day04: Add part 1

I'm not sure why this is so much to write. It feels like there should be
a more generic way to check this instead of duplicating those lines, but
maybe it's not worth it for 2 ranges ...

day04/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day04"
+version = "0.1.0"

day04/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "day04"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day04/src/main.rs

@@ -0,0 +1,25 @@
+use std::error;
+use std::fs::File;
+use std::io::BufRead;
+use std::io::BufReader;
+use std::ops;
+
+fn main() -> Result<(), Box<dyn error::Error>> {
+	let f = File::open("input.txt")?;
+	let reader = BufReader::new(f);
+
+	let mut count: u32 = 0;
+	for line in reader.lines() {
+		let line = line?;
+		let range_strs = line.split(",").collect::<Vec<&str>>();
+		let range_a_u32 = range_strs[0].split("-").map(|x| x.parse::<u32>().unwrap()).collect::<Vec<u32>>();
+		let range_a = ops::Range { start: range_a_u32[0], end: range_a_u32[1] + 1 };
+		let range_b_u32 = range_strs[1].split("-").map(|x| x.parse::<u32>().unwrap()).collect::<Vec<u32>>();
+		let range_b = ops::Range { start: range_b_u32[0], end: range_b_u32[1] + 1 };
+		if (range_a.contains(&range_b.start) && range_a.contains(&(range_b.end - 1))) || (range_b.contains(&range_a.start) && range_b.contains(&(range_a.end - 1))) {
+			count += 1;
+		}
+	}
+	println!("Found {} completely contained ranges.", count);
+    Ok(())
+}