Add second approach using reverse search

2023/day5/src/main.rs

@@ -109,6 +109,15 @@ impl Map {
             None
         }
     }
+
+    fn map_reverse(&self, value: usize) -> Option<usize> {
+        if self.destination_range.contains(&value) {
+            let index = value - self.destination_range.start;
+            Some(self.source_range.clone().nth(index).unwrap())
+        } else {
+            None
+        }
+    }
 }
 
 #[derive(Debug)]
@@ -142,6 +151,15 @@ impl MapList {
         }
         value
     }
+
+    fn map_reverse(&self, value: usize) -> usize {
+        for map in &self.maps {
+            if let Some(mapped_value) = map.map_reverse(value) {
+                return mapped_value;
+            }
+        }
+        value
+    }
 }
 
 fn part1(input: &str) -> Result<usize, String> {
@@ -169,7 +187,36 @@ fn part2(input: &str) -> Result<usize, String> {
         println!("parsing rest found: {rest}");
         panic!();
     }
-    let lowest_location = almanac
+
+    #[derive(PartialEq, Eq)]
+    #[allow(dead_code)]
+    enum Approach {
+        Reverse,
+        BruteForce,
+    }
+
+    let approach = Approach::BruteForce;
+
+    let lowest_location = match approach {
+        Approach::Reverse => {
+            let mut i = 0;
+            loop {
+                let mut reverse_val = i;
+                for map_list in almanac.map_lists.iter().rev() {
+                    reverse_val = map_list.map_reverse(reverse_val);
+                }
+                if almanac
+                    .seeds
+                    .0
+                    .iter()
+                    .any(|seed| seed.contains(&reverse_val))
+                {
+                    break i;
+                }
+                i += 1;
+            }
+        }
+        Approach::BruteForce => almanac
             .seeds
             .0
             .into_par_iter()
@@ -191,7 +238,9 @@ fn part2(input: &str) -> Result<usize, String> {
                 result
             })
             .min()
-        .unwrap();
+            .unwrap(),
+    };
+
     Ok(lowest_location)
 }