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

# threshold-exactly4outof8

Exactly-4-out-of-8 detector. Fires when precisely half the inputs are active. The tie detector.

## Circuit

```
  x₀ x₁ x₂ x₃ x₄ x₅ x₆ x₇
   │  │  │  │  │  │  │  │
   └──┴──┴──┴──┼──┴──┴──┴──┘
               │
       ┌───────┴───────┐
       ▼               ▼
  ┌─────────┐     ┌─────────┐
  │  ≥ 4    │     │  ≤ 4    │
  │ b = -4  │     │ b = +4  │
  └─────────┘     └─────────┘
       │               │
       └───────┬───────┘
               ▼
          ┌─────────┐
          │   AND   │
          └─────────┘
               │
               ▼
            tie?
```

## The Center of the Distribution

HW = 4 is special:

- **Maximum entropy**: C(8,4) = 70 is the largest binomial coefficient for n=8
- **Perfect balance**: 4 ones, 4 zeros
- **Self-complementary**: Flipping all bits maps HW=4 to itself (though not necessarily the same pattern)

This circuit fires on 70 of 256 inputs - the mode of the distribution.

## Neither Majority Nor Minority

| Circuit | Condition | Fires at HW=4? |
|---------|-----------|----------------|
| Majority | HW ≥ 5 | No |
| Minority | HW ≤ 3 | No |
| **Exactly4** | HW = 4 | **Yes** |

Exactly4 catches what both Majority and Minority miss. It's the gap between them - the deadlock.

## Threshold Arithmetic

**AtLeast4**: Sum all inputs with weight +1, subtract 4.
```
x₀ + x₁ + x₂ + x₃ + x₄ + x₅ + x₆ + x₇ - 4 ≥ 0
```
Fires when 4 or more inputs are active.

**AtMost4**: Sum all inputs with weight -1, add 4.
```
-x₀ - x₁ - x₂ - x₃ - x₄ - x₅ - x₆ - x₇ + 4 ≥ 0
```
Fires when 4 or fewer inputs are active.

Their intersection is exactly 4.

## The Binomial Peak

| HW | C(8,k) | Exactly4? |
|----|--------|-----------|
| 0 | 1 | - |
| 1 | 8 | - |
| 2 | 28 | - |
| 3 | 56 | - |
| **4** | **70** | **YES** |
| 5 | 56 | - |
| 6 | 28 | - |
| 7 | 8 | - |
| 8 | 1 | - |

The distribution is symmetric around 4. This circuit sits at the apex.

## Parameters

| Component | Weights | Bias |
|-----------|---------|------|
| AtLeast4 | all +1 | -4 |
| AtMost4 | all -1 | +4 |
| AND | [+1, +1] | -2 |

**Total: 3 neurons, 21 parameters, 2 layers**

## Usage

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

w = load_file('model.safetensors')

def exactly4(bits):
    inp = torch.tensor([float(b) for b in bits])
    atleast = int((inp * w['atleast.weight']).sum() + w['atleast.bias'] >= 0)
    atmost = int((inp * w['atmost.weight']).sum() + w['atmost.bias'] >= 0)
    comb = torch.tensor([float(atleast), float(atmost)])
    return int((comb * w['and.weight']).sum() + w['and.bias'] >= 0)

# Balanced: alternating pattern
bits = [1, 0, 1, 0, 1, 0, 1, 0]
print(exactly4(bits))  # 1

# Majority (5) - not a tie
bits = [1, 1, 1, 0, 1, 0, 1, 0]
print(exactly4(bits))  # 0
```

## Files

```
threshold-exactly4outof8/
├── model.safetensors
├── model.py
├── config.json
└── README.md
```

## License

MIT