Struct TruncateUpdater 

pub struct TruncateUpdater {
    pub max_rank: Option<usize>,
    pub svd_policy: Option<SvdTruncationPolicy>,
}

Truncation updater for nsite=2 sweeps.

This updater performs SVD-based truncation on two-site subtrees, compressing bond dimensions while preserving the tensor train structure.

Algorithm

For each step with nodes [A, B] where B is the new center:

1. Contract tensors A and B into a single tensor AB
2. Factorize AB using SVD with truncation (left indices = A’s external + bond to A’s other neighbors)
3. The left tensor becomes the new A, the right tensor becomes the new B
4. B is the orthogonality center (isometry pointing towards B)

Usage

use tensor4all_core::{DynIndex, TensorDynLen};
use tensor4all_treetn::{apply_local_update_sweep, LocalUpdateSweepPlan, TreeTN, TruncateUpdater};

let s0 = DynIndex::new_dyn(2);
let bond = DynIndex::new_dyn(1);
let s1 = DynIndex::new_dyn(2);
let t0 = TensorDynLen::from_dense(vec![s0, bond.clone()], vec![1.0, 0.0])?;
let t1 = TensorDynLen::from_dense(vec![bond, s1], vec![1.0, 0.0])?;
let mut treetn = TreeTN::<TensorDynLen, usize>::from_tensors(vec![t0, t1], vec![0, 1])?;
treetn.canonicalize_mut(std::iter::once(0usize), Default::default())?;

let plan = LocalUpdateSweepPlan::from_treetn(&treetn, &0usize, 2).unwrap();
let mut updater = TruncateUpdater::new(
    Some(4),
    Some(tensor4all_core::SvdTruncationPolicy::new(1e-10)),
);
apply_local_update_sweep(&mut treetn, &plan, &mut updater)?;

assert_eq!(treetn.node_count(), 2);

Fields

max_rank: Option<usize>

Maximum bond dimension after truncation.

svd_policy: Option<SvdTruncationPolicy>

Explicit SVD truncation policy.

Implementations

impl TruncateUpdater

pub fn new( max_rank: Option<usize>, svd_policy: Option<SvdTruncationPolicy>, ) -> Self

Create a new truncation updater.

Arguments

• max_rank - Maximum bond dimension (None for no limit)
• svd_policy - SVD truncation policy override (None uses the global default)

Trait Implementations

impl Clone for TruncateUpdater

fn clone(&self) -> TruncateUpdater

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for TruncateUpdater

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T, V> LocalUpdater<T, V> for TruncateUpdater

where T: TensorLike, <T::Index as IndexLike>::Id: Clone + Hash + Eq + Ord + Debug + Send + Sync, V: Clone + Hash + Eq + Ord + Send + Sync + Debug,

fn update( &mut self, subtree: TreeTN<T, V>, step: &LocalUpdateStep<V>, _full_treetn: &TreeTN<T, V>, ) -> Result<TreeTN<T, V>>

Update a local subtree.

fn before_step( &mut self, _step: &LocalUpdateStep<V>, _full_treetn_before: &TreeTN<T, V>, ) -> Result<()>

Optional hook called before performing an update step.

fn after_step( &mut self, _step: &LocalUpdateStep<V>, _full_treetn_after: &TreeTN<T, V>, ) -> Result<()>

Optional hook called after an update step has been applied to the full TreeTN.