figcraft/crates/render/src/boolean.rs

//! Boolean group results for drawing, cached so editing one operand stays cheap.
//!
//! A boolean group's result is its operands folded bottom to top, each combined with the result
//! of those below it by its own operation (`Node::bool_op`, else the group's). Every prefix of the
//! fold is cached under a rolling hash of the operands it covers, so when an operand changes (a
//! drag, a point edit) the fold restarts from the longest unchanged prefix: the operands below the
//! edited one are never recombined, and groups whose operands didn't change aren't recomputed at
//! all. The caches drop their least recently used half when full, so a long drag doesn't evict
//! every other group's result.

use std::collections::HashMap;
use std::hash::{DefaultHasher, Hash, Hasher};
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::{Arc, Mutex, OnceLock};

use figcraft_doc::BoolOp;
use figcraft_geom::BezPath;
use figcraft_pathops::{FoldBudget, PathOpsError};

use crate::draw::hash_path;

/// A bounded map of computed paths; the least recently used half goes when it is full.
pub(crate) struct PathCache {
    map: HashMap<u64, (Arc<BezPath>, u64)>,
    tick: u64,
    cap: usize,
}

impl PathCache {
    pub(crate) fn new(cap: usize) -> Self {
        Self { map: HashMap::new(), tick: 0, cap: cap.max(2) }
    }

    pub(crate) fn get(&mut self, key: u64) -> Option<Arc<BezPath>> {
        self.tick += 1;
        let tick = self.tick;
        self.map.get_mut(&key).map(|(p, used)| {
            *used = tick;
            p.clone()
        })
    }

    pub(crate) fn insert(&mut self, key: u64, path: Arc<BezPath>) {
        if self.map.len() >= self.cap {
            let mut ticks: Vec<u64> = self.map.values().map(|(_, t)| *t).collect();
            // Keep the newer half: drop every entry used at or before the median tick.
            let mid = (ticks.len() / 2).saturating_sub(1);
            let (_, cut, _) = ticks.select_nth_unstable(mid);
            let cut = *cut;
            self.map.retain(|_, (_, t)| *t > cut);
        }
        self.tick += 1;
        self.map.insert(key, (path, self.tick));
    }
}

/// The shared cache of a cache-returning `OnceLock`, recovering from a poisoned lock.
pub(crate) fn locked(c: &Mutex<PathCache>) -> std::sync::MutexGuard<'_, PathCache> {
    c.lock().unwrap_or_else(std::sync::PoisonError::into_inner)
}

fn cache() -> &'static Mutex<PathCache> {
    static C: OnceLock<Mutex<PathCache>> = OnceLock::new();
    C.get_or_init(|| Mutex::new(PathCache::new(4096)))
}

thread_local! {
    static STEPS: std::cell::Cell<u64> = const { std::cell::Cell::new(0) };
}

/// How many boolean steps (one operand combined with the result below it) this thread has
/// computed: what a cache miss costs. For tests and benchmarks.
pub fn boolean_steps() -> u64 {
    STEPS.with(std::cell::Cell::get)
}

/// The folds refused as too complex to combine, so a refused group costs nothing the next time it
/// is drawn (an empty path marks each).
fn refused() -> &'static Mutex<PathCache> {
    static C: OnceLock<Mutex<PathCache>> = OnceLock::new();
    C.get_or_init(|| Mutex::new(PathCache::new(256)))
}

static WARNED: AtomicBool = AtomicBool::new(false);

/// The result of folding `operands` (bottom to top, each with its operation; the bottom one's is
/// unused), reusing the longest cached prefix. Invalid geometry gives an empty path. `None`: the
/// fold is past the path operations' caps ([`PathOpsError::TooComplex`]), so the group draws
/// without its result, as layers past the render's memory budget are left out (with a warning,
/// once).
pub fn fold_cached(operands: &[(BezPath, BoolOp)]) -> Option<BezPath> {
    let keys = fold_keys(operands);
    let Some(&last) = keys.last() else { return Some(BezPath::new()) };
    if locked(refused()).get(last).is_some() {
        return None;
    }
    let folded =
        if operands.len() < 2 { figcraft_pathops::boolean_ops(operands.iter().map(|(p, op)| (p, *op))) } else { fold_from_cache(operands, &keys) };
    match folded {
        Ok(p) => Some(p),
        Err(PathOpsError::TooComplex) => {
            if !WARNED.swap(true, Ordering::Relaxed) {
                log::warn!("render: a boolean group's result is left out: {}", PathOpsError::TooComplex);
            }
            locked(refused()).insert(last, Arc::default());
            None
        }
        Err(PathOpsError::BadInput) => Some(BezPath::new()),
    }
}

/// `keys[i]`: the cache key of the fold of operands `0..=i`.
fn fold_keys(operands: &[(BezPath, BoolOp)]) -> Vec<u64> {
    let mut h = DefaultHasher::new();
    operands
        .iter()
        .enumerate()
        .map(|(i, (p, op))| {
            if i > 0 {
                (*op as u8).hash(&mut h);
            }
            hash_path(p, &mut h);
            9u8.hash(&mut h);
            h.finish()
        })
        .collect()
}

/// The fold of two or more `operands` from the longest cached prefix (`keys`: [`fold_keys`]); the
/// steps it computes count against one [`FoldBudget`].
fn fold_from_cache(operands: &[(BezPath, BoolOp)], keys: &[u64]) -> Result<BezPath, PathOpsError> {
    let hit = {
        let mut c = locked(cache());
        keys.iter().enumerate().skip(1).rev().find_map(|(i, k)| c.get(*k).map(|p| (i, p)))
    };
    let (start, mut acc) = match hit {
        Some((i, p)) if i + 1 == operands.len() => return Ok((*p).clone()),
        Some((i, p)) => (i, (*p).clone()),
        None => match operands.first() {
            Some((p, _)) => (0, p.clone()),
            None => return Ok(BezPath::new()),
        },
    };
    let mut budget = FoldBudget::default();
    let mut fresh = Vec::new();
    for ((p, op), key) in operands.iter().zip(keys).skip(start + 1) {
        STEPS.with(|s| s.set(s.get() + 1));
        budget.spend(&acc, p)?;
        acc = figcraft_pathops::boolean_step(&acc, p, *op)?;
        fresh.push((*key, Arc::new(acc.clone())));
    }
    let mut c = locked(cache());
    for (k, p) in fresh {
        c.insert(k, p);
    }
    Ok(acc)
}