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use std::collections::{HashMap, HashSet};

type ProgramId = usize;

#[derive(Debug, Clone)]
struct Program<'a> {
    id: ProgramId,
    name: &'a str,
    weight: u32,
    children: Vec<ProgramId>,
}

struct ProgramTree<'a> {
    nodes: Vec<Program<'a>>,
    root_node: ProgramId,
}

impl<'a> ProgramTree<'a> {
    fn total_weight(&self, program_id: ProgramId) -> u32 {
        let program = &self.nodes[program_id];
        program.weight
            + program
                .children
                .iter()
                .map(|&child| self.total_weight(child))
                .sum::<u32>()
    }
}

impl<'a> ProgramTree<'a> {
    fn parse(input_string: &'a str) -> Result<ProgramTree, String> {
        let mut nodes = Vec::new();
        let mut name_to_node: HashMap<&str, ProgramId> = HashMap::new();

        for (line_index, line) in input_string.lines().enumerate() {
            let parts: Vec<&str> = line.split(" -> ").collect();
            let name_weight_parts: Vec<&str> = parts[0].split(' ').collect();
            if name_weight_parts.len() != 2 {
                return Err(format!(
                    "Line {}: Invalid format, expected '$NAME ($WEIGHT)'",
                    line_index + 1
                ));
            }
            let name = name_weight_parts[0];
            let weight = name_weight_parts[1]
                .replace("(", "")
                .replace(")", "")
                .parse::<u32>()
                .map_err(|error| {
                    format!(
                        "Line {}: Invalid weight ({})",
                        line_index + 1,
                        error.to_string()
                    )
                })?;

            let program_id = nodes.len();
            let program = Program {
                id: program_id,
                name,
                weight,
                children: Vec::new(),
            };
            nodes.push(program);
            name_to_node.insert(name, program_id);
        }

        for line in input_string.lines() {
            let parts: Vec<&str> = line.split(" -> ").collect();
            let name_weight_parts: Vec<&str> = parts[0].split(' ').collect();
            if parts.len() == 2 {
                let name = name_weight_parts[0];
                let parent = *name_to_node.get_mut(name).unwrap();
                for child_name in parts[1].trim().split(", ") {
                    let child_ref = *name_to_node.get(child_name).unwrap();
                    nodes[parent].children.push(child_ref);
                }
            }
        }

        let all_program_ids: HashSet<&ProgramId> = name_to_node.values().collect();
        let children: HashSet<&ProgramId> = name_to_node
            .values()
            .flat_map(|&child_program_id| nodes[child_program_id].children.iter())
            .collect();

        let roots: Vec<&&ProgramId> = all_program_ids.difference(&children).collect();
        let root_node = **roots[0];
        if roots.len() == 1 {
            Ok(Self { nodes, root_node })
        } else {
            Err("No single root found".to_string())
        }
    }
}

pub fn part1(input_string: &str) -> Result<String, String> {
    let tree = ProgramTree::parse(input_string)?;
    Ok(tree.nodes[tree.root_node].name.to_string())
}

fn fixup_weight(program_id: ProgramId, tree: &ProgramTree) -> Option<u32> {
    let program = &tree.nodes[program_id];
    if program.children.len() > 1 {
        if let (Some(lone_weight), desired_weight) = program
            .children
            .iter()
            .fold(HashMap::new(), |mut acc, &child_id| {
                *acc.entry(tree.total_weight(child_id)).or_insert(0) += 1;
                acc
            })
            .iter()
            .fold((None, 0), |acc, (&weight, &occurrences)| {
                if occurrences == 1 {
                    (Some(weight), acc.1)
                } else {
                    (acc.0, weight)
                }
            })
        {
            if let Some(&child_id) = program
                .children
                .iter()
                .find(|&&p| tree.total_weight(p) == lone_weight)
            {
                return fixup_weight(child_id, tree).or_else(|| {
                    let total_weight = tree.total_weight(child_id);
                    let child = &tree.nodes[child_id];
                    Some(desired_weight - (total_weight - child.weight))
                });
            }
        }
    } else if !program.children.is_empty() {
        if let Some(value) = fixup_weight(program.children[0], tree) {
            return Some(value);
        }
    }
    None
}

pub fn part2(input_string: &str) -> Result<u32, String> {
    let tree = ProgramTree::parse(input_string)?;
    fixup_weight(tree.root_node, &tree).ok_or_else(|| "No solution found".to_string())
}

#[test]
fn test_part1() {
    assert_eq!(
        Ok("veboyvy".to_string()),
        part1(include_str!("day07_input.txt"))
    );
}

#[test]
fn test_part2() {
    assert_eq!(Ok(749), part2(include_str!("day07_input.txt")));
}