From d827d155c9fc7b4008bc8f8ed8b9c5158eb1f9b3 Mon Sep 17 00:00:00 2001
From: MS <ms@taler.net>
Date: Tue, 23 May 2023 12:03:42 +0200
Subject: Conversion service.

Checking that conversion never produces negative amounts.
---
 sandbox/src/main/kotlin/tech/libeufin/sandbox/CircuitApi.kt | 13 +++++++++++--
 .../main/kotlin/tech/libeufin/sandbox/ConversionService.kt  | 13 +++++++++++--
 2 files changed, 22 insertions(+), 4 deletions(-)

(limited to 'sandbox/src/main/kotlin/tech/libeufin')

diff --git a/sandbox/src/main/kotlin/tech/libeufin/sandbox/CircuitApi.kt b/sandbox/src/main/kotlin/tech/libeufin/sandbox/CircuitApi.kt
index 70642899..e44dc227 100644
--- a/sandbox/src/main/kotlin/tech/libeufin/sandbox/CircuitApi.kt
+++ b/sandbox/src/main/kotlin/tech/libeufin/sandbox/CircuitApi.kt
@@ -166,10 +166,19 @@ fun applyCashoutRatioAndFee(
 ): BigDecimal {
     // Normal case, when the calculation starts from the regional amount.
     if (!fromCredit) {
-        return ((amount * ratiosAndFees.sell_at_ratio.toBigDecimal()) -
+        val maybeCashoutAmount =  ((amount * ratiosAndFees.sell_at_ratio.toBigDecimal()) -
                 ratiosAndFees.sell_out_fee.toBigDecimal()).roundToTwoDigits()
+        // throws 500, since bank should not allow to get negative fiat amounts.
+        if (maybeCashoutAmount < BigDecimal.ZERO) {
+            logger.error("Cash-out operation caused a negative fiat output." +
+                    "  Regional amount was '$amount', cash-out ratio is '${ratiosAndFees.sell_at_ratio}," +
+                    " cash-out fee is '${ratiosAndFees.sell_out_fee}''"
+            )
+            throw internalServerError("Applying cash-out fees yielded negative fiat amount.")
+        }
+        return maybeCashoutAmount
     }
-    // UI convenient case, when the calculation start from the
+    // UI convenient case, when the calculation starts from the
     // desired fiat amount that the user wants eventually be paid.
     return ((amount + ratiosAndFees.sell_out_fee.toBigDecimal()) /
             ratiosAndFees.sell_at_ratio.toBigDecimal()).roundToTwoDigits()
diff --git a/sandbox/src/main/kotlin/tech/libeufin/sandbox/ConversionService.kt b/sandbox/src/main/kotlin/tech/libeufin/sandbox/ConversionService.kt
index f19718fa..a080dc80 100644
--- a/sandbox/src/main/kotlin/tech/libeufin/sandbox/ConversionService.kt
+++ b/sandbox/src/main/kotlin/tech/libeufin/sandbox/ConversionService.kt
@@ -110,9 +110,18 @@ fun downloadLoop(block: () -> Unit) {
 private fun applyBuyinRatioAndFees(
     amount: BigDecimal,
     ratiosAndFees: RatioAndFees
-): BigDecimal =
-    ((amount * ratiosAndFees.buy_at_ratio.toBigDecimal())
+): BigDecimal {
+    val maybeBuyinAmount = ((amount * ratiosAndFees.buy_at_ratio.toBigDecimal())
             - ratiosAndFees.buy_in_fee.toBigDecimal()).roundToTwoDigits()
+    // Bank's fault, as buying in should never lead to negative.
+    if (maybeBuyinAmount < BigDecimal.ZERO) {
+        logger.error("Negative buy-in scenario: input fiat amount was '${amount}'" +
+                ", buy-in ratio was '${ratiosAndFees.buy_at_ratio}'," +
+                " buy-in fee was '${ratiosAndFees.buy_in_fee}'")
+        throw internalServerError("Applying buy-in fees yielded negative regional amount")
+    }
+    return maybeBuyinAmount
+}
 
 private fun ensureDisabledRedirects(client: HttpClient) {
     client.config {
-- 
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