Fix payout calculation for correct bet in bet panel.

web/components/bet-panel.tsx

@@ -18,7 +18,7 @@ import {
   getProbability,
   calculateShares,
   getProbabilityAfterBet,
-  calculatePayout,
+  calculatePayoutAfterCorrectBet,
 } from '../lib/calculate'
 import { firebaseLogin } from '../lib/firebase/users'
 import { OutcomeLabel } from './outcome-label'
@@ -179,11 +179,13 @@ export function BetPanel(props: { contract: Contract; className?: string }) {
         </div>
         <div>
           {formatMoney(
-            calculatePayout(
-              contract,
-              { outcome: betChoice, amount: betAmount ?? 0, shares } as Bet,
-              betChoice
-            )
+            betAmount
+              ? calculatePayoutAfterCorrectBet(contract, {
+                  outcome: betChoice,
+                  amount: betAmount,
+                  shares,
+                } as Bet)
+              : 0
           )}
         </div>
       </AdvancedPanel>

web/lib/calculate.ts

@@ -62,6 +62,25 @@ export function calculatePayout(
   return (1 - fees) * (amount + ((shares - amount) / total) * winningsPool)
 }
 
+export function calculatePayoutAfterCorrectBet(contract: Contract, bet: Bet) {
+  const { amount, outcome, shares } = bet
+  const { totalShares, totalBets } = contract
+
+  const startPool = contract.startPool.YES + contract.startPool.NO
+  const truePool = amount + contract.pool.YES + contract.pool.NO - startPool
+
+  const totalBetsOutcome = totalBets[outcome] + amount
+  const totalSharesOutcome = totalShares[outcome] + shares
+
+  if (totalBetsOutcome >= truePool)
+    return (amount / totalBetsOutcome) * truePool
+
+  const total = totalSharesOutcome - totalBetsOutcome
+  const winningsPool = truePool - totalBetsOutcome
+
+  return (1 - fees) * (amount + ((shares - amount) / total) * winningsPool)
+}
+
 function calculateMktPayout(contract: Contract, bet: Bet) {
   const p =
     contract.pool.YES ** 2 / (contract.pool.YES ** 2 + contract.pool.NO ** 2)