manifold/common/sell-bet.ts

import { Bet } from "./bet";
import { Contract } from "./contract";
import { CREATOR_FEE, PLATFORM_FEE } from "./fees";
import { User } from "./user";

export const getSellBetInfo = (
  user: User,
  bet: Bet,
  contract: Contract,
  newBetId: string
) => {
  const { id: betId, amount, shares, outcome } = bet;

  const { YES: yesPool, NO: noPool } = contract.pool;
  const { YES: yesStart, NO: noStart } = contract.startPool;
  const { YES: yesShares, NO: noShares } = contract.totalShares;
  const { YES: yesBets, NO: noBets } = contract.totalBets;

  const [y, n, s] = [yesPool, noPool, shares];

  const shareValue =
    outcome === "YES"
      ? // https://www.wolframalpha.com/input/?i=b+%2B+%28b+n%5E2%29%2F%28y+%28-b+%2B+y%29%29+%3D+c+solve+b
        (n ** 2 +
          s * y +
          y ** 2 -
          Math.sqrt(
            n ** 4 + (s - y) ** 2 * y ** 2 + 2 * n ** 2 * y * (s + y)
          )) /
        (2 * y)
      : (y ** 2 +
          s * n +
          n ** 2 -
          Math.sqrt(
            y ** 4 + (s - n) ** 2 * n ** 2 + 2 * y ** 2 * n * (s + n)
          )) /
        (2 * n);

  const startPool = yesStart + noStart;
  const pool = yesPool + noPool - startPool;

  const probBefore = yesPool ** 2 / (yesPool ** 2 + noPool ** 2);

  const f = pool / (probBefore * yesShares + (1 - probBefore) * noShares);

  const myPool = outcome === "YES" ? yesPool - yesStart : noPool - noStart;

  const adjShareValue = Math.min(Math.min(1, f) * shareValue, myPool);

  const newPool =
    outcome === "YES"
      ? { YES: yesPool - adjShareValue, NO: noPool }
      : { YES: yesPool, NO: noPool - adjShareValue };

  const newTotalShares =
    outcome === "YES"
      ? { YES: yesShares - shares, NO: noShares }
      : { YES: yesShares, NO: noShares - shares };

  const newTotalBets =
    outcome === "YES"
      ? { YES: yesBets - amount, NO: noBets }
      : { YES: yesBets, NO: noBets - amount };

  const probAfter = newPool.YES ** 2 / (newPool.YES ** 2 + newPool.NO ** 2);

  const creatorFee = CREATOR_FEE * adjShareValue;
  const saleAmount = (1 - CREATOR_FEE - PLATFORM_FEE) * adjShareValue;

  console.log(
    "SELL M$",
    amount,
    outcome,
    "for M$",
    saleAmount,
    "M$/share:",
    f,
    "creator fee: M$",
    creatorFee
  );

  const newBet: Bet = {
    id: newBetId,
    userId: user.id,
    contractId: contract.id,
    amount: -adjShareValue,
    shares: -shares,
    outcome,
    probBefore,
    probAfter,
    createdTime: Date.now(),
    sale: {
      amount: saleAmount,
      betId,
    },
  };

  const newBalance = user.balance + saleAmount;

  return {
    newBet,
    newPool,
    newTotalShares,
    newTotalBets,
    newBalance,
    creatorFee,
  };
};