From 99b5bda691b5089050ee180fcbdce771a5e9aa6a Mon Sep 17 00:00:00 2001
From: Maxime Augier <max@xolus.net>
Date: Mon, 11 Dec 2023 19:57:29 +0100
Subject: [PATCH] Finally, two parts working

---
 day5/src/main.rs | 128 +++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 106 insertions(+), 22 deletions(-)

diff --git a/day5/src/main.rs b/day5/src/main.rs
index 3322450..b725fb8 100644
--- a/day5/src/main.rs
+++ b/day5/src/main.rs
@@ -1,16 +1,17 @@
-use std::{collections::BTreeMap, io::stdin, ops::{Mul, Range}};
-use anyhow::{Result, bail};
+use std::{io::stdin, ops::Range};
+use anyhow::{Result, anyhow, bail, Context};
+use itertools::Itertools;
 
 mod parse;
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy)]
 struct MapRange {
     base: usize,
     target: usize,
 }
 
 /// Stores a map as a series of contiguous offset ranges.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 struct Map {
     from: String,
     to: String,
@@ -20,6 +21,12 @@ struct Map {
 
 impl Map {
 
+    fn identity(from: String) -> Self{
+        let to = from.clone();
+        Self { from, to, ranges: vec![MapRange { base: 0, target: 0 }] } 
+    }
+
+    /// Build a compact segment map from a list of (target, source, length) tuples
     fn from_ranges(from: String, to: String, mut v: Vec<(usize,usize,usize)>) -> Self {
         v.sort_by_key(|r| r.1);
 
@@ -37,40 +44,117 @@ impl Map {
         Self { from, to, ranges }
     }
 
+    /// Gives the segment index relevant for a given source location in the map
     fn range_idx(&self, target: usize) -> usize {
         self.ranges.binary_search_by_key(&target, |r| r.base)
-            .unwrap_or_else(|i| i-1)
+            .unwrap_or_else(|i| i-1) //INVARIANT: `i`` can never be 0 because we always have a range starting at 0
     }
 
+    /// Computes the image of a location over the map
     fn lookup(&self, src: usize) -> usize {
         let range = &self.ranges[self.range_idx(src)];
         (src + range.target) - range.base
     }
+
+    fn segments(&self) -> impl Iterator<Item=(Range<usize>, Range<usize>)> + '_ {
+        self.ranges.iter()
+            .zip(self.ranges.iter().dropping(1)) 
+            .map(|(a,b)| (a.base..b.base, a.target..(b.base-a.base)))
+    }
+
+    /// Composes two maps
+    fn compose(&self, rhs: &Map) -> Result<Map> {
+        if self.to != rhs.from { bail!("incompatible maps {self:?} and {rhs:?}")};
+
+        let mut ranges = Vec::with_capacity(self.ranges.len() + rhs.ranges.len());
+
+        for i in 0..self.ranges.len() {
+            let mine = &self.ranges[i];
+            // because we don't store ends of ranges, we have to check the next one
+            let stop = self.ranges.get(i+1)
+                .map(|next| next.base + mine.target - mine.base); // watch for order of
+                                                                             // operations because
+                                                                             // unsigned
+
+            let start_index = rhs.range_idx(mine.target);
+            // stop index must be inclusive because our stop point may not match rhs boundary
+            let stop_index = stop.map(|s| rhs.range_idx(s-1)).unwrap_or(rhs.ranges.len()-1);
+
+            // The first target range begins before our own start, treat it specially
+            let first_range = &rhs.ranges[start_index];
+            ranges.push(MapRange { base: mine.base, target: first_range.target + mine.target - first_range.base });
+
+            for other in &rhs.ranges[start_index+1 ..= stop_index] {
+                ranges.push(MapRange { base: mine.base + other.base - mine.target, target: other.target }); 
+            }
+        }
+
+        Ok(Map {
+            from: self.from.to_owned(),
+            to: rhs.to.to_owned(),
+            ranges,
+        })
+    }
+
+}
+
+fn intersect(a: &Range<usize>, b: &Range<usize>) -> Range<usize> {
+   a.start.max(b.start) .. a.end.min(b.end)
+}
+
+fn vec_to_ranges(s: &[usize]) -> Result<Vec<Range<usize>>> {
+    let mut i = s.iter();
+    let mut rs = Vec::with_capacity(s.len() / 2);
+    loop {
+        let Some(&start) = i.next() else { break };
+        let len = i.next().ok_or(anyhow!("odd number of values"))?;
+        rs.push(start .. start+len);
+    }
+    Ok(rs)
 }
 
 fn main() -> Result<()> {
     let (seeds, maps) = parse::input(stdin().lock())?;
 
-    eprintln!("{seeds:?}, {maps:?}");
+    eprintln!("seeds: {seeds:?}");
 
-    let mut current= "seed";
+    let map = maps.iter()
+        .try_fold(Map::identity("seed".to_owned()), |lhs, rhs| lhs.compose(rhs))
+        .context(anyhow!("Building combined map"))?;
 
-    for map in &maps {
-        if map.from != current { bail!("Wrong map order, expected {current}") }
-        current = &map.to;
+    eprintln!("built combined map from {} to {} ({} segments)", map.from, map.to, map.ranges.len());
+
+    let best = seeds.iter().copied()
+                    .map(|v| map.lookup(v))
+                    .min();
+
+    println!("Part 1: lowest location: {best:?}");
+
+
+    let part2_ranges = vec_to_ranges(&seeds)?;
+    let mut segment_index: Vec<_> = map.segments().collect();
+    segment_index.sort_by_key(|s| s.1.start);
+    let mut best2: Option<usize> = None;
+
+    for (src_range, dst_range) in &segment_index {
+        if let Some(b) = best2 {
+            if b <= dst_range.start { break }
+        }
+        for seeds in &part2_ranges {
+            let inter = intersect(seeds, src_range);
+            if !inter.is_empty() {
+                let candidate = inter.start + dst_range.start - src_range.start;
+                if let Some(b) = best2 {
+                    if b < candidate { continue }
+                }
+                best2 = Some(candidate);
+            }
+        }
     }
 
-    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:?}");
+    let best2 = best2.ok_or(anyhow!("No intersect inside known range"))?;
+
+    println!("Part 2: lowest location {best2}");
+
     Ok(())
 }