Reapply "feat(loyalty): EnakPoint & EnakCoin" (#32)

This reverts commit 4e24f9bbb0.

internal/processor/earning_calculator.go

@@ -0,0 +1,85 @@
+package processor
+
+import (
+	"math"
+
+	"apskel-pos-be/internal/entities"
+	"apskel-pos-be/internal/models"
+)
+
+// EarningLine is what an order earns in one currency, and the settings that produced
+// it. The settings are kept so the ledger row can freeze them: a later change of
+// setting must not change what an earning meant, and a reversal is computed with the
+// same numbers (docs/prd-point-coin.md F3).
+type EarningLine struct {
+	Amount   int64
+	Settings models.LoyaltyEarnSettings
+	// True when max_per_order cut the amount down.
+	Capped bool
+}
+
+// EarningResult is what an order earns.
+type EarningResult struct {
+	// subtotal − discount − the part paid with EnakPoint, in rupiah, never negative.
+	// Tax and anything else added on top of the subtotal are not part of it (Q1).
+	Basis float64
+	Point EarningLine
+	Coin  EarningLine
+}
+
+// Metadata is the snapshot stored on the currency's EARN ledger row.
+func (r EarningResult) Metadata(line EarningLine) entities.Metadata {
+	m := entities.Metadata{
+		"basis":            r.Basis,
+		"earn_per_amount":  line.Settings.EarnPerAmount,
+		"earn_value":       line.Settings.EarnValue,
+		"min_order_amount": line.Settings.MinOrderAmount,
+		"capped":           line.Capped,
+	}
+	if line.Settings.MaxPerOrder != nil {
+		m["max_per_order"] = *line.Settings.MaxPerOrder
+	}
+	return m
+}
+
+// CalculateEarning applies the earning formula of docs/prd-point-coin.md F1:
+//
+//	basis  = subtotal − discount_amount − paid with EnakPoint
+//	amount = 0                                  if basis < min_order_amount
+//	amount = floor(basis / earn_per_amount) × earn_value
+//	amount = min(amount, max_per_order)         if max_per_order is set
+//
+// The part paid with EnakPoint earns nothing (Q10). Money is handled in whole cents so
+// floor never lands one short on a value like 87500.00 that float64 cannot hold
+// exactly. It has no side effects.
+func CalculateEarning(order *entities.Order, pointPaidAmount float64, settings models.OutletLoyaltySettings) EarningResult {
+	basisCents := toCents(order.Subtotal) - toCents(order.DiscountAmount) - toCents(pointPaidAmount)
+	if basisCents < 0 {
+		basisCents = 0
+	}
+	return EarningResult{
+		Basis: float64(basisCents) / 100,
+		Point: earnLine(basisCents, settings.Point),
+		Coin:  earnLine(basisCents, settings.Coin),
+	}
+}
+
+func earnLine(basisCents int64, s models.LoyaltyEarnSettings) EarningLine {
+	line := EarningLine{Settings: s}
+	if !s.Enabled || s.EarnPerAmount <= 0 || s.EarnValue <= 0 {
+		return line
+	}
+	if basisCents < s.MinOrderAmount*100 {
+		return line
+	}
+	line.Amount = basisCents / (s.EarnPerAmount * 100) * s.EarnValue
+	if s.MaxPerOrder != nil && line.Amount > *s.MaxPerOrder {
+		line.Amount = *s.MaxPerOrder
+		line.Capped = true
+	}
+	return line
+}
+
+func toCents(rupiah float64) int64 {
+	return int64(math.Round(rupiah * 100))
+}