Demonstrate solving the first problems from 2021

.gitignore

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 .direnv
+target

problems/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "problems"
+version = "0.1.0"

problems/Cargo.toml

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

problems/data/day1a.txt

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+199
+200
+208
+210
+200
+207
+240
+269
+260
+263

problems/src/day1.rs

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+const PART1: &str = include_str!("../data/day1a.txt");
+
+pub fn part1() -> String {
+    let lines = PART1.lines();
+
+    let depths: Vec<i32> = lines.map(|l| l.parse::<i32>().unwrap()).collect();
+
+    let depths_a = depths.iter();
+    let depths_b = depths.iter().skip(1);
+    let depth_cmp = depths_a.zip(depths_b);
+
+    let increases = depth_cmp.fold(0, |cur, (a, b)| {
+        println!("{} < {}? {}", a, b, a < b);
+        if a < b {
+            cur + 1
+        } else {
+            cur
+        }
+    });
+    format!("{}", increases)
+}
+
+pub fn part2() -> String {
+    "no response".to_owned()
+}

problems/src/main.rs

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+mod day1;
+
+fn main() {
+    let day = std::env::args().skip(1).next();
+    println!("day: {:?}", day);
+
+    let result = match day.as_ref().map(|v| v.as_ref()) {
+        Some("day1a") => day1::part1(),
+        Some("day1b") => day1::part2(),
+        _ => panic!("unrecognized day"),
+    };
+
+    println!("{:?} Result: {}", day, result);
+}