Day 4 part 1

2019/src/bin/day4.rs

@@ -0,0 +1,42 @@
+use std::ops::RangeInclusive;
+
+const INPUT: RangeInclusive<u32> = 356261..=846303;
+
+// However, they do remember a few key facts about the password:
+
+// * It is a six-digit number.
+// * The value is within the range given in your puzzle input.
+// * Two adjacent digits are the same (like 22 in 122345).
+// * Going from left to right, the digits never decrease; they only ever increase or stay the same
+//   (like 111123 or 135679).
+
+fn main() {
+    let count = INPUT.filter(meets_criteria).count();
+    println!("Part 1: {}", count);
+}
+
+fn meets_criteria(number: &u32) -> bool {
+    let mut number = *number;
+    let mut digits = [0; 6];
+    digits.iter_mut().rev().for_each(|digit| {
+        *digit = number % 10;
+        number /= 10;
+    });
+
+    // adjacent digits and increasing digits
+    digits.windows(2).any(|pair| pair[0] == pair[1])
+        && digits.windows(2).all(|pair| pair[1] >= pair[0])
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_meets_criteria() {
+        assert!(meets_criteria(&111123));
+        assert!(meets_criteria(&135669));
+        assert!(!meets_criteria(&223450));
+        assert!(!meets_criteria(&123789));
+    }
+}