fix: refactor two commands in saving grade to prevent race conditions

src/modules/cards/infrastructure/cards.repository.ts

@@ -1,4 +1,5 @@
-import { Injectable, InternalServerErrorException, Logger } from '@nestjs/common'
+import { Injectable, InternalServerErrorException, Logger, NotFoundException } from '@nestjs/common'
+import { pick } from 'remeda'
 
 import {
   createPrismaOrderBy,
@@ -224,4 +225,73 @@ export class CardsRepository {
       throw new InternalServerErrorException(e?.message)
     }
   }
+
+  private async getSmartRandomCard(cards: Array<CardWithGrade>): Promise<CardWithGrade> {
+    const selectionPool: Array<CardWithGrade> = []
+
+    cards.forEach(card => {
+      // Calculate the average grade for the card
+      const averageGrade =
+        card.grades.length === 0
+          ? 0
+          : card.grades.reduce((acc, grade) => acc + grade.grade, 0) / card.grades.length
+      // Calculate weight for the card, higher weight for lower grade card
+      const weight = 6 - averageGrade
+
+      // Add the card to the selection pool `weight` times
+      for (let i = 0; i < weight; i++) {
+        selectionPool.push(card)
+      }
+    })
+
+    return selectionPool[Math.floor(Math.random() * selectionPool.length)]
+  }
+
+  private async getNotDuplicateRandomCard(
+    cards: Array<CardWithGrade>,
+    previousCardId: string
+  ): Promise<CardWithGrade> {
+    const randomCard = await this.getSmartRandomCard(cards)
+
+    if (!randomCard) {
+      this.logger.error(`No cards found in deck}`, {
+        previousCardId,
+        randomCard,
+        cards,
+      })
+      throw new NotFoundException(`No cards found in deck`)
+    }
+    if (randomCard.id === previousCardId && cards.length !== 1) {
+      return this.getNotDuplicateRandomCard(cards, previousCardId)
+    }
+
+    return randomCard
+  }
+
+  async getRandomCardInDeck(deckId: string, userId: string, previousCardId: string) {
+    const cards = await this.findCardsByDeckIdWithGrade(userId, deckId)
+
+    if (!cards.length) {
+      throw new NotFoundException(`No cards found in deck with id ${deckId}`)
+    }
+
+    const smartRandomCard = await this.getNotDuplicateRandomCard(cards, previousCardId)
+
+    return {
+      ...pick(smartRandomCard, [
+        'id',
+        'question',
+        'answer',
+        'deckId',
+        'questionImg',
+        'answerImg',
+        'questionVideo',
+        'answerVideo',
+        'created',
+        'updated',
+        'shots',
+      ]),
+      grade: smartRandomCard.grades[0]?.grade || 0,
+    }
+  }
 }