day5: problem 2 solution. parallelized!

5/src/main.rs

@@ -2,6 +2,9 @@ use std::fs::File;
 use std::io::{BufRead, BufReader, Lines};
 use std::ops::Range;
 
+use itertools::Itertools;
+use rayon::prelude::*;
+
 // --- Day 5: If You Give A Seed A Fertilizer ---
 
 // You take the boat and find the gardener right where you were told he would be:
@@ -278,7 +281,12 @@ impl Almanac {
 // What is the lowest location number that corresponds to any of the initial seed numbers?
 
 fn problem1_lowest_location(almanac: &Almanac) -> u64 {
-    almanac.seeds.iter().map(|seed| almanac.lookup(*seed)).min().unwrap()
+    almanac
+        .seeds
+        .iter()
+        .map(|seed| almanac.lookup(*seed))
+        .min()
+        .unwrap()
 }
 
 fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
@@ -287,15 +295,60 @@ fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
 }
 
 // PROBLEM 2 solution
+
+// Everyone will starve if you only plant such a small number of seeds. Re-reading the
+// almanac, it looks like the seeds: line actually describes ranges of seed numbers.
+
+// The values on the initial seeds: line come in pairs. Within each pair, the first
+// value is the start of the range and the second value is the length of the range. So,
+// in the first line of the example above:
+
+// seeds: 79 14 55 13
+
+// This line describes two ranges of seed numbers to be planted in the garden. The first
+// range starts with seed number 79 and contains 14 values: 79, 80, ..., 91, 92. The
+// second range starts with seed number 55 and contains 13 values: 55, 56, ..., 66, 67.
+
+// Now, rather than considering four seed numbers, you need to consider a total of 27
+// seed numbers.
+
+// In the above example, the lowest location number can be obtained from seed number 82,
+// which corresponds to soil 84, fertilizer 84, water 84, light 77, temperature 45,
+// humidity 46, and location 46. So, the lowest location number is 46.
+
+// Consider all of the initial seed numbers listed in the ranges on the first line of
+// the almanac. What is the lowest location number that corresponds to any of the
+// initial seed numbers?
+
+fn problem2_lowest_location(almanac: &Almanac) -> u64 {
+    let seed_ranges = almanac
+        .seeds
+        .iter()
+        .tuples()
+        .map(|(range_start, length)| *range_start..*range_start + *length);
+
+    seed_ranges
+        .map(|range| {
+            range
+                .into_par_iter()
+                .map(|x| almanac.lookup(x))
+                .min()
+                .unwrap()
+        })
+        .min()
+        .unwrap()
+}
+
 fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
-    0
+    let almanac = Almanac::from(input);
+    problem2_lowest_location(&almanac)
 }
 
 #[cfg(test)]
 mod tests {
     use std::io::{BufRead, Cursor};
 
-    use crate::{Almanac, AlmanacMapping, problem1_lowest_location};
+    use crate::*;
 
     const EXAMPLE_ALMANAC: &str = &"seeds: 79 14 55 13
 
@@ -359,4 +412,11 @@ humidity-to-location map:
         let a = Almanac::from(c.lines());
         assert_eq!(problem1_lowest_location(&a), 35);
     }
+
+    #[test]
+    fn test_problem2_example() {
+        let c = Cursor::new(EXAMPLE_ALMANAC);
+        let a = Almanac::from(c.lines());
+        assert_eq!(problem2_lowest_location(&a), 46);
+    }
 }