Mycelium/mycelium/src/hmap.rs

/* Mycelium Scheme
 * Copyright (C) 2025 Ava Affine
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

use alloc::slice;
use alloc::vec::Vec;
use alloc::vec;
use alloc::boxed::Box;
use alloc::string::String;

/* Use a prime number so that the modulus operation
 * provides better avalanche effect
 */
const INDEXED_BUCKETS: u8 = 199;

/* This only has to work to make quasi unique indexes from
 * variable names. Any given program will not have so many
 * symbols that this becomes a bottleneck in runtime.
 *
 * Priorities:
 *   - SPEED in embedded code
 *   - avalanche effect
 *
 * Not a priority: minimal collisions
 *
 * Just to make sure this is not misused I keep it private.
 * And yes, I am sure a B-Tree would be better.
 *
 * TODO: Make sure that the obvious timing attacks
 *     dont create risk for scheme crypto libraries...
 *     or more likely rip and replace with a better nostd hashmap
 */
#[inline]
fn string_hash(input: &String) -> u8 {
    input
        .chars()
        // each letter and number get a digit
        .map(|c| c.to_digit(36)
            // all else is 0
            .or_else(|| Some(0))
            .unwrap())
        // modulo reduction
        .reduce(|acc, i| (acc + i) % INDEXED_BUCKETS as u32)
        // TODO: some analysis on which cases end up here
        .or_else(|| Some(0))
        .unwrap() as u8
}

#[derive(Clone)]
pub struct Bucket<T: Clone>(Vec<(String, T)>);
#[derive(Clone)]
pub struct QuickMap<T: Clone>(Box<[Bucket<T>; INDEXED_BUCKETS as usize]>);

impl<'a, T: Clone> QuickMap<T> {
    const ARRAY_REPEAT_VALUE: Bucket<T> = Bucket(vec![]);

    pub fn new() -> QuickMap<T> {
        QuickMap(Box::new([QuickMap::ARRAY_REPEAT_VALUE; INDEXED_BUCKETS as usize]))
    }

    pub fn get(&self, arg: &String) -> Option<&T> {
        let idx = string_hash(&arg);
        for kv in self.0[idx as usize].0.iter() {
            if &kv.0 == arg {
                return Some(&kv.1);
            }
        }

        return None;
    }

    pub fn remove(&mut self, arg: &String) -> Option<T> {
        let idx = string_hash(&arg);
        let len = self.0[idx as usize].0.len();
        for i in 0..len {
            if &self
                .0[idx as usize]
                .0[i as usize]
                .0 == arg {
                    return Some(self.0[idx as usize].0.swap_remove(i).1);
            }
        }

        return None;
    }

    pub fn contains_key(&self, arg: &String) -> bool {
        let idx = string_hash(arg);
        for kv in self.0[idx as usize].0.iter() {
            if &kv.0 == arg {
                return true;
            }
        }

        return false;
    }

    pub fn insert(&mut self, k: String, v: T) -> Option<T> {
        let idx = string_hash(&k);
        for kv in self.0[idx as usize].0.iter_mut() {
            if kv.0 == k {
                let tmp = kv.1.clone();
                kv.1 = v;
                return Some(tmp);
            }
        }

        self.0[idx as usize].0.push((k, v));
        return None
    }

    pub fn iter(&'a self) -> QuickMapIter<'a, T> {
        QuickMapIter::<'a, T>{
            buckets: &self.0,
            bucket_cursor: 0,
            vec_iter: self.0[0].0.iter(),
        }
    }
}

#[derive(Clone)]
pub struct QuickMapIter<'a, T: Clone> {
    buckets: &'a [Bucket<T>; INDEXED_BUCKETS as usize],
    bucket_cursor: usize,
    vec_iter: slice::Iter<'a, (String, T)>,
}

impl<'a, T: Clone> Iterator for QuickMapIter<'a, T> {
    type Item = &'a (String, T);

    fn next(&mut self) -> Option<Self::Item> {
        self.vec_iter
            .next()
            .or_else(|| {
                self.bucket_cursor += 1;
                if self.bucket_cursor == INDEXED_BUCKETS as usize{
                    None
                } else {
                    self.vec_iter = self.buckets[self.bucket_cursor].0.iter();
                    self.next()
                }
            })
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn add_fetch_and_remove_simple() {
        let mut q = QuickMap::<u8>::new();
        let key = String::from("test");
        q.insert(String::from("test"), 1);
        assert_eq!(*q.get(&key).unwrap(), 1);
        assert!(q.contains_key(&key));
        assert_eq!(
            q.remove(&key),
            Some(1),
        );
        assert_eq!(q.contains_key(&key), false);
        assert_eq!(q.get(&key), None);
    }

    #[test]
    fn iter_test() {
        let mut q = QuickMap::<u8>::new();
        let k1 = String::from("test1");
        let k2 = String::from("test1@"); // will be in same bucket
        let k3 = String::from("test2");
        let k4 = String::from("test2--"); // will be in same bucket
        q.insert(k1.clone(), 1);
        q.insert(k2.clone(), 2);
        q.insert(k3.clone(), 3);
        q.insert(k4.clone(), 4);

        // test k1 and k2 in same bucket but that other keys are not
        assert_eq!(q.0[string_hash(&k1) as usize].0.len(), 2);
        // test k3 and k4 in same bucket but that other keys are not
        assert_eq!(q.0[string_hash(&k3) as usize].0.len(), 2);

        let mut i = q.iter();
        let entry1 = i.next().unwrap();
        let entry2 = i.next().unwrap();
        let entry3 = i.next().unwrap();
        let entry4 = i.next().unwrap();

        assert_eq!(i.next(), None);
        assert_eq!(entry1.0, k1);
        assert_eq!(entry1.1, 1);
        assert_eq!(entry2.0, k2);
        assert_eq!(entry2.1, 2);
        assert_eq!(entry3.0, k3);
        assert_eq!(entry3.1, 3);
        assert_eq!(entry4.0, k4);
        assert_eq!(entry4.1, 4);
    }
}