,72 @@
+tiles = {}
+
+directions = {'se', 'sw', 'ne', 'nw', 'e', 'w'}
+
+function navigate(v, h, direction)
+    if direction == 'e' then
+        return v, h+1
+    elseif direction == 'w' then
+        return v, h-1
+    end
+
+    local even = v % 2 == 0
+    local new_v = v
+    local new_h = h
+
+    if direction:sub(1, 1) == 'n' then
+        new_v = new_v - 1
+    else
+        new_v = new_v + 1
+    end
+
+
+    if even and direction:sub(2, 2) == 'e' then
+        new_h = new_h + 1
+    elseif not even and direction:sub(2, 2) == 'w' then
+        new_h = new_h - 1
+    end
+
+    return new_v, new_h
+end
+
+function parse_tile(line)
+    local vertical = ''
+    local tile = {}
+    for i = 1, #line do
+        local current = line:sub(i, i)
+        if current == 'n' or current == 's' then
+            vertical = current
+        else
+            table.insert(tile, vertical .. current)
+            vertical = ''
+        end
+    end
+
+    return tile
+end
+
+for line in io.lines('input.txt') do
+    local tile = parse_tile(line)
+    local v = 0
+    local h = 0
+
+    for _, direction in pairs(tile) do
+        v, h = navigate(v, h, direction)
+    end
+
+    if not tiles[v] then tiles[v] = {} end
+    if not tiles[v][h] then tiles[v][h] = 0 end
+
+    tiles[v][h] = tiles[v][h] + 1
+end
+
+count = 0
+for _, row in pairs(tiles) do
+    for _, flips in pairs(row) do
+        if flips % 2 == 1 then
+            count = count + 1
+        end
+    end
+end
+
+print(count)
-- 
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