---
license: mit
tags:
- pytorch
- safetensors
- threshold-logic
- neuromorphic
- modular-arithmetic
---

# threshold-mod3

Computes Hamming weight mod 3 for 8-bit inputs. Multi-layer network using thermometer encoding.

## Circuit

```
  x₀ x₁ x₂ x₃ x₄ x₅ x₆ x₇
   │  │  │  │  │  │  │  │
   └──┴──┴──┴──┼──┴──┴──┴──┘
               ▼
       ┌─────────────┐
       │ Thermometer │  Layer 1: 9 neurons
       │  Encoding   │  Fires when HW ≥ k
       └─────────────┘
               │
               ▼
       ┌─────────────┐
       │  MOD-3      │  Layer 2: 2 neurons
       │  Detection  │  Weight pattern (1,1,-2)
       └─────────────┘
               │
               ▼
       ┌─────────────┐
       │  Classify   │  Output: 3 classes
       └─────────────┘
               │
               ▼
          {0, 1, 2}
```

## Algebraic Insight

The weight pattern `(1, 1, -2)` causes cumulative sums to cycle mod 3:

```
HW=0: sum=0 → 0 mod 3
HW=1: sum=1 → 1 mod 3
HW=2: sum=2 → 2 mod 3
HW=3: sum=0 → 0 mod 3  (reset: 1+1-2=0)
HW=4: sum=1 → 1 mod 3
...
```

The key: `1 + 1 + (1-3) = 0`. Every 3 increments, the sum resets.

This generalizes to MOD-m: use `(1, 1, ..., 1, 1-m)` with m-1 ones.

## Architecture

| Layer | Neurons | Function |
|-------|---------|----------|
| Input | 8 | Binary bits |
| Hidden 1 | 9 | Thermometer: fires when HW ≥ k |
| Hidden 2 | 2 | MOD-3 detection |
| Output | 3 | One-hot classification |

**Total: 14 neurons, 110 parameters**

## Output Distribution

| Class | HW values | Count/256 |
|-------|-----------|-----------|
| 0 | 0, 3, 6 | 85 |
| 1 | 1, 4, 7 | 86 |
| 2 | 2, 5, 8 | 85 |

## Usage

```python
from safetensors.torch import load_file
import torch

w = load_file('model.safetensors')

def forward(x):
    x = x.float()
    x = (x @ w['layer1.weight'].T + w['layer1.bias'] >= 0).float()
    x = (x @ w['layer2.weight'].T + w['layer2.bias'] >= 0).float()
    out = x @ w['output.weight'].T + w['output.bias']
    return out.argmax(dim=-1)

inp = torch.tensor([[1,0,1,1,0,0,1,0]])  # HW=4
print(forward(inp).item())  # 1 (4 mod 3 = 1)
```

## Files

```
threshold-mod3/
├── model.safetensors
├── model.py
├── config.json
└── README.md
```

## License

MIT