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//! Inversion (Descrete Math)
use crate::{
array_compression_unique_binary_search::ArrayCompression,
fenwick_tree_usize_add_1_indexed::Fenwick,
};
pub fn inversion_number<T: Ord + Clone>(a: &[T]) -> usize {
let a = ArrayCompression::once(a);
let mut fw = Fenwick::new(a.len());
let mut cnt = 0;
for x in a.into_iter().rev() {
cnt += fw.get(x);
fw.add(x, 1);
}
cnt as usize
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test() {
let a = vec![0, -1, 3, 5, 2];
assert_eq!(inversion_number(&a), 3);
}
}