// Copyright 2019. The Tari Project
//
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//! Data structure describing a fixed set of size _n_.

use alloc::vec::Vec;
use core::ops::Add;

/// Data structure for fixed set of size _n_.
#[derive(Clone, Debug)]
pub struct FixedSet<T> {
    items: Vec<Option<T>>,
}

impl<T: Clone + PartialEq + Default> FixedSet<T> {
    /// Creates a new fixed set of size n.
    pub fn new(n: usize) -> FixedSet<T> {
        FixedSet { items: vec![None; n] }
    }

    /// Returns the size of the fixed set, NOT the number of items that have been set.
    pub fn size(&self) -> usize {
        self.items.len()
    }

    /// Set the `index`-th item to `val`. Any existing item is overwritten. The set takes ownership of `val`.
    pub fn set_item(&mut self, index: usize, val: T) -> bool {
        if index >= self.items.len() {
            return false;
        }
        self.items[index] = Some(val);
        true
    }

    /// Return a reference to the `index`-th item, or `None` if that item has not been set yet.
    pub fn get_item(&self, index: usize) -> Option<&T> {
        match self.items.get(index) {
            None => None,
            Some(option) => option.as_ref(),
        }
    }

    /// Delete an item from the set by setting the `index`-th value to `None`.
    pub fn clear_item(&mut self, index: usize) {
        if index < self.items.len() {
            self.items[index] = None;
        }
    }

    /// Returns true if every item in the set has been set. An empty set returns true as well.
    pub fn is_full(&self) -> bool {
        self.items.iter().all(Option::is_some)
    }

    /// Return the first index of the given item in the set by performing a linear search through the set.
    pub fn search(&self, val: &T) -> Option<usize> {
        self.items
            .iter()
            .position(|item| item.as_ref().map(|i| i == val).unwrap_or(false))
    }

    /// Produces the sum of the values in the set, provided the set is full.
    pub fn sum(&self) -> Option<T>
    where for<'a> &'a T: Add<&'a T, Output = T> {
        // This function uses HTRB to work: See https://doc.rust-lang.org/nomicon/hrtb.html
        // or here https://users.rust-lang.org/t/lifetimes-for-type-constraint-where-one-reference-is-local/11087
        if self.size() == 0 {
            return Some(T::default());
        }
        if !self.is_full() {
            return None;
        }
        let mut iter = self.items.iter().filter_map(Option::as_ref);
        // Take the first item
        // unwrap wont fail as we know there is a first item.
        let mut sum = iter.next().unwrap().clone();
        for v in iter {
            sum = &sum + v;
        }
        Some(sum)
    }

    /// Collects all non-empty elements of the set into a Vec instance.
    pub fn into_vec(self) -> Vec<T> {
        self.items.into_iter().flatten().collect()
    }

    /// Returns an iterator that yields exactly `n` elements of the FixedSet. An element may be not be set which yields
    /// a `None`.
    pub fn iter(&self) -> impl Iterator<Item = Option<&T>> + '_ {
        self.items.iter().map(|e| e.as_ref())
    }
}

//----------------------------------------------         Tests         ----------------------------------------------//

#[cfg(test)]
mod test {
    use alloc::string::String;

    use super::{FixedSet, *};

    #[derive(Eq, PartialEq, Clone, Debug, Default)]
    struct Foo {
        baz: String,
    }

    #[test]
    fn zero_sized_fixed_set() {
        let mut s = FixedSet::<usize>::new(0);
        assert!(s.is_full(), "Set should be full");
        assert!(!s.set_item(1, 1), "Should not be able to set item");
        assert_eq!(s.get_item(0), None, "Should not return a value");
        assert_eq!(s.sum(), Some(0));
    }

    fn data(s: &str) -> Foo {
        match s {
            "patrician" => Foo {
                baz: "The Patrician".into(),
            },
            "rincewind" => Foo {
                baz: "Rincewind".into(),
            },
            "vimes" => Foo {
                baz: "Commander Vimes".into(),
            },
            "librarian" => Foo {
                baz: "The Librarian".into(),
            },
            "carrot" => Foo {
                baz: "Captain Carrot".into(),
            },
            _ => Foo { baz: "None".into() },
        }
    }

    #[test]
    fn small_set() {
        let mut s = FixedSet::<Foo>::new(3);
        // Set is empty
        assert!(!s.is_full());
        // Add an item
        assert!(s.set_item(1, data("patrician")));
        assert!(!s.is_full());
        // Add an item
        assert!(s.set_item(0, data("vimes")));
        assert!(!s.is_full());
        // Replace an item
        assert!(s.set_item(1, data("rincewind")));
        assert!(!s.is_full());
        // Add item, filling set
        assert!(s.set_item(2, data("carrot")));
        assert!(s.is_full());
        // Try add an invalid item
        assert!(!s.set_item(3, data("librarian")));
        assert!(s.is_full());
        // Clear an item
        s.clear_item(1);
        assert!(!s.is_full());
        // Check contents
        assert_eq!(s.get_item(0).unwrap().baz, "Commander Vimes");
        assert!(s.get_item(1).is_none());
        assert_eq!(s.get_item(2).unwrap().baz, "Captain Carrot");
        // Size is 3
        assert_eq!(s.size(), 3);
        // Slow search
        assert_eq!(s.search(&data("carrot")), Some(2));
        assert_eq!(s.search(&data("vimes")), Some(0));
        assert_eq!(s.search(&data("librarian")), None);
        assert_eq!(s.search(&data("random")), None);
    }

    #[test]
    fn sum_values() {
        let mut s = FixedSet::<usize>::new(4);
        s.set_item(0, 5);
        assert_eq!(s.sum(), None);
        s.set_item(1, 4);
        assert_eq!(s.sum(), None);
        s.set_item(2, 3);
        assert_eq!(s.sum(), None);
        s.set_item(3, 2);
        assert_eq!(s.sum(), Some(14));
        s.set_item(1, 0);
        assert_eq!(s.sum(), Some(10));
    }

    #[test]
    fn iterator() {
        let mut s = FixedSet::<usize>::new(5);
        s.set_item(0, 3);
        s.set_item(3, 2);
        s.set_item(1, 1);
        let elems = s.iter().collect::<Vec<_>>();
        assert_eq!(elems, vec![Some(&3), Some(&1), None, Some(&2), None]);
    }

    #[test]
    fn into_vec() {
        let mut s = FixedSet::<usize>::new(5);
        s.set_item(1, 5);
        s.set_item(3, 3);
        assert_eq!(s.into_vec(), vec![5, 3]);
    }
}