From fb535480020886dfbf41f682f1b9f7b284249912 Mon Sep 17 00:00:00 2001
From: Justin Wernick <justin@worthe-it.co.za>
Date: Thu, 5 Dec 2019 00:29:40 +0200
Subject: Day 4 - some quick iteration

---
 inputs/day_4.txt |  2 ++
 src/bin/day_4.rs | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)
 create mode 100644 inputs/day_4.txt
 create mode 100644 src/bin/day_4.rs

diff --git a/inputs/day_4.txt b/inputs/day_4.txt
new file mode 100644
index 0000000..f62320b
--- /dev/null
+++ b/inputs/day_4.txt
@@ -0,0 +1,2 @@
+138241
+674034
diff --git a/src/bin/day_4.rs b/src/bin/day_4.rs
new file mode 100644
index 0000000..3cb8d38
--- /dev/null
+++ b/src/bin/day_4.rs
@@ -0,0 +1,59 @@
+use std::io;
+use std::io::prelude::*;
+
+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 stdin = io::stdin();
+    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
+    }
+}
-- 
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