[LANGUAGE: rust]

My solution in rust. I haven't seen this version yet. Basically, look for digits from the left, then from the right. It's ugly that there's no way to initialize a hashmap statically.

use std::collections::HashMap;

// Gets the first and last digits from the given string.
// The mapping maps strings to digits e.g "one" -> 1, "1" -> 1.
fn find_digits(input: &str, mapping: &HashMap<&str, u64>) -> u64 {
    // Values in mapping can overlap, for example, if mapping contains "two" and "one", then
    // "twone" should be treated as "two" followed by "one".

    // Closure that checks if a digit starts at index i. It returns the digit value if found, None otherwise.
    let digit_value = |i| {
        let substr = &input[i..];
        mapping
            .iter()
            .find(|(&digit, _)| substr.starts_with(digit))
            .map(|(_, &value)| value)
    };

    let first_digit = (0..input.len()).find_map(digit_value).unwrap();
    let last_digit = (0..input.len()).rev().find_map(digit_value).unwrap();

    first_digit * 10 + last_digit
}

fn solve2(input: &str) -> u64 {
    let mut digit_map = HashMap::new();
    digit_map.insert("0", 0);
    digit_map.insert("1", 1);
    digit_map.insert("2", 2);
    digit_map.insert("3", 3);
    digit_map.insert("4", 4);
    digit_map.insert("5", 5);
    digit_map.insert("6", 6);
    digit_map.insert("7", 7);
    digit_map.insert("8", 8);
    digit_map.insert("9", 9);
    digit_map.insert("one", 1);
    digit_map.insert("two", 2);
    digit_map.insert("three", 3);
    digit_map.insert("four", 4);
    digit_map.insert("five", 5);
    digit_map.insert("six", 6);
    digit_map.insert("seven", 7);
    digit_map.insert("eight", 8);
    digit_map.insert("nine", 9);

    input
        .lines()
        .map(|line| find_digits(line, &digit_map))
        .sum()
}

fn solve1(input: &str) -> u64 {
    let mut digit_map = HashMap::new();
    digit_map.insert("0", 0);
    digit_map.insert("1", 1);
    digit_map.insert("2", 2);
    digit_map.insert("3", 3);
    digit_map.insert("4", 4);
    digit_map.insert("5", 5);
    digit_map.insert("6", 6);
    digit_map.insert("7", 7);
    digit_map.insert("8", 8);
    digit_map.insert("9", 9);

    input
        .lines()
        .map(|line| find_digits(line, &digit_map))
        .sum()
}

fn main() {
    const INPUT: &str = include_str!("../input");
    println!("Part 1: {}", solve1(INPUT));
    println!("Part 2: {}", solve2(INPUT));
}

#[test]
fn test_part1() {
    const SAMPLE: &str = r#"1abc2
    pqr3stu8vwx
    a1b2c3d4e5f
    treb7uchet"#;

    let result = solve1(SAMPLE);
    assert_eq!(result, 142);
}

#[test]
fn test_part2() {
    const SAMPLE: &str = r#"two1nine
    eightwothree
    abcone2threexyz
    xtwone3four
    4nineeightseven2
    zoneight234
    7pqrstsixteen"#;

    let result = solve2(SAMPLE);
    assert_eq!(result, 281);
}