1 files changed, 55 insertions, 0 deletions
diff --git a/2019/src/bin/day_4.rs b/2019/src/bin/day_4.rs
new file mode 100644
index 0000000..d7d6b69
--- /dev/null
+++ b/2019/src/bin/day_4.rs
@@ -0,0 +1,55 @@
+use structopt::StructOpt;
+
+#[derive(Debug, StructOpt)]
+#[structopt(name = "Day 4: Secure Container")]
+/// Calculates how many possible lock values there are
+///
+/// See https://adventofcode.com/2019/day/4 for details.
+struct Opt {
+    /// Repeated digits must be exactly 2 long
+    #[structopt(long = "larger-range-rule")]
+    larger_range_rule: bool,
+    min: u32,
+    max: u32,
+}
+
+fn main() {
+    let opt = Opt::from_args();
+
+    println!(
+        "{}",
+        valid_combinations(opt.min, opt.max, opt.larger_range_rule)
+    )
+}
+
+fn valid_combinations(min: u32, max: u32, larger_range_rule: bool) -> u32 {
+    (min..max)
+        .filter(|x| {
+            two_adjacent_identical_digits(*x, larger_range_rule) && digits_never_decrease(*x)
+        })
+        .count() as u32
+}
+
+fn two_adjacent_identical_digits(x: u32, larger_range_rule: bool) -> bool {
+    if larger_range_rule {
+        (0..5).any(|d| {
+            digit(x, d) == digit(x, d + 1)
+                && digit(x, d) != digit(x, d + 2)
+                && digit(x, d) != digit(x, d - 1)
+        })
+    } else {
+        (0..5).any(|d| digit(x, d) == digit(x, d + 1))
+    }
+}
+
+fn digits_never_decrease(x: u32) -> bool {
+    (0..5).all(|d| digit(x, d) >= digit(x, d + 1))
+}
+
+fn digit(x: u32, digit: i32) -> u32 {
+    if digit < 0 {
+        0
+    } else {
+        (x / 10u32.pow(digit as u32)) % 10
+    }
+}