Day 2 part 2

Cargo.toml

@@ -10,3 +10,7 @@ bstr = "1.11.0"
 clap = { version = "4.5.21", features = ["derive"] }
 color-eyre = "0.6.3"
 humantime = "2.1.0"
+
+[profile.release]
+debug = true
+

src/bin/day2.rs

@@ -1,4 +1,4 @@
-use std::time::Instant;
+use std::{collections::HashSet, time::Instant};
 
 use aoc_2025::{load, print_res};
 use bstr::BString;
@@ -27,6 +27,55 @@ pub fn parsing(input: &BString) -> color_eyre::Result<Parsed<'_>> {
         .collect()
 }
 
+fn id_sum(start: u64, end: u64, first_only: bool) -> u64 {
+    let mut sum = 0;
+    let mut seen = HashSet::new();
+
+    let mag = start.ilog10() + 1;
+
+    assert_eq!(
+        mag,
+        end.ilog10() + 1,
+        "Only the same magnitude is supported"
+    );
+
+    for part_len in (1..mag)
+        .rev()
+        .filter(|&part_len| mag.is_multiple_of(part_len))
+    {
+        let part_count = mag / part_len;
+        let pow_part = 10u64.pow(part_len);
+
+        for part in (pow_part / 10)..pow_part {
+            let mut id = 0;
+            for _ in 0..part_count {
+                id = id * pow_part + part;
+            }
+
+            if id < start {
+                continue;
+            }
+
+            if id > end {
+                break;
+            }
+
+            if seen.contains(&id) {
+                continue;
+            }
+
+            seen.insert(id);
+            sum += id;
+        }
+
+        if first_only {
+            break;
+        }
+    }
+
+    sum
+}
+
 #[inline(never)]
 pub fn part1(input: Parsed) {
     let mut invalid_id_sum = 0;
@@ -58,24 +107,7 @@ pub fn part1(input: Parsed) {
             panic!();
         }
 
-        let half_mag = mag_s / 2;
-        let pow10 = 10u64.pow(half_mag);
-
-        let high_start = s / pow10;
-        let high_end = e / pow10;
-
-        for invalid_part in high_start..=high_end {
-            let invalid = invalid_part * pow10 + invalid_part;
-            if invalid < start {
-                continue
-            }
-
-            if invalid > end {
-                break
-            }
-
-            invalid_id_sum += invalid;
-        }
+        invalid_id_sum += id_sum(s, e, true);
     }
 
     print_res!("Sum of invalid IDs: {invalid_id_sum}");
@@ -83,7 +115,26 @@ pub fn part1(input: Parsed) {
 
 #[inline(never)]
 pub fn part2(input: Parsed) {
-    todo!("todo part2")
+    let mut invalid_id_sum = 0;
+
+    for (start, end) in input {
+        let mut s = start;
+        let mag_e = end.ilog10() + 1;
+        loop {
+            let mag_s = s.ilog10() + 1;
+
+            if mag_s != mag_e {
+                let next_start = 10u64.pow(mag_s);
+                invalid_id_sum += id_sum(s, next_start - 1, false);
+                s = next_start;
+            } else {
+                invalid_id_sum += id_sum(s, end, false);
+                break;
+            }
+        }
+    }
+
+    print_res!("Sum of invalid IDs: {invalid_id_sum}");
 }
 
 pub fn main() -> color_eyre::Result<()> {