duster/src/renderer/walking.rs

use crate::renderer::context_element::ContextElement;
use crate::renderer::context_element::IntoContextElement;
use crate::renderer::WalkError;
use std::borrow::Borrow;

enum WalkResult<'a> {
    NoWalk,
    PartialWalk,
    FullyWalked(&'a dyn IntoContextElement),
}

fn walk_path_from_single_level<'a, P, C>(context: &'a C, path: &[P]) -> WalkResult<'a>
where
    P: Borrow<str>,
    C: Borrow<dyn IntoContextElement + 'a>,
{
    if path.is_empty() {
        return WalkResult::FullyWalked(context.borrow());
    }

    let mut walk_failure = WalkResult::NoWalk;
    let mut output = context.borrow();
    for elem in path.iter() {
        match output.borrow().walk(elem.borrow()) {
            Err(WalkError::CantWalk { .. }) => {
                return walk_failure;
            }
            Ok(new_val) => {
                walk_failure = WalkResult::PartialWalk;
                output = new_val;
            }
        }
    }

    WalkResult::FullyWalked(output)
}

pub fn get_first_non_pseudo_element<'a, B>(breadcrumbs: &'a Vec<B>) -> Option<&B>
where
    B: Borrow<dyn IntoContextElement + 'a>,
{
    breadcrumbs
        .iter()
        .rev()
        .filter(|b| !(*b).borrow().is_pseudo_element())
        .next()
}

pub fn walk_path<'a, B, P>(
    breadcrumbs: &'a Vec<B>,
    path: &Vec<P>,
) -> Result<&'a dyn IntoContextElement, WalkError>
where
    B: Borrow<dyn IntoContextElement + 'a>,
    P: Borrow<str>,
{
    if breadcrumbs.is_empty() {
        // This happens when you use a section with an explicit
        // context path, where both the path and the explicit context
        // path fail, leaving you with absolutely no context.
        return Err(WalkError::CantWalk);
    }
    if path.is_empty() {
        return Ok(breadcrumbs
            .last()
            .expect("Breadcrumbs should never be empty.")
            .borrow());
    }
    if path
        .first()
        .expect("Already proved path is not empty")
        .borrow()
        == "."
    {
        let first_non_pseudo_element = get_first_non_pseudo_element(breadcrumbs);
        return match first_non_pseudo_element {
            None => Err(WalkError::CantWalk),
            Some(current_context) => {
                match walk_path_from_single_level(current_context, &path[1..]) {
                    // If no walking was done at all or we partially walked
                    // then stop trying to find anything because '.' restricts
                    // us to the current scope
                    WalkResult::NoWalk | WalkResult::PartialWalk => Err(WalkError::CantWalk),
                    WalkResult::FullyWalked(new_context) => Ok(new_context),
                }
            }
        };
    }
    for context in breadcrumbs.iter().rev() {
        match walk_path_from_single_level(context, path) {
            // If no walking was done at all, keep looping
            WalkResult::NoWalk => {}
            // If we partially walked then stop trying to find
            // anything
            WalkResult::PartialWalk => {
                return Err(WalkError::CantWalk);
            }
            WalkResult::FullyWalked(new_context) => return Ok(new_context),
        }
    }
    Err(WalkError::CantWalk)
}