From 8ac325848b2e69892233b6bb8eb841a7b8fa07d5 Mon Sep 17 00:00:00 2001
From: William Brawner <me@wbrawner.com>
Date: Fri, 8 Dec 2023 22:42:41 -0700
Subject: [PATCH] Add solution for day 8

---
 src/Day08.lua | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++
 src/util.lua  | 15 +++++++++
 2 files changed, 107 insertions(+)
 create mode 100644 src/Day08.lua
 create mode 100644 src/util.lua

diff --git a/src/Day08.lua b/src/Day08.lua
new file mode 100644
index 0000000..86421db
--- /dev/null
+++ b/src/Day08.lua
@@ -0,0 +1,92 @@
+---@diagnostic disable: lowercase-global
+require "util"
+
+function parse_node(line)
+    local matches = {}
+    for match in line:gmatch("%w+") do
+        matches[#matches + 1] = match
+    end
+    return matches[1], { L = matches[2], R = matches[3] }
+end
+
+function values(src)
+    local dest = {}
+    for _,v in pairs(src) do
+        dest[#dest + 1] = v
+    end
+    return dest
+end
+
+function part1(input)
+    local steps = input[1]
+    local nodes = {}
+    for i = 3, #input do
+        local node, children = parse_node(input[i])
+        nodes[node] = children
+    end
+    local current_node = "AAA"
+    local step_index = 1
+    local step_count = 0
+    while current_node ~= "ZZZ" do
+        local nextStep = steps:sub(step_index, step_index)
+        current_node = nodes[current_node][nextStep]
+        step_count = step_count + 1
+        step_index = step_index + 1
+        if step_index > #steps then
+            step_index = 1
+        end
+    end
+    return step_count
+end
+
+function find_lcm(first, second)
+    local max = math.max(first, second)
+    local fcm = first * second
+    local cm = max
+    io.write(string.format("finding lcm of %s and %s... ", first, second))
+    while cm <= fcm do
+        if (cm % first == 0 and cm % second == 0) then
+            print(cm)
+            return cm
+        end
+        cm = cm + max
+    end
+end
+
+function part2(input)
+    local steps = input[1]
+    local nodes = {}
+    local current_nodes = {}
+    for i = 3, #input do
+        local node, children = parse_node(input[i])
+        nodes[node] = children
+        if string.sub(node, #node, #node) == "A" then
+            current_nodes[node] = 0
+        end
+    end
+    for node, _ in pairs(current_nodes) do
+        local step_index = 1
+        local current_node = node
+        while current_node:sub(#current_node, #current_node) ~= "Z" do
+            local nextStep = steps:sub(step_index, step_index)
+            current_node = nodes[current_node][nextStep]
+            current_nodes[node] = current_nodes[node] + 1
+            step_index = step_index + 1
+            if step_index > #steps then
+                step_index = 1
+            end
+        end
+    end
+    local node_steps = values(current_nodes)
+    local lcm = find_lcm(node_steps[1], node_steps[2])
+    for i = 3, #node_steps do
+        lcm=find_lcm(lcm, node_steps[i])
+    end
+    return lcm
+end
+
+local test_input = readInput("day08_test.txt")
+local input = readInput("day08.txt")
+print(part1(input))
+assert_equals(part2(test_input), 6)
+print(part2(input))
diff --git a/src/util.lua b/src/util.lua
new file mode 100644
index 0000000..6970376
--- /dev/null
+++ b/src/util.lua
@@ -0,0 +1,15 @@
+function readInput(file)
+    local lines = {}
+    for line in io.lines(file) do
+        lines[#lines + 1] = line
+    end
+    return lines
+end
+
+function assert_equals(actual, expected, message)
+    if actual ~= expected then
+        print(string.format("assert failed: %s", message or ""))
+        print(string.format("\texpected: %s", expected))
+        print(string.format("\tactual: %s", actual))
+    end
+end