HASA — History-Aware Sampling Algorithm

A lightweight, pip-installable PyTorch library for robust deep learning training under noise.

HASA tracks each training sample's loss trajectory over a sliding window and uses loss variance as a noise indicator. Clean samples stabilise quickly (low variance); noisy/mislabelled samples oscillate (high variance). After a warm-up phase the algorithm masks out high-variance samples so that gradients are computed only from likely-clean data.

Installation

pip install hasa

Or install directly from GitHub:

pip install git+https://github.com/msc35/hasa-py.git

For development:

git clone https://github.com/msc35/hasa-py.git
cd hasa-py
pip install -e ".[dev]"

Quick Start

1. Wrap your dataset to return sample indices

HASA needs to map each loss value back to a specific dataset sample. The simplest approach is an IndexedDataset wrapper:

from torch.utils.data import Dataset

class IndexedDataset(Dataset):
    def __init__(self, dataset):
        self.dataset = dataset

    def __len__(self):
        return len(self.dataset)

    def __getitem__(self, idx):
        x, y = self.dataset[idx]
        return idx, x, y

2. Train with HASA selection

import torch
import torch.nn as nn
from torch.utils.data import DataLoader
from hasa import HASA

dataset = IndexedDataset(my_dataset)
loader = DataLoader(dataset, batch_size=128, shuffle=True)

model = MyModel()
optimizer = torch.optim.Adam(model.parameters(), lr=1e-3)
criterion = nn.CrossEntropyLoss(reduction='none')  # MUST be unreduced

selector = HASA(num_samples=len(dataset), window_size=15, select_ratio=0.8)

for epoch in range(150):
    for indices, x, y in loader:
        logits = model(x)
        losses = criterion(logits, y)

        mask = selector.step(indices, losses.detach())

        # Divide by mask.sum() (not batch_size) to preserve gradient magnitude
        loss = (losses * mask).sum() / mask.sum()
        loss.backward()
        optimizer.step()
        optimizer.zero_grad()

    selector.end_epoch()

3. (Optional) Langevin noise injection

Motivated by interpreting SGD as approximate Bayesian inference (Mandt, Hoffman & Blei, 2018):

selector = HASA(
    num_samples=len(dataset),
    window_size=15,
    select_ratio=0.8,
    langevin_noise=1e-4,
)

# Inside the training loop, after optimizer.step():
selector.inject_langevin_noise(model)

How It Works

1. Warm-up phase (first window_size epochs): all samples are used — the per-sample loss history buffer fills up.
2. Selection phase (after warm-up): for each batch, compute per-sample loss variance from the history buffer. Keep only the select_ratio fraction with the lowest variance. High-variance samples (likely mislabelled) are masked out.

              Warm-up (epochs 0..T-1)          Selection (epochs T+)
             ┌─────────────────────┐     ┌──────────────────────────────┐
  per-sample │  Record losses into │     │  Var(loss history) per sample│
  losses ───>│  ring buffer, train │────>│  Keep lowest-variance k%     │
             │  on ALL samples     │     │  Mask out the rest           │
             └─────────────────────┘     └──────────────────────────────┘

Hyperparameters

Checkpointing

HASA supports full state serialisation for resuming training:

state = selector.state_dict()
torch.save(state, "hasa_checkpoint.pt")

# Restore later
selector.load_state_dict(torch.load("hasa_checkpoint.pt"))

HASATrainer (convenience wrapper)

For a simpler API that handles the full training loop:

from hasa.callbacks import HASATrainer

trainer = HASATrainer(model, optimizer, criterion, selector, device="cuda")
for epoch in range(num_epochs):
    metrics = trainer.train_epoch(dataloader)
    print(f"Epoch {epoch}: loss={metrics['loss']:.4f}, selected={metrics['selected_frac']:.2%}")

Works with any PyTorch model

HASA is model-agnostic — it only looks at per-sample loss values. It works with any architecture (CNNs, Transformers, MLPs, etc.) and any loss function that supports reduction='none':

• nn.CrossEntropyLoss(reduction='none') — classification
• nn.MSELoss(reduction='none') — regression
• nn.BCEWithLogitsLoss(reduction='none') — binary classification
• Any custom loss returning per-sample values

API Reference

HASA(num_samples, window_size=15, select_ratio=0.8, langevin_noise=0.0, device="cpu")

• step(sample_indices, losses) -> BoolTensor — update history, return selection mask.
• end_epoch() — advance the epoch counter. Must be called once per epoch.
• inject_langevin_noise(model) — add Gaussian noise to parameters.
• state_dict() / load_state_dict(d) — checkpoint support.
• epoch — current epoch (read-only property).
• in_warmup — True during the warm-up phase.

LossHistoryBuffer(num_samples, window_size, device)

• update(indices, losses) — write losses into the ring buffer.
• variance(indices) -> Tensor — compute per-sample loss variance.
• is_ready(epoch) -> bool — True after warm-up.

hard_select(variances, select_ratio) -> BoolTensor

Returns a mask keeping the lowest select_ratio fraction of samples by variance.

Running Tests

pip install -e ".[dev]"
pytest --cov=hasa

License

MIT