day08 Add part 1

Took multiple seconds to run, so I wonder if there's a more efficient
way to do this. Maybe would be better to have a Vec<Vec<usize>> instead
of putting all lines after each other.

day08/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day08"
+version = "0.1.0"

day08/Cargo.toml

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+[package]
+name = "day08"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day08/src/main.rs

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+use std::error;
+use std::fs::File;
+use std::io::BufRead;
+use std::io::BufReader;
+
+fn main() -> Result<(), Box<dyn error::Error>> {
+	let f = File::open("input.txt")?;
+	let reader = BufReader::new(f);
+
+	let mut map = vec![];
+	let mut width = 0;
+	for line in reader.lines() {
+		let line = line?;
+
+		width = line.len();
+		map.extend(line.chars().map(|c| c.to_digit(10)).flatten().map(|d| d as usize));
+	}
+
+	let width = width;
+	let height = map.len() / width;
+
+	let mut visible_count = 0;
+	for i in 0..map.len() {
+		let line = i / height;
+		let column = i % width + 1; // we need + 1 here, because there is no modulo 0
+
+		// println!("line: {}, column: {}, self: {}", line, column, map[i]);
+		let left = map[(line * width)..i].iter().max();
+		let right = map[i..((line + 1) * width)].iter()
+			.enumerate()
+			.filter_map(|(j, x)| if (j + i) / height == line && j > 0 { Some(x) } else { None })
+			.max();
+		// let right = map[(i + 1)..((line + 1) * width)].iter().max();
+		let up = map[0..i].iter()
+			.enumerate()
+			//.inspect(|(j, x)| println!("{j} {x}"))
+			.filter_map(|(j, x)| if (j + 1) % width == column { Some(x) } else { None })
+			.max();
+		let down = map[i..].iter()
+			.enumerate()
+			.filter_map(|(j, x)| if (i + j + 1) % width == column && j > 0 { Some(x) } else { None })
+			.max();
+		// println!("left: {:?}, right: {:?}, up: {:?}, down: {:?}", &left, &right, &up, &down);
+
+		for direction_max in vec![left, right, up, down] {
+			match direction_max {
+				Some(x) if x < &map[i] => {
+					visible_count += 1;
+					break;
+				},
+				None => {
+					visible_count += 1;
+					break;
+				},
+				_ => ()
+			}
+		}
+		// println!("visible: {visible_count}");
+	}
+
+    println!("Found {visible_count} visible trees.");
+    Ok(())
+}