transfers: introduce basic calculations for N-day transfer limits

Issue: moov-io#368

internal/transfers/limits.go

@@ -0,0 +1,105 @@
+// Copyright 2020 The Moov Authors
+// Use of this source code is governed by an Apache License
+// license that can be found in the LICENSE file.
+
+package transfers
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/moov-io/paygate/internal"
+)
+
+var (
+	sevenDayLimit = func() string {
+		if v := os.Getenv("TRANSFERS_SEVEN_DAY_SOFT_LIMIT"); v != "" {
+			return v
+		}
+		return "10000.00"
+	}()
+	thirtyDayLimit = func() string {
+		if v := os.Getenv("TRANSFERS_THIRTY_DAY_SOFT_LIMIT"); v != "" {
+			return v
+		}
+		return "25000.00"
+	}()
+)
+
+func ParseLimits(sevenDays, thirtyDays string) (*Limits, error) {
+	seven, err := internal.NewAmount("USD", sevenDays)
+	if err != nil {
+		return nil, err
+	}
+	thirty, err := internal.NewAmount("USD", thirtyDays)
+	if err != nil {
+		return nil, err
+	}
+	return &Limits{
+		PreviousSevenDays: seven,
+		PreviousThityDays: thirty,
+	}, nil
+}
+
+type Limits struct {
+	PreviousSevenDays *internal.Amount
+	PreviousThityDays *internal.Amount
+}
+
+func UnderLimits(existing []*internal.Transfer, pending *internal.Transfer, limits *Limits) error {
+	if len(existing) == 0 {
+		return nil
+	}
+	if err := previousSevenDaysUnderLimit(existing, pending, limits.PreviousSevenDays); err != nil {
+		return err
+	}
+	if err := previousThityDaysUnderLimit(existing, pending, limits.PreviousThityDays); err != nil {
+		return err
+	}
+	return nil
+}
+
+func previousSevenDaysUnderLimit(existing []*internal.Transfer, pending *internal.Transfer, limit *internal.Amount) error {
+	newerThan := time.Now().Add(-7 * 24 * time.Hour).Truncate(24 * time.Hour)
+	total, err := sumTransfers(existing, newerThan).Plus(pending.Amount)
+	if err != nil {
+		return fmt.Errorf("limits: total error: %v", err)
+	}
+
+	if total.Int() >= limit.Int() {
+		amt, err := internal.NewAmountFromInt("USD", total.Int()-limit.Int())
+		if err != nil {
+			return fmt.Errorf("limits: error=%v", err)
+		}
+		return fmt.Errorf("previous seven days would transfer %v over limit", amt)
+	}
+	return nil
+}
+
+func previousThityDaysUnderLimit(existing []*internal.Transfer, pending *internal.Transfer, limit *internal.Amount) error {
+	newerThan := time.Now().Add(-30 * 24 * time.Hour).Truncate(24 * time.Hour)
+	total, err := sumTransfers(existing, newerThan).Plus(pending.Amount)
+	if err != nil {
+		return fmt.Errorf("limits: total error: %v", err)
+	}
+
+	if total.Int() >= limit.Int() {
+		amt, err := internal.NewAmountFromInt("USD", total.Int()-limit.Int())
+		if err != nil {
+			return fmt.Errorf("limits: error=%v", err)
+		}
+		return fmt.Errorf("previous thirty days would transfer %v over limit", amt)
+	}
+	return nil
+}
+
+func sumTransfers(existing []*internal.Transfer, newerThan time.Time) *internal.Amount {
+	sum, _ := internal.NewAmount("USD", "0.00")
+	for i := range existing {
+		if existing[i].Created.After(newerThan) {
+			sum.Plus(existing[i].Amount)
+		}
+	}
+	return sum
+}