I'm dumb.

src/days/day6.rs

@@ -1,3 +1,4 @@
+
 use std::{path::Path, str::FromStr, iter::zip};
 
 use itertools::Itertools;
@@ -77,22 +78,23 @@ impl FromStr for RecordsOverview {
     }
 }
 
+pub fn number_of_record_beating_chances(max_time: i128, record: i128) -> u32 {
+    let sqrt = ((max_time.pow(2) - 4 * record) as f64).sqrt();
+
+    let answer1 = -((-max_time as f64) + sqrt) / 2f64;
+    let answer2 = -((-max_time as f64) - sqrt) / 2f64;
+
+    (answer2.ceil() - answer1.ceil()) as u32
+}
+
 pub fn one() -> Result<u32, AdventError> {
     let overview = read_into::<RecordsOverview>(Path::new("resources/input6.txt"))?;
     Ok(overview.records.iter().fold(1, |acc, record| {
-        acc * ((0..(record.time + 1)).into_iter().map(|x| {
-            x * (record.time - x)
-        }).filter(|x| {
-            *x > record.distance
-        }).count() as u32)
+        acc * number_of_record_beating_chances(record.time.into(), record.distance.into())
     }))
 }
 
 pub fn two() -> Result<u32, AdventError> {
     let record = read_into::<Record>(Path::new("resources/input6.txt"))?;
-    Ok((0..(record.time + 1)).into_iter().map(|x| {
-        x * (record.time - x)
-    }).filter(|x| {
-        *x > record.distance
-    }).count() as u32) 
+    Ok(number_of_record_beating_chances(record.time.into(), record.distance.into())) 
 }

src/main.rs

@@ -19,7 +19,7 @@ fn main() -> Result<(), AdventError> {
         (day6::one as fn() -> Result<u32, AdventError>, day6::two as fn() -> Result<u32, AdventError>),
     ];
 
-    if env::args().len() != 3 {
+    if env::args().len() == 1 {
         Ok(days.into_iter().map(|(one, two)| {
             let one_result = one()?;
             let two_result = two()?;
@@ -31,12 +31,17 @@ fn main() -> Result<(), AdventError> {
             };
         }).fold((), |acc, _| { acc }))
     } else {
-        let (one, two) = days[env::args().nth(1).ok_or(AdventError("Failed to parse day".into()))?.parse::<usize>()? - 1];
+        let day_index = env::args().nth(1).ok_or(AdventError("Failed to parse day".into()))?.parse::<usize>()? - 1;
+        let (one, two) = days[day_index];
+        if env::args().len() == 3 {
             match env::args().nth(2).unwrap().parse::<usize>()? {
                 1 => Ok(println!("1: {}", one()?)),
                 2 => Ok(println!("2: {}", two()?)),
                 _ => return Err(AdventError("part must be either 1 or 2".into()))
             }
+        } else {
+            Ok(println!("{}: ({}, {})", day_index + 1, one()?, two()?))
+        }
     }
     
 }