flashcards-api/src/modules/decks/use-cases/get-random-card-in-deck-use-case.ts

import { ForbiddenException, Logger, NotFoundException } from '@nestjs/common'
import { CommandHandler, ICommandHandler } from '@nestjs/cqrs'
import { Prisma } from '@prisma/client'
import { pick } from 'remeda'

import { CardsRepository } from '../../cards/infrastructure/cards.repository'
import { DecksRepository } from '../infrastructure/decks.repository'

export class GetRandomCardInDeckCommand {
  constructor(
    public readonly userId: string,
    public readonly deckId: string,
    public readonly previousCardId: string
  ) {}
}

type CardWithGrade = Prisma.cardGetPayload<{ include: { grades: true } }>

@CommandHandler(GetRandomCardInDeckCommand)
export class GetRandomCardInDeckHandler implements ICommandHandler<GetRandomCardInDeckCommand> {
  logger = new Logger(GetRandomCardInDeckHandler.name)
  constructor(
    private readonly cardsRepository: CardsRepository,
    private readonly decksRepository: DecksRepository
  ) {}

  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 execute(command: GetRandomCardInDeckCommand) {
    const deck = await this.decksRepository.findDeckById(command.deckId)

    if (!deck) throw new NotFoundException(`Deck with id ${command.deckId} not found`)

    if (deck.userId !== command.userId && deck.isPrivate) {
      throw new ForbiddenException(`You can't get a private deck that you don't own`)
    }
    const cards = await this.cardsRepository.findCardsByDeckIdWithGrade(
      command.userId,
      command.deckId
    )

    if (!cards.length) {
      throw new NotFoundException(`No cards found in deck with id ${command.deckId}`)
    }

    const smartRandomCard = await this.getNotDuplicateRandomCard(cards, command.previousCardId)

    return {
      ...pick(smartRandomCard, [
        'id',
        'question',
        'answer',
        'deckId',
        'questionImg',
        'answerImg',
        'questionVideo',
        'answerVideo',
        'created',
        'updated',
        'shots',
      ]),
      grade: smartRandomCard.grades[0]?.grade || 0,
    }
  }
}