Implement day 14.

2018/inputs/14.txt

@@ -0,0 +1 @@
+637061

2018/src/common.rs

@@ -1,6 +1,8 @@
 use std::collections::HashMap;
 use std::hash::Hash;
 use std::io;
+use std::io::Read;
+use std::str::FromStr;
 
 /// Apply Erathostenes's sieve to the supplied array
 ///
@@ -50,6 +52,19 @@ pub fn trim_back(input: &mut Vec<u8>) {
     }
 }
 
+/// Read the entire input as one value.
+///
+/// This function loads the input into a string and then attempts to parse it.
+pub fn read_single_input<T>(input: &mut Read) -> T
+    where T: FromStr,
+          <T as FromStr>::Err: std::fmt::Debug
+{
+    let mut buf = String::new();
+    input.read_to_string(&mut buf).unwrap();
+
+    buf.trim().parse().unwrap()
+}
+
 
 /// An interface to count elements in particular categories.
 pub trait GroupingCount {

2018/src/day14.rs

@@ -1,6 +1,68 @@
 use std::io::Read;
 
 use common::Solution;
+use common::read_single_input;
+
+fn skill_after(n: usize) -> u64 {
+    let mut state = vec![3u8, 7];
+    let mut elves = [0, 1];
+
+    while state.len() < n + 10 {
+        let result: u8 = elves.iter().map(|x| state[*x]).sum();
+        if result >= 10 {
+            state.push(result / 10);
+        }
+        state.push(result % 10);
+
+        for elf in elves.iter_mut() {
+            *elf = (*elf + state[*elf] as usize + 1) % state.len();
+        }
+    }
+
+    let mut skill = 0;
+    for d in state.into_iter().skip(n).take(10) {
+        skill *= 10;
+        skill += d as u64;
+    }
+
+    skill
+}
+
+fn update_current(mut current: usize, by: usize, base: usize) -> usize {
+    current *= 10;
+    current %= base;
+    current += by;
+    current
+}
+
+fn find_first(n: usize, len: usize) -> usize {
+    println!("{} {}", n, len);
+    let mut state = vec![3u8, 7];
+    let mut elves = [0, 1];
+    let mut current = 37;
+    let mod_base = 10usize.pow(len as u32);
+
+    loop {
+        let result: u8 = elves.iter().map(|x| state[*x]).sum();
+        if result >= 10 {
+            current = update_current(current, result as usize / 10, mod_base);
+            if current == n {
+                return state.len() - 5;
+            }
+            state.push(result / 10);
+        }
+        current = update_current(current, result as usize % 10, mod_base);
+        if current == n {
+            return state.len() - 5 + 1;
+        }
+        state.push(result % 10);
+
+        for elf in elves.iter_mut() {
+            *elf = (*elf + state[*elf] as usize + 1) % state.len();
+        }
+    }
+}
+
 
 #[derive(Default)]
 pub struct Day14 {}
@@ -12,14 +74,37 @@ impl Day14 {
 }
 
 impl Solution for Day14 {
-    fn part1(&mut self, _input: &mut Read) -> String {
+    fn part1(&mut self, input: &mut Read) -> String {
-        unimplemented!()
+        let input = read_single_input(input);
+        format!("{:010}", skill_after(input))
     }
 
-    fn part2(&mut self, _input: &mut Read) -> String {
+    fn part2(&mut self, input: &mut Read) -> String {
-        unimplemented!()
+        let mut buf = String::new();
+        input.read_to_string(&mut buf).unwrap();
+
+        let input = buf.trim().parse().unwrap();
+        format!("{}", find_first(input, buf.trim().len()))
     }
 }
 
 #[cfg(test)]
-mod tests {}
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_skill_after() {
+        assert_eq!(5158916779, skill_after(9));
+        assert_eq!(124515891, skill_after(5));
+        assert_eq!(9251071085, skill_after(18));
+        assert_eq!(5941429882, skill_after(2018));
+    }
+
+    #[test]
+    fn test_find_first() {
+        assert_eq!(9, find_first(51589, 5));
+        assert_eq!(5, find_first(1245, 5));
+        assert_eq!(18, find_first(92510, 5));
+        assert_eq!(2018, find_first(59414, 5));
+    }
+}