day06: Add part 1

Borrow-checker hindered me to keep the same HashSet around all the time
and just clear it, so it feels a little inefficient to have a new set
every time.

day06/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day06"
+version = "0.1.0"

day06/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "day06"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day06/src/main.rs

@@ -0,0 +1,37 @@
+use std::collections::HashSet;
+use std::error;
+use std::fs::File;
+use std::io::prelude::*;
+
+fn main() -> Result<(), Box<dyn error::Error>> {
+	let mut f = File::open("input.txt")?;
+
+	let mut buf = [0; 32];
+	let mut pos = 0;
+	let mut found = false;
+	let mut n = f.read(&mut buf[3..])?;
+	while n > 0 {
+		for i in 4..(4 + n) {
+			pos += 1;
+			// println!("{}", std::str::from_utf8(&buf[(i - 4)..i]).unwrap());
+			// skip if we read uninitialized data from first run
+			if buf[i - 4] == 0 {
+				continue;
+			}
+			let set: HashSet<&u8> = HashSet::from_iter(&buf[(i - 4)..i]);
+			if set.len() == 4 {
+				found = true;
+				break;
+			}
+		}
+		if found {
+			break;
+		}
+		for i in 0..3 {
+			buf[i] = buf[29 + i];
+		}
+		n = f.read(&mut buf[3..])?;
+	}
+    println!("Found 4 different chars at position {}", pos);
+    Ok(())
+}