Function partial_contract 

pub fn partial_contract<V>(
    a: &TreeTN<TensorDynLen, V>,
    b: &TreeTN<TensorDynLen, V>,
    spec: &PartialContractionSpec<DynIndex>,
    center: &V,
    options: ContractionOptions,
) -> Result<TreeTN<TensorDynLen, V>>
where
    V: Clone + Hash + Eq + Send + Sync + Debug + Ord,
    <DynIndex as IndexLike>::Id: Clone + Hash + Eq + Ord + Debug + Send + Sync,

Partially contract two TreeTNs according to the given specification.

Arguments

• a - First tensor network
• b - Second tensor network
• spec - Which site indices to contract versus link through diagonal structure
• center - Canonical center node for the result
• options - Contraction algorithm options

Index handling

• contract_pairs: Both indices are traced over (inner product). Neither appears in the result.
• diagonal_pairs: The two indices are linked through explicit diagonal structure so that only matching values contribute, while the left-hand site index remains in the result.
• Unmentioned indices: Pass through unchanged as external legs.

Examples

use tensor4all_core::{DynIndex, TensorDynLen};
use tensor4all_treetn::{
    contraction::ContractionOptions,
    partial_contract,
    PartialContractionSpec,
    TreeTN,
};

let idx_a = DynIndex::new_dyn(2);
let idx_b = DynIndex::new_dyn(2);
let a = TreeTN::<TensorDynLen, usize>::from_tensors(
    vec![TensorDynLen::from_dense(vec![idx_a.clone()], vec![1.0, 2.0]).unwrap()],
    vec![0],
).unwrap();
let b = TreeTN::<TensorDynLen, usize>::from_tensors(
    vec![TensorDynLen::from_dense(vec![idx_b.clone()], vec![3.0, 4.0]).unwrap()],
    vec![0],
).unwrap();

let spec = PartialContractionSpec {
    contract_pairs: vec![(idx_a.clone(), idx_b.clone())],
    diagonal_pairs: vec![],
    output_order: None,
};

let result = partial_contract(&a, &b, &spec, &0usize, ContractionOptions::default()).unwrap();
assert_eq!(result.node_count(), 1);