day 10 solutions

Signed-off-by: Stephen Waits <swaits@amazon.com>

data/input-10.txt

@@ -0,0 +1,101 @@
+144
+10
+75
+3
+36
+80
+143
+59
+111
+133
+1
+112
+23
+62
+101
+137
+41
+24
+8
+121
+35
+105
+161
+69
+52
+21
+55
+29
+135
+142
+38
+108
+141
+115
+68
+7
+98
+82
+9
+72
+118
+27
+153
+140
+61
+90
+158
+102
+28
+134
+91
+2
+17
+81
+31
+15
+120
+20
+34
+56
+4
+44
+74
+14
+147
+11
+49
+128
+16
+99
+66
+47
+125
+155
+130
+37
+67
+54
+60
+48
+136
+89
+119
+154
+122
+129
+163
+73
+100
+85
+95
+30
+76
+162
+22
+79
+88
+150
+53
+63
+92

data/output-10.txt

@@ -0,0 +1,2 @@
+2240
+99214346656768

src/day10.rs

@@ -0,0 +1,72 @@
+// ==== day 10
+
+use crate::utils;
+use anyhow::Result;
+
+pub fn count_diff1_diff3(diffs: &Vec<i64>) -> usize {
+    let diff_1 = diffs.iter().filter(|n| **n == 1).count();
+    let diff_3 = diffs.iter().filter(|n| **n == 3).count();
+    diff_1 * diff_3
+}
+
+// the number of paths from a contiguous sequence [i+0, i+1, i+N-1] of size N
+// follows the pattern 0, 1, 1, 2, 4, where the total paths for a runlength N
+// is sum of the paths in N-3, N-2, and N-1.
+fn paths_by_runlength(runlength: usize) -> usize {
+    let mut p: Vec<usize> = vec![0, 1, 1]; // vec of most recent 4 in sequence
+    const L: usize = 3; // length of p Vec
+    for i in L..runlength + 1 {
+        p[i % L] = p[(i - 3) % L] + p[(i - 2) % L] + p[(i - 1) % L];
+    }
+    p[runlength % L]
+}
+
+fn count_paths(diffs: &Vec<i64>) -> usize {
+    // count the runs of 1s
+    let mut last_diff: i64 = 0;
+    let mut runlength: usize = 1;
+    let mut runs_of_ones: Vec<usize> = Vec::new();
+    diffs.iter().for_each(|d| {
+        if *d == last_diff {
+            runlength += 1;
+        } else {
+            if last_diff == 1 {
+                runs_of_ones.push(runlength + 1);
+            }
+            runlength = 1;
+            last_diff = *d;
+        }
+    });
+
+    // figure out how many path options we have for each run and multiply
+    runs_of_ones
+        .iter()
+        .map(|r| paths_by_runlength(*r))
+        .product()
+}
+
+pub fn run() -> Result<(usize, usize)> {
+    let mut data = utils::read_i64s("data/input-10.txt")?;
+    let answers = utils::read_i64s("data/output-10.txt")?;
+
+    // add the adapters on either end of the sequence and sort
+    let max = data.iter().max().unwrap().clone();
+    data.push(0); // add the (0) adapter at my seat
+    data.push(max + 3); // add the (max in the list + 3) adapter in my device
+    data.sort_unstable(); // paths are always sorted
+
+    // create a vec of the diffs from data[i] to data[i+1]
+    let diffs: Vec<i64> = data
+        .iter()
+        .enumerate()
+        .take(data.len() - 1)
+        .map(|(i, _)| (&data[i] - &data[i + 1]).abs())
+        .collect();
+
+    let p1 = count_diff1_diff3(&diffs);
+    let p2 = count_paths(&diffs);
+
+    assert_eq!(p1, answers[0] as usize);
+    assert_eq!(p2, answers[1] as usize);
+    Ok((p1, p2))
+}

src/main.rs

@@ -9,6 +9,7 @@ mod day06;
 mod day07;
 mod day08;
 mod day09;
+mod day10;
 mod utils;
 
 // TODO
@@ -31,5 +32,6 @@ fn main() -> Result<()> {
     println!("Day 07: {:?}", day07::run()?);
     println!("Day 08: {:?}", day08::run()?);
     println!("Day 09: {:?}", day09::run()?);
+    println!("Day 10: {:?}", day10::run()?);
     Ok(())
 }