megashtein

megashtein

In their paper "Deep Squared Euclidean Approximation to the Levenshtein Distance for DNA Storage", Guo et al. explore techniques for using a neural network to embed sequences in such a way that the squared Euclidean distance between embeddings approximates the Levenshtein distance between the original sequences. This implementation also takes techniques from "Levenshtein Distance Embeddings with Poisson Regression for DNA Storage" by Wei et al. (2023).

This is valuable because there are excellent libraries for doing fast GPU accelerated searches for the K nearest neighbors of vectors, like faiss. Algorithms like HNSW allow us to do these searches in logarithmic time, where a brute force levenshtein distance based fuzzy search would need to run in exponential time.

This repo contains a PyTorch implementation of the core ideas from Guo's paper, adapted for ASCII sequences rather than DNA sequences. The implementation includes:

• A convolutional neural network architecture for sequence embedding
• Training using Poisson regression loss (PNLL) as described in the paper
• Synthetic data generation with controlled edit distance relationships
• Model saving and loading functionality

The trained model learns to embed ASCII strings such that the squared Euclidean distance between embeddings approximates the true Levenshtein distance between the strings.

Model Architecture

• Base Architecture: Convolutional Neural Network with embedding layer
• Input: ASCII sequences up to 80 characters (padded with null characters)
• Output: 80-dimensional dense embeddings
• Vocab Size: 128 (ASCII character set)
• Embedding Dimension: 140

The model uses a 5-layer CNN with average pooling followed by fully connected layers to produce fixed-size embeddings from variable-length ASCII sequences.

Usage

import torch
from models import EditDistanceModel

# Load the model
model = EditDistanceModel(embedding_dim=140)
model.load_state_dict(torch.load('megashtein_trained_model.pth'))
model.eval()

# Embed strings
def embed_string(text, max_length=80):
    # Pad and convert to tensor
    padded = (text + '\0' * max_length)[:max_length]
    indices = [min(ord(c), 127) for c in padded]
    tensor = torch.tensor(indices, dtype=torch.long).unsqueeze(0)

    with torch.no_grad():
        embedding = model(tensor)
    return embedding

# Example usage
text1 = "hello world"
text2 = "hello word"

emb1 = embed_string(text1)
emb2 = embed_string(text2)

# Compute approximate edit distance
approx_distance = torch.sum((emb1 - emb2) ** 2).item()
print(f"Approximate edit distance: {approx_distance}")

Training Details

The model is trained using:

• Loss Function: Poisson Negative Log-Likelihood (PNLL)
• Optimizer: AdamW with learning rate 0.000817
• Batch Size: 32
• Sequence Length: 80 characters (fixed)
• Synthetic Data: Pairs of ASCII strings with known Levenshtein distances

Use Cases

• Fuzzy String Search: Find similar strings in large text collections
• Text Clustering: Group similar texts based on edit distance
• Data Deduplication: Identify near-duplicate text entries
• Approximate String Matching: Fast similarity search with controllable accuracy

Limitations

• Fixed Length: Input sequences must be exactly 80 characters (padded/truncated)
• ASCII Only: Limited to ASCII character set (0-127)
• Approximation: Provides approximate rather than exact edit distances