Constant zebra_chain::transaction::zip317::MIN_MEMPOOL_TX_FEE_RATE

pub const MIN_MEMPOOL_TX_FEE_RATE: usize = 100;

The minimum fee per kilobyte for Zebra mempool transactions. Also used as the minimum fee for a mempool transaction.

Based on DEFAULT_MIN_RELAY_TX_FEE in zcashd: https://github.com/zcash/zcash/blob/f512291ff20098291442e83713de89bcddc07546/src/main.h#L71-L72

This is a usize to simplify transaction size-based calculation code.