From 97767da4b887d59864560389cc946cde2ac37a87 Mon Sep 17 00:00:00 2001
From: "Eric N. Vander Weele" <ericvw@gmail.com>
Date: Mon, 12 Feb 2024 16:45:15 -0500
Subject: [PATCH] 2019: Day 4

---
 2019/input_04.txt         |  1 +
 2019/src/bin/puzzle_04.rs | 73 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+)
 create mode 100644 2019/input_04.txt
 create mode 100644 2019/src/bin/puzzle_04.rs

diff --git a/2019/input_04.txt b/2019/input_04.txt
new file mode 100644
index 0000000..51c032b
--- /dev/null
+++ b/2019/input_04.txt
@@ -0,0 +1 @@
+231832-767346
diff --git a/2019/src/bin/puzzle_04.rs b/2019/src/bin/puzzle_04.rs
new file mode 100644
index 0000000..22dfb7e
--- /dev/null
+++ b/2019/src/bin/puzzle_04.rs
@@ -0,0 +1,73 @@
+use std::io;
+
+const BASE: u32 = 10;
+
+/// Going left to right, check if digits never decrease.
+fn digits_never_decrease(num: u32) -> bool {
+    let mut prev_digit = num % BASE;
+    let mut x = num / BASE;
+    while x > 0 {
+        let digit = x % BASE;
+        if prev_digit < digit {
+            return false;
+        }
+        prev_digit = digit;
+        x /= BASE;
+    }
+
+    true
+}
+
+fn contains_adjacent_digits_same(num: u32) -> bool {
+    debug_assert!(digits_never_decrease(num));
+
+    let mut prev_digit = num % BASE;
+    let mut x = num / BASE;
+    while x > 0 {
+        let digit = x % BASE;
+        if digit == prev_digit {
+            return true;
+        }
+        prev_digit = digit;
+        x /= BASE;
+    }
+
+    false
+}
+
+fn contains_strict_adjacent_digit_pair(num: u32) -> bool {
+    debug_assert!(digits_never_decrease(num));
+
+    let mut counts = [0; BASE as usize];
+
+    let mut x = num;
+    while x > 0 {
+        counts[(x % BASE) as usize] += 1;
+        x /= BASE;
+    }
+
+    counts.into_iter().any(|x| x == 2)
+}
+
+fn main() {
+    let input_range: Vec<u32> = io::read_to_string(io::stdin())
+        .unwrap()
+        .trim()
+        .split('-')
+        .map(|x| x.parse().unwrap())
+        .collect();
+
+    assert!(input_range.len() == 2, "Only two numbers expected!");
+    let password_range = input_range[0]..=input_range[1];
+
+    let never_decreasing = password_range.filter(|&x| digits_never_decrease(x));
+    let at_least_2_digits_same = never_decreasing.filter(|&x| contains_adjacent_digits_same(x));
+
+    println!("Part 1: {}", at_least_2_digits_same.clone().count());
+
+    let strict_digit_pairs = at_least_2_digits_same
+        .into_iter()
+        .filter(|&x| contains_strict_adjacent_digit_pair(x));
+
+    println!("Part 2: {}", strict_digit_pairs.count());
+}