Working

day5/src/main.rs

@@ -3,22 +3,74 @@ use anyhow::{Result, bail};
 
 mod parse;
 
+#[derive(Debug)]
+struct MapRange {
+    base: usize,
+    target: usize,
+}
+
 /// Stores a map as a series of contiguous offset ranges.
 #[derive(Debug)]
 struct Map {
     from: String,
     to: String,
-    ranges: Vec<(Range<usize>, Range<usize>)>,
+    ranges: Vec<MapRange>,
 }
 
 
 impl Map {
 
+    fn from_ranges(from: String, to: String, mut v: Vec<(usize,usize,usize)>) -> Self {
+        v.sort_by_key(|r| r.1);
+
+        let mut ranges = Vec::with_capacity(2*v.len() + 1);
+        ranges.push(MapRange {base: 0, target: 0 } );
+
+        for (target, base, len) in v {
+            if ranges.last().unwrap().base == base {
+                ranges.pop();
+            }
+            ranges.push(MapRange { base, target });
+            ranges.push(MapRange { base: base+len, target: base+len });
+        }
+
+        Self { from, to, ranges }
+    }
+
+    fn range_idx(&self, target: usize) -> usize {
+        self.ranges.binary_search_by_key(&target, |r| r.base)
+            .unwrap_or_else(|i| i-1)
+    }
+
     fn lookup(&self, src: usize) -> usize {
-        todo!()
+        let range = &self.ranges[self.range_idx(src)];
+        (src + range.target) - range.base
     }
 }
 
 fn main() -> Result<()> {
+    let (seeds, maps) = parse::input(stdin().lock())?;
+
+    eprintln!("{seeds:?}, {maps:?}");
+
+    let mut current= "seed";
+
+    for map in &maps {
+        if map.from != current { bail!("Wrong map order, expected {current}") }
+        current = &map.to;
+    }
+
+    let best = seeds.iter()
+                .map(|&s| {
+                    let mut cur = s;
+                    eprint!("{cur}");
+                    for m in &maps {
+                        cur = m.lookup(cur);
+                        eprint!(" -> {cur}")
+                    }
+                    eprintln!();
+                    cur
+                }).min();
+    println!("lowest location: {best:?}");
     Ok(())
 }

day5/src/parse.rs

@@ -32,12 +32,11 @@ fn map<R: BufRead>(s: &mut Lines<R>) -> Option<Result<Map>> {
         loop {
             let line = s.next().ok_or(anyhow!("Unterminated map"))??;
             if line.is_empty() { break }
-            let (dst, src, len) = range(&line)?;
+            ranges.push(range(&line)?);
 
-            ranges.push((src..src+len, dst..dst+len));
         }
 
-        Ok(Map { from: from.to_owned(), to: to.to_owned(), ranges })
+        Ok(Map::from_ranges(from.to_owned(), to.to_owned(), ranges))
 
     })())
 }
@@ -50,7 +49,7 @@ pub fn input<R: BufRead>(s: R) -> Result<( Vec<usize>, Vec<Map>)> {
         bail!("Missing empty line after seeds")
     }
 
-    let maps = unfold(s, map).collect::<Result<_>>()?;
+    let maps = unfold(s, map).collect::<Result<Vec<_>>>()?;
 
     Ok((seeds, maps))
 }