From 34f5a96e1ecb01e40605fca13cb761367bcfcda0 Mon Sep 17 00:00:00 2001
From: logaritmisk <anders.e.olsson@gmail.com>
Date: Wed, 21 Dec 2016 14:32:08 +0100
Subject: [PATCH] 2016/18

---
 2016/18/Cargo.lock  |   4 ++
 2016/18/Cargo.toml  |   6 +++
 2016/18/src/main.rs | 101 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 111 insertions(+)
 create mode 100644 2016/18/Cargo.lock
 create mode 100644 2016/18/Cargo.toml
 create mode 100644 2016/18/src/main.rs

diff --git a/2016/18/Cargo.lock b/2016/18/Cargo.lock
new file mode 100644
index 0000000..5b16d80
--- /dev/null
+++ b/2016/18/Cargo.lock
@@ -0,0 +1,4 @@
+[root]
+name = "18"
+version = "0.1.0"
+
diff --git a/2016/18/Cargo.toml b/2016/18/Cargo.toml
new file mode 100644
index 0000000..8659fc6
--- /dev/null
+++ b/2016/18/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "18"
+version = "0.1.0"
+authors = ["logaritmisk <anders.e.olsson@gmail.com>"]
+
+[dependencies]
diff --git a/2016/18/src/main.rs b/2016/18/src/main.rs
new file mode 100644
index 0000000..b3dde43
--- /dev/null
+++ b/2016/18/src/main.rs
@@ -0,0 +1,101 @@
+fn is_safe(l: char, c: char, r: char) -> bool {
+    // Rule #1.
+    if l == '^' && c == '^' && r == '.' {
+        return false;
+    }
+
+    // Rule #2.
+    if l == '.' && c == '^' && r == '^' {
+        return false;
+    }
+
+    // Rule #3.
+    if l == '^' && c == '.' && r == '.' {
+        return false;
+    }
+
+    // Rule #4.
+    if l == '.' && c == '.' && r == '^' {
+        return false;
+    }
+
+    true
+}
+
+pub fn evaluate(input: &str) -> String {
+    let input = input.chars().collect::<Vec<char>>();
+    let mut output = String::new();
+
+    for i in 0..input.len() {
+        let c = if i == 0 {
+            if is_safe('.', input[i], input[i + 1]) {
+                '.'
+            } else {
+                '^'
+            }
+        } else if i == input.len() - 1 {
+            if is_safe(input[i - 1], input[i], '.') {
+                '.'
+            } else {
+                '^'
+            }
+        } else {
+            if is_safe(input[i - 1], input[i], input[i + 1]) {
+                '.'
+            } else {
+                '^'
+            }
+        };
+
+        output.push(c);
+    }
+
+    output
+}
+
+fn main() {
+    let mut map = Vec::new();
+
+    map.push(".^^^^^.^^.^^^.^...^..^^.^.^..^^^^^^^^^^..^...^^.^..^^^^..^^^^...^.^.^^^^^^^^....^..^^^^^^.^^^.^^^.^^".to_owned());
+
+    while map.len() < 400000 {
+        let next = {
+            let last = map.last().unwrap();
+
+            evaluate(last)
+        };
+
+        map.push(next);
+    }
+
+    let count: usize = map.iter()
+        .map(|row| {
+            row.chars()
+                .filter(|c| *c == '.')
+                .count()
+        })
+        .sum();
+
+    println!("safe_tiles={}", count);
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn example_01() {
+        assert_eq!(".^^^^".to_owned(), evaluate("..^^."));
+    }
+
+    #[test]
+    fn example_02() {
+        assert_eq!("^^..^".to_owned(), evaluate(".^^^^"));
+    }
+
+    #[test]
+    fn example_03() {
+        assert_eq!("^^^...^..^".to_owned(), evaluate(".^^.^.^^^^"));
+    }
+}