feat(u/riking/aoc/day01): add day01 solution

Tests with the puzzle's sample inputs are included.

Change-Id: I32cd59b9e3894bde3d67ebdc3a977961b17bdb49
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2223
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Tested-by: BuildkiteCI

users/riking/adventofcode-2020/day01/src/main.rs

@@ -0,0 +1,85 @@
+use anyhow::anyhow;
+use std::fs::File;
+use std::io::prelude::*;
+use std::io::BufReader;
+
+const PART_2: bool = true;
+
+fn day01(is_part2: bool, numbers: &Vec<i64>) -> Result<String, anyhow::Error> {
+    //println!("{:?}", numbers);
+
+    for n1 in numbers.iter() {
+        for n2 in numbers.iter() {
+            if is_part2 {
+                for n3 in numbers.iter() {
+                    if n1 + n2 + n3 == 2020 {
+                        return Ok((n1 * n2 * n3).to_string());
+                    }
+                }
+            } else {
+                if n1 + n2 == 2020 {
+                    return Ok((n1 * n2).to_string());
+                }
+            }
+        }
+    }
+
+    Err(anyhow!("no solution found"))
+}
+
+fn parse(filename: &str) -> Result<Vec<i64>, anyhow::Error> {
+    let f = File::open(filename)?;
+    let mut reader = BufReader::new(f);
+
+    let mut values = Vec::<i64>::new();
+
+    let mut line = String::new();
+    loop {
+        line.clear();
+        reader.read_line(&mut line)?;
+        let trimmed_line = line.trim();
+        if trimmed_line.is_empty() {
+            break;
+        }
+
+        values.push(trimmed_line.parse()?);
+    }
+    Ok(values)
+}
+
+fn main() -> anyhow::Result<()> {
+    let args: Vec<String> = std::env::args().collect();
+
+    //println!("{:?}", args);
+    if args.len() != 2 {
+        return Err(anyhow!("usage: day01 input_file"));
+    }
+    let filename = args.into_iter().skip(1).next().expect("args len == 1");
+
+    let numbers = parse(&filename)?;
+
+    println!("{}", day01(PART_2, &numbers)?);
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::day01;
+
+    #[test]
+    fn test_part1() {
+        let vec = vec![1721, 979, 366, 299, 675, 1456];
+        let result = day01(false, &vec).unwrap();
+
+        assert_eq!(result, 514579.to_string());
+    }
+
+    #[test]
+    fn test_part2() {
+        let vec = vec![1721, 979, 366, 299, 675, 1456];
+        let result = day01(true, &vec).unwrap();
+
+        assert_eq!(result, 241861950.to_string());
+    }
+}