digits-2023-10-15/digits-work-sample-go-bree-lapointe.d/analyzer.go

package analyzer

import (
	"fmt"
	"sort"

	"gonum.org/v1/gonum/stat"
)

// Analyzer processes Transactions and generates reports.
type Analyzer struct {
	// The list of Transactions available to analyze.
	transactions Transactions
}

// Add adds unique transactions to Analyzer.
func (anz *Analyzer) Add(transactions Transactions) int {
	added := 0
	for i := range transactions {
		dupe := false
		for j := range anz.transactions {
			if transactions[i].equals(anz.transactions[j]) {
				dupe = true
				break
			}
		}
		if !dupe {
			added += 1
			anz.transactions = append(anz.transactions, transactions[i])
		}
	}
	return added
}

// TransactionCount is the number of unique Transactions.
func (anz *Analyzer) TransactionCount() int {
	return len(anz.transactions)
}

// TransactionsAmount is the total amount of all the transactions.
func (anz *Analyzer) TransactionsAmount() Amount {
	return anz.transactions.Sum()
}

// LargestTransaction is the single largest transaction, ranked by
// the absolute value of the transaction amount. Returns an error if there
// are no transactions.
//
//   - Note: Large negative transactions are still large--it just means the
//     money moved the other direction.
func (anz *Analyzer) LargestTransaction() (Transaction, error) {
	if anz == nil || len(anz.transactions) == 0 {
		return Transaction{}, ErrNoTransactionsToAnalyze
	}
	largestIdx := 0
	for i := 1; i < len(anz.transactions); i++ {
		if anz.transactions[largestIdx].Amount.AbsoluteValue() < anz.transactions[i].Amount.AbsoluteValue() {
			largestIdx = i
		}
	}
	return anz.transactions[largestIdx], nil
}

// byCardholder groups the transactions by cardholder and returns a map of
// cardholder name to the list of transactions that are part of that cardholder.
func (anz *Analyzer) byCardholder() map[string]Transactions {
	result := map[string]Transactions{}
	for i := range anz.transactions {
		transactionsForCardholder := result[anz.transactions[i].stringifyCardholder()]
		transactionsForCardholder = append(transactionsForCardholder, anz.transactions[i])
		result[anz.transactions[i].stringifyCardholder()] = transactionsForCardholder
	}
	return result
}

// ByCategory groups the transactions by category and returns a map of
// category name to the list of transactions that are part of that category.
func (anz *Analyzer) ByCategory() map[string]Transactions {
	result := map[string]Transactions{}
	for i := range anz.transactions {
		transactionsInCategory := result[anz.transactions[i].Category]
		transactionsInCategory = append(transactionsInCategory, anz.transactions[i])
		result[anz.transactions[i].Category] = transactionsInCategory
	}
	return result
}

type cardholderAndSum struct {
	cardholder string
	sum        Amount
}
type cardholderAndSums []cardholderAndSum

func (cardholderAndSums cardholderAndSums) Len() int {
	return len(cardholderAndSums)
}

func (cardholderAndSums cardholderAndSums) Less(i, j int) bool {
	if cardholderAndSums[i].sum == cardholderAndSums[j].sum {
		return cardholderAndSums[i].cardholder > cardholderAndSums[j].cardholder
	}
	return cardholderAndSums[i].sum > cardholderAndSums[j].sum
}

func (cardholderAndSums cardholderAndSums) Swap(i, j int) {
	cardholderAndSums[i], cardholderAndSums[j] = cardholderAndSums[j], cardholderAndSums[i]
}

// BigSpenderReport generates our Big Spender Report.
//
// "Big spenders" are defined as those people who, in aggregate
// across their transactions, have spent more than 2 standard
// deviations above the mean spender.
//
// The method determines who they are and generates a well-formatted
// report to call them out.
func (anz *Analyzer) BigSpenderReport() string {
	if len(anz.transactions) == 0 {
		return anz.formatCardholderAmountsAsUSD("Digits Big Spenders Report", nil)
	}

	meanSpend, stddevSpend := anz.spendMeanStdDevPerCardholder()
	cardholderAndSums := make(cardholderAndSums, 0)
	for cardholder, transactionsForCardholder := range anz.byCardholder() {
		cardholderSpendSum := transactionsForCardholder.Sum()
		if cardholderSpendSum-2*stddevSpend > meanSpend {
			cardholderAndSums = append(cardholderAndSums, cardholderAndSum{cardholder: cardholder, sum: cardholderSpendSum})
		}
	}
	sort.Slice(cardholderAndSums, cardholderAndSums.Less)

	return anz.formatCardholderAmountsAsUSD("Digits Big Spenders Report", cardholderAndSums)
}

func (anz *Analyzer) spendMeanStdDevPerCardholder() (mean, stdddev Amount) {
	spendPerCardholder := anz.spendPerCardholder()
	meanf64, stdf64 := stat.MeanStdDev(spendPerCardholder, nil)
	return Amount(meanf64), Amount(stdf64)
}

func (anz *Analyzer) spendPerCardholder() []float64 {
	values := []float64{}
	for _, transactions := range anz.byCardholder() {
		values = append(values, float64(transactions.Sum()))
	}
	return values
}

func (anz *Analyzer) formatCardholderAmountsAsUSD(title string, cardholderAndSums []cardholderAndSum) string {
	report := fmt.Sprintf(`
%s
------------------------------------------------------
Name                          Amount
------------------------------------------------------`, title)
	for i := range cardholderAndSums {
		report = fmt.Sprintf("%s\n%-30s%s",
			report,
			cardholderAndSums[i].cardholder,
			cardholderAndSums[i].sum.FormatUSD(),
		)
	}
	return fmt.Sprintf("%s\n------------------------------------------------------", report)
}

// New creates a new Analyzer with an empty set of transactions
func New() *Analyzer {
	return &Analyzer{transactions: Transactions{}}
}