2021 day1/rust: add solution

.gitignore

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+target/

2021/day1/day1_rs/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day1_rs"
+version = "0.1.0"
+dependencies = [
+ "itertools",
+]
+
+[[package]]
+name = "either"
+version = "1.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e78d4f1cc4ae33bbfc157ed5d5a5ef3bc29227303d595861deb238fcec4e9457"
+
+[[package]]
+name = "itertools"
+version = "0.10.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "69ddb889f9d0d08a67338271fa9b62996bc788c7796a5c18cf057420aaed5eaf"
+dependencies = [
+ "either",
+]

2021/day1/day1_rs/Cargo.toml

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+[package]
+name = "day1_rs"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+itertools = "0.10.1"

2021/day1/day1_rs/src/main.rs

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+use itertools::Itertools;
+use std::io::{self, BufRead};
+
+fn count_increasing(i: impl Iterator<Item = impl Ord + Clone>) -> usize {
+    i.tuple_windows().filter(|(x, y)| x < y).count()
+}
+
+fn main() {
+    let numbers: Vec<usize> = io::stdin()
+        .lock()
+        .lines()
+        .map(|l| str::parse(&l.unwrap()).unwrap())
+        .collect();
+
+    println!("{}", count_increasing(numbers.iter()));
+    println!(
+        "{}",
+        count_increasing(
+            numbers
+                .windows(3)
+                .map(|window| -> usize { window.iter().sum() })
+        )
+    );
+}