ads2_2022/code/rust/src/graphs/tarjan.rs

// ----------------------------------------------------------------
// IMPORTS
// ----------------------------------------------------------------

use std::fmt::Display;
use std::hash::Hash;
use std::collections::HashMap;

use crate::core::utils;
use crate::core::log::log_debug;
use crate::stacks::stack::Stack;
use crate::graphs::graph::Graph;

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// CONSTANTS
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#[derive(Clone, Copy, PartialEq)]
enum State {
    UNTOUCHED,
    PENDING,
    FINISHED,
}

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// METHOD Tarjan Algorithm
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


/// # Tarjan Algorithm #
/// Runs the Tarjan-Algorithm to compute the strongly connected components.
pub fn tarjan_algorithm<T>(gph: &Graph<T>, debug: bool) -> Vec<Vec<T>>
    where T: Eq + Hash + Clone + Copy + Display
{
    let mut ctx = Context::new(&gph, debug);

    for u in gph.nodes.iter() {
        tarjan_visit(gph, u, &mut ctx);
    }
    return ctx.components;
}

/// recursive depth-first search algorithm to compute components
fn tarjan_visit<T>(gph: &Graph<T>, v: &T, ctx: &mut Context<T>)
    where T: Eq + Hash + Clone + Copy + Display
{
    if !(ctx.get_state(v) == State::UNTOUCHED) {
        return;
    }

    ctx.max_index += 1;
    ctx.push(v);
    ctx.set_root(v, ctx.max_index);
    ctx.set_index(v, ctx.max_index);
    ctx.set_state(v, State::PENDING);

    // depth first search:
    for u in gph.successors(&v) {
        tarjan_visit(gph, &u, ctx);
        // remains relevant for v, provided u still in Stack:
        if ctx.stack.elements.contains(&u) {
            let root = utils::min(
                ctx.get_root(&u),
                ctx.get_root(v)
            );
            ctx.set_root(v, root);
        }
    }
    ctx.set_state(v, State::FINISHED);
    ctx.log_info(&v);

    if ctx.get_index(v) == ctx.get_root(v) {
        let mut component: Vec<T> = Vec::new();
        loop {
            let u = ctx.top();
            ctx.pop();
            component.push(u.clone());
            if u == *v { break; }
        }
        ctx.components.push(component);
    }
}

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// AUXILIARY context variables for algorithm
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#[derive(Clone, Copy)]
struct NodeInformation<T> {
    node: T,
    root: usize,
    index: usize,
    state: State,
}

struct Context<T> {
    stack: Stack<T>,
    max_index: usize,
    infos: HashMap<T, NodeInformation<T>>,
    components: Vec<Vec<T>>,
    debug: bool,
}

impl<T> Context<T>
    where T: Eq + Hash + Clone + Copy + Display
{
    fn new(gph: &Graph<T>, debug: bool) -> Context<T> {
        let mut infos = HashMap::<T, NodeInformation<T>>::new();
        for u in gph.nodes.iter() {
            infos.entry(u.clone()).or_insert(NodeInformation::<T>::new(&u));
        }
        return Context {
            stack: Stack::new(),
            max_index: 0,
            infos: infos,
            components: vec![],
            debug: debug,
        };
    }

    fn push(self: &mut Self, u: &T) {
        self.stack.push(u.clone());
    }

    fn top(self: &mut Self) -> T {
        return self.stack.top();
    }

    fn pop(self: &mut Self) -> T {
        return self.stack.pop();
    }

    fn update_infos(self: &mut Self, u: &T, info: NodeInformation<T>) {
        self.infos.insert(u.clone(), info);
    }

    fn set_state(self: &mut Self, u: &T, state: State) {
        let mut info = *self.infos.get(u).unwrap();
        info.state = state;
        self.update_infos(u, info);
    }

    fn set_root(self: &mut Self, u: &T, root: usize) {
        let mut info = *self.infos.get(u).unwrap();
        info.root = root;
        self.update_infos(u, info);
    }

    fn set_index(self: &mut Self, u: &T, index: usize) {
        let mut info = *self.infos.get(u).unwrap();
        info.index = index;
        self.update_infos(u, info);
    }

    fn get_info(self: &Self, u: &T) -> NodeInformation<T> {
        return *self.infos.get(u).unwrap();
    }

    fn get_state(self: &Self, u: &T) -> State {
        return self.get_info(u).state;
    }

    fn get_root(self: &Self, u: &T) -> usize {
        return self.get_info(u).root;
    }

    fn get_index(self: &Self, u: &T) -> usize {
        return self.get_info(u).index;
    }

    fn log_info(self: &Self, u: &T) {
        if !self.debug { return; }
        let info = self.get_info(&u);
        log_debug!("node, index, component: ({}, {}, {})", info.node, info.index, info.root);
    }
}

impl<T> NodeInformation<T>
    where T: Clone + Copy
{
    fn new(node: &T) -> NodeInformation<T> {
        return NodeInformation {
            node: node.clone(),
            root: 0,
            index: 0,
            state: State::UNTOUCHED,
        };
    }
}