phanerozoic / threshold-halfsubtractor

threshold-halfsubtractor

Subtracts two 1-bit inputs, producing difference and borrow. The subtraction counterpart to the half adder.

Circuit

      a       b                    a   b
      │       │                    │   │
      ├───┬───┤                    └─┬─┘
      │   │   │                      │
      ▼   │   ▼                      ▼
  ┌──────┐│┌──────┐              ┌───────┐
  │  OR  │││ NAND │              │ ¬a ∧ b│
  │w:1,1 │││w:-1,-1│             │w:-1,1 │
  │b: -1 │││b: +1 │              │b: -1  │
  └──────┘│└──────┘              └───────┘
      │   │   │                      │
      └───┼───┘                      │
          ▼                          │
      ┌──────┐                       │
      │ AND  │                       │
      │w: 1,1│                       │
      │b: -2 │                       │
      └──────┘                       │
          │                          │
          ▼                          ▼
        diff                      borrow

The diff output uses XOR (2 layers). The borrow output detects when we subtract 1 from 0.

Truth Table

Binary: a - b = diff - (borrow × 2)

Mechanism

Diff (XOR): Same as the half adder's sum. The difference bit is 1 when exactly one input is 1.

Borrow: Uses weights [-1, +1] with bias -1. This fires only when a=0 and b=1:

• a=0, b=0: -0 + 0 - 1 = -1 < 0 → no borrow
• a=0, b=1: -0 + 1 - 1 = 0 ≥ 0 → borrow
• a=1, b=0: -1 + 0 - 1 = -2 < 0 → no borrow
• a=1, b=1: -1 + 1 - 1 = -1 < 0 → no borrow

The borrow is NOT(a) AND b - we borrow when subtracting 1 from 0.

Components

Total: 4 neurons, 12 parameters

Contrast with Half Adder

The only difference: borrow has an inhibitory weight on a.

Usage

from safetensors.torch import load_file
import torch

w = load_file('model.safetensors')

def half_subtractor(a, b):
    inp = torch.tensor([float(a), float(b)])

    # XOR for diff
    l1 = (inp @ w['xor.layer1.weight'].T + w['xor.layer1.bias'] >= 0).float()
    diff = int((l1 @ w['xor.layer2.weight'].T + w['xor.layer2.bias'] >= 0).item())

    # Borrow
    borrow = int((inp @ w['borrow.weight'].T + w['borrow.bias'] >= 0).item())

    return diff, borrow

Files

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

License

MIT