mempool/backend/src/api/fee-api.ts

import { MempoolBlock } from '../mempool.interfaces';
import { Common } from './common';
import mempool from './mempool';
import projectedBlocks from './mempool-blocks';

interface RecommendedFees {
  fastestFee: number,
  halfHourFee: number,
  hourFee: number,
  economyFee: number,
  minimumFee: number,
}

class FeeApi {
  constructor() { }

  defaultFee = Common.isLiquid() ? 0.1 : 1;
  minimumIncrement = Common.isLiquid() ? 0.1 : 1;

  public getRecommendedFee(): RecommendedFees {
    const pBlocks = projectedBlocks.getMempoolBlocks();
    const mPool = mempool.getMempoolInfo();
    const minimumFee = this.roundUpToNearest(mPool.mempoolminfee * 100000, this.minimumIncrement);
    const defaultMinFee = Math.max(minimumFee, this.defaultFee);

    if (!pBlocks.length) {
      return {
        'fastestFee': defaultMinFee,
        'halfHourFee': defaultMinFee,
        'hourFee': defaultMinFee,
        'economyFee': minimumFee,
        'minimumFee': minimumFee,
      };
    }

    const firstMedianFee = this.optimizeMedianFee(pBlocks[0], pBlocks[1]);
    const secondMedianFee = pBlocks[1] ? this.optimizeMedianFee(pBlocks[1], pBlocks[2], firstMedianFee) : this.defaultFee;
    const thirdMedianFee = pBlocks[2] ? this.optimizeMedianFee(pBlocks[2], pBlocks[3], secondMedianFee) : this.defaultFee;

    // explicitly enforce a minimum of ceil(mempoolminfee) on all recommendations.
    // simply rounding up recommended rates is insufficient, as the purging rate
    // can exceed the median rate of projected blocks in some extreme scenarios
    // (see https://bitcoin.stackexchange.com/a/120024)
    return {
      'fastestFee': Math.max(minimumFee, firstMedianFee),
      'halfHourFee': Math.max(minimumFee, secondMedianFee),
      'hourFee': Math.max(minimumFee, thirdMedianFee),
      'economyFee': Math.max(minimumFee, Math.min(2 * minimumFee, thirdMedianFee)),
      'minimumFee': minimumFee,
    };
  }

  private optimizeMedianFee(pBlock: MempoolBlock, nextBlock: MempoolBlock | undefined, previousFee?: number): number {
    const useFee = previousFee ? (pBlock.medianFee + previousFee) / 2 : pBlock.medianFee;
    if (pBlock.blockVSize <= 500000) {
      return this.defaultFee;
    }
    if (pBlock.blockVSize <= 950000 && !nextBlock) {
      const multiplier = (pBlock.blockVSize - 500000) / 500000;
      return Math.max(Math.round(useFee * multiplier), this.defaultFee);
    }
    return this.roundUpToNearest(useFee, this.minimumIncrement);
  }

  private roundUpToNearest(value: number, nearest: number): number {
    return Math.ceil(value / nearest) * nearest;
  }
}

export default new FeeApi();