Solution for 2022/day15-part2

2022/day15-part2/Cargo.toml

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+[package]
+name = "day15-part2"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2022/day15-part2/src/main.rs

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+use std::collections::VecDeque;
+
+#[derive(Debug, Copy, Clone)]
+struct Sensor {
+    sx: isize,
+    sy: isize,
+    bx: isize,
+    by: isize,
+}
+
+impl Sensor {
+    // Determine the distance between sensor and beacon, using Manhattan metric.
+    fn beacon_distance(&self) -> isize {
+        (self.sx - self.bx).abs() + (self.sy - self.by).abs()
+    }
+
+    // fn beacon_impossible(&self, x: isize, y: isize) -> bool {
+    //     let d = (self.sx - x).abs() + (self.sy - y).abs();
+    //     d <= self.beacon_distance()
+    // }
+
+    // Return a range for a given y-coordinate where no beacon could
+    // possibly be present for this sensor.
+    fn covered_in_line(&self, y: isize) -> Option<(isize, isize)> {
+        let d = self.beacon_distance() - (self.sy - y).abs();
+        if d < 0 {
+            None
+        } else {
+            Some(((self.sx - d), (self.sx + d)))
+        }
+    }
+}
+
+// Collapse multiple ranges into the minimal set of ranges.
+fn collapse_ranges(ranges: &mut VecDeque<(isize, isize)>) {
+    // First, sort by minimal value..
+    ranges
+        .make_contiguous()
+        .sort_unstable_by(|a, b| a.0.cmp(&b.0));
+
+    // Then continually merge pairs throughout the VecDeque.
+    let mut idx: usize = 0;
+    while idx + 1 < ranges.len() {
+        // Grab the current pair.
+        let a = ranges[idx];
+        let b = ranges[idx + 1];
+        // Check if the minimal vaue of b is contained in a.
+        // In that case we can merge the two.
+        if a.0 <= b.0 && b.0 <= a.1 + 1 {
+            // Remove the first of the pair.
+            ranges.remove(idx);
+            // And update the second element.
+            // (which just moved from idx+1 to idx)
+            ranges[idx] = (a.0, a.1.max(b.1));
+        } else {
+            // If no merge took place, move on to the next pair.
+            idx += 1;
+        }
+    }
+}
+
+fn main() {
+    // Use command line arguments to specify the input filename.
+    let args: Vec<String> = std::env::args().collect();
+    if args.len() < 2 {
+        panic!("Usage: ./main <input-file>\nNo input file provided. Exiting.");
+    }
+
+    // Next, read the contents of the input file into a string for easier processing.
+    let input = std::fs::read_to_string(&args[1]).expect("Error opening file");
+
+    // --- TASK BEGIN ---
+
+    // Parse the input.
+    let mut sensors: Vec<Sensor> = Vec::new();
+    for line in input.lines() {
+        let line = line
+            .split('=')
+            .skip(1)
+            .map(|x| {
+                x.chars()
+                    .take_while(|c| *c != ',' && *c != ':')
+                    .collect::<String>()
+            })
+            .map(|e| e.parse::<isize>().unwrap())
+            .collect::<Vec<_>>();
+        sensors.push(Sensor {
+            sx: line[0],
+            sy: line[1],
+            bx: line[2],
+            by: line[3],
+        })
+    }
+
+    // Iterate over all possible lines.
+    for y in 0..4000000 {
+        // Collect all the ranges in line y where no beacons could be.
+        let mut ranges: VecDeque<(isize, isize)> = VecDeque::new();
+        for s in &sensors {
+            let r = s.covered_in_line(y);
+            // Only collect non-empty ranges, ofc.
+            if let Some(sr) = r {
+                ranges.push_back(sr);
+            }
+        }
+        // Next up, sort + collapse the ranges.
+        collapse_ranges(&mut ranges);
+        // Check if there is a gap in the ranges.
+        // This is likely the spot we're looking for.
+        if ranges.len() > 1 {
+            // dbg!(&ranges);
+            let x = ranges[0].1 + 1;
+            println!("Gap spotted: ({},{})", x, y);
+            println!("Tuning frequency: {}", x * 4000000 + y);
+        }
+    }
+}