Update day 4 with part 1 solution

Signed-off-by: Alek Ratzloff <alekr@jsausa.com>

day04/Day04.rs

@@ -1,11 +1,67 @@
-//use std::io::{self, Read};
-
-//type Result<T> = std::result::Result<T, Box<dyn std::error::Error>>;
-
 fn main() {
     part1(246515, 739105);
+    part2(246515, 739105);
+}
+
+fn digit_count(n: usize) -> u32 {
+    if n == 0 { 1 } else { (n as f64).log10().floor() as u32 + 1 }
+}
+
+fn digit_n(digit: u32, n: usize) -> usize {
+    let digit_count = digit_count(n);
+    println!("{} {} {}", n, digit_count, digit);
+    let place = 10usize.pow(digit_count - digit);
+    (n / place) % 10
+}
+
+fn is_increasing(n: usize) -> bool {
+    for digit in 0 .. 5 {
+        let this = digit_n(digit, n);
+        let next = digit_n(digit + 1, n);
+        if next < this {
+            return false;
+        }
+    }
+    true
+}
+
+fn has_double(n: usize) -> bool {
+    let digit_count = digit_count(n);
+    for digit in 0 .. digit_count {
+        let this = digit_n(digit, n);
+        let next = digit_n(digit + 1, n);
+        if next == this {
+            return true;
+        }
+    }
+    false
+}
+
+fn has_strict_double(n: usize) -> bool {
+    false
 }
 
 fn part1(lower: usize, upper: usize) {
-
+    let mut count = 0;
+    for n in lower ..= upper {
+        if is_increasing(n) && has_double(n) {
+            count += 1;
+        }
+    }
+    println!("Part 1: {}", count);
+}
+
+fn part2(lower: usize, upper: usize) {
+    let mut count = 0;
+
+    for n in &[112233, 123444, 111122, 144455, 114444] {
+        println!("{} has_strict_double: {}", n, has_strict_double(*n));
+    }
+
+    for n in lower ..= upper {
+        if is_increasing(n) && has_strict_double(n) {
+            count += 1;
+        }
+    }
+    println!("Part 2: {}", count);
 }