🚀 Day 2: MASSIVE 10x speedup & 99% memory reduction! Iterator magic FTW!

README.md

@@ -12,12 +12,12 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | Day | Part 1 | Part 2 |
 | :---: | :---: | :---:  |
 | [Day 1](./src/bin/01.rs) | `102.5µs` | `123.5µs` |
-| [Day 2](./src/bin/02.rs) | `1.1ms` | `3.8ms` |
+| [Day 2](./src/bin/02.rs) | `131.1µs` | `314.3µs` |
 | [Day 3](./src/bin/03.rs) | `85.1µs` | `121.6µs` |
 | [Day 4](./src/bin/04.rs) | `244.2µs` | `4.1ms` |
 | [Day 5](./src/bin/05.rs) | `196.7µs` | `66.3µs` |
 
-**Total: 9.94ms**
+**Total: 5.49ms**
 <!--- benchmarking table --->
 
 ---

src/bin/02.rs

@@ -12,38 +12,11 @@ fn parse_range(s: &str) -> Option<(u64, u64)> {
         .and_then(|(a, b)| Some((a?, b?)))
 }
 
-/// Generate all invalid numbers (pattern repeated exactly twice) within [min, max]
-fn generate_invalid_part1(min: u64, max: u64) -> Vec<u64> {
-    let mut result = Vec::new();
-    let max_digits = max.ilog10() as usize + 1;
+/// Iterator over invalid numbers (pattern repeated exactly twice) within [min, max]
+fn iter_invalid_part1(min: u64, max: u64) -> impl Iterator<Item = u64> {
+    let max_digits = if max == 0 { 1 } else { max.ilog10() as usize + 1 };
 
-    // Pattern length k produces numbers with 2k digits
-    for pattern_len in 1..=max_digits / 2 {
-        let pattern_min = if pattern_len == 1 {
-            1
-        } else {
-            10u64.pow(pattern_len as u32 - 1)
-        };
-        let pattern_max = 10u64.pow(pattern_len as u32) - 1;
-
-        for pattern in pattern_min..=pattern_max {
-            // Construct number by repeating pattern twice
-            let num = pattern * 10u64.pow(pattern_len as u32) + pattern;
-            if num >= min && num <= max {
-                result.push(num);
-            }
-        }
-    }
-    result
-}
-
-/// Generate all invalid numbers (pattern repeated at least twice) within [min, max]
-fn generate_invalid_part2(min: u64, max: u64) -> Vec<u64> {
-    let mut seen = HashSet::new();
-    let max_digits = max.ilog10() as usize + 1;
-
-    // For each pattern length
-    for pattern_len in 1..=max_digits / 2 {
+    (1..=max_digits / 2).flat_map(move |pattern_len| {
         let pattern_min = if pattern_len == 1 {
             1
         } else {
@@ -52,36 +25,78 @@ fn generate_invalid_part2(min: u64, max: u64) -> Vec<u64> {
         let pattern_max = 10u64.pow(pattern_len as u32) - 1;
         let multiplier = 10u64.pow(pattern_len as u32);
 
-        // For each number of repetitions (at least 2)
-        for reps in 2..=max_digits / pattern_len {
-            for pattern in pattern_min..=pattern_max {
-                // Build number by repeating pattern
+        (pattern_min..=pattern_max).filter_map(move |pattern| {
+            let num = pattern * multiplier + pattern;
+            if num >= min && num <= max {
+                Some(num)
+            } else {
+                None
+            }
+        })
+    })
+}
+
+/// Iterator over invalid numbers (pattern repeated at least twice) within [min, max]
+fn iter_invalid_part2(min: u64, max: u64) -> impl Iterator<Item = u64> {
+    let max_digits = if max == 0 { 1 } else { max.ilog10() as usize + 1 };
+
+    (1..=max_digits / 2).flat_map(move |pattern_len| {
+        let pattern_min = if pattern_len == 1 {
+            1
+        } else {
+            10u64.pow(pattern_len as u32 - 1)
+        };
+        let pattern_max = 10u64.pow(pattern_len as u32) - 1;
+        let multiplier = 10u64.pow(pattern_len as u32);
+
+        (2..=max_digits / pattern_len).flat_map(move |reps| {
+            (pattern_min..=pattern_max).filter_map(move |pattern| {
                 let mut num = 0u64;
                 for _ in 0..reps {
                     num = num * multiplier + pattern;
                 }
-
                 if num >= min && num <= max {
-                    seen.insert(num);
+                    Some(num)
+                } else {
+                    None
                 }
+            })
+        })
+    })
+}
+
+/// Merge overlapping ranges to avoid duplicate counting
+fn merge_ranges(ranges: &[(u64, u64)]) -> Vec<(u64, u64)> {
+    if ranges.is_empty() {
+        return Vec::new();
     }
+
+    let mut sorted: Vec<_> = ranges.to_vec();
+    sorted.sort_by_key(|r| r.0);
+
+    let mut merged = Vec::with_capacity(sorted.len());
+    merged.push(sorted[0]);
+
+    for &(start, end) in &sorted[1..] {
+        let last = merged.last_mut().unwrap();
+        if start <= last.1 + 1 {
+            last.1 = last.1.max(end);
+        } else {
+            merged.push((start, end));
         }
     }
 
-    seen.into_iter().collect()
+    merged
 }
 
 pub fn part_one(input: &str) -> Option<u64> {
     let ranges: Vec<(u64, u64)> = input.split(',').filter_map(parse_range).collect();
+    let merged = merge_ranges(&ranges);
 
-    let global_min = ranges.iter().map(|r| r.0).min().unwrap_or(0);
-    let global_max = ranges.iter().map(|r| r.1).max().unwrap_or(0);
-
-    let invalid_nums = generate_invalid_part1(global_min, global_max);
-
-    let sum = invalid_nums
+    // Sum invalid numbers directly from iterator, checking merged ranges
+    let sum: u64 = merged
         .iter()
-        .filter(|&&n| ranges.iter().any(|&(start, end)| n >= start && n <= end))
+        .flat_map(|&(start, end)| iter_invalid_part1(start, end))
         .sum();
 
     Some(sum)
@@ -89,15 +104,15 @@ pub fn part_one(input: &str) -> Option<u64> {
 
 pub fn part_two(input: &str) -> Option<u64> {
     let ranges: Vec<(u64, u64)> = input.split(',').filter_map(parse_range).collect();
+    let merged = merge_ranges(&ranges);
 
-    let global_min = ranges.iter().map(|r| r.0).min().unwrap_or(0);
-    let global_max = ranges.iter().map(|r| r.1).max().unwrap_or(0);
-
-    let invalid_nums = generate_invalid_part2(global_min, global_max);
-
-    let sum = invalid_nums
+    // Use a small HashSet only for deduplication within merged ranges
+    // (needed because same number can match multiple pattern lengths)
+    let mut seen = HashSet::new();
+    let sum: u64 = merged
         .iter()
-        .filter(|&&n| ranges.iter().any(|&(start, end)| n >= start && n <= end))
+        .flat_map(|&(start, end)| iter_invalid_part2(start, end))
+        .filter(|&n| seen.insert(n))
         .sum();
 
     Some(sum)