Bitcoin Transaction Fee Checker

This C++ tool parses a Bitcoin block in JSON format (from Blockstream Esplora API), calculates the total transaction fees and checks if the coinbase output is valid or not.

What it does

• Reads a JSON array of transactions from stdin.
• Calculates total input and output amounts for all transactions (except of course the coinbase).
• Calculates the total transaction fees with this simple formula:
  fee = sum(inputs) - sum(outputs)
• Ignore the coinbase transaction
• Parses the coinbase_output value (first transaction)
• Computes the maximum allowed reward :max_reward = block_reward + total_fees
• Verifies that the coinbase output does not exceed the sum of (block reward + total fees),
• so basically it checks if the checkd block is valid or not.

Assumptions

• Reward is hardcoded, supposed to be at july 2025 at 6.25 BTC (625000000 satoshi), (corresponding to halving #3 (starting from block height 630000)).
• JSON is in the format returned by Blockstream Esplora API.

Build (Cross Platform Linux/Windows)

```bash
mkdir build && cd build
cmake ..
make

How to run:

curl https://blockstream.info/api/block/000000007bc154e0fa7ea32218a72fe2c1bb9f86cf8c9ebf9a715ed27fdb229a/txs/0 | ./txfees Expected output: 0

Output

The program prints the total transaction fees (in satoshi) If the coinbase output exceeds the maximum allowed value, it prints a warning and exits with code 1

Exit Codes

Code	Meaning
0	Block is valid
1	Coinbase output exceeds allowed reward

Notes

The tool uses nlohmann/json for parsing, and is fully self-contained. json.hpp is downloaded from here: https://github.com/nlohmann/json/releases/download/v3.11.3/json.hpp and save it in: external/json.hpp

No external runtime dependencies.

Compatible with Linux and Windows.