From 378855142edc575c55b446f34d748ef3c4fe74f7 Mon Sep 17 00:00:00 2001
From: Christian Grothoff <christian@grothoff.org>
Date: Thu, 24 Aug 2023 20:50:47 +0200
Subject: implement STEFAN value calculation

---
 src/include/taler_exchange_service.h | 55 ++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

(limited to 'src/include/taler_exchange_service.h')

diff --git a/src/include/taler_exchange_service.h b/src/include/taler_exchange_service.h
index 471263f78..a1a1e3997 100644
--- a/src/include/taler_exchange_service.h
+++ b/src/include/taler_exchange_service.h
@@ -825,6 +825,61 @@ void
 TALER_EXCHANGE_keys_decref (struct TALER_EXCHANGE_Keys *keys);
 
 
+/**
+ * Use STEFAN curve in @a keys to convert @a brut to @a net.  Computes the
+ * expected minimum (!) @a net amount that should for sure arrive in the
+ * target amount at cost of @a brut to the wallet. Note that STEFAN curves by
+ * design over-estimate actual fees and a wallet may be able to achieve the
+ * same @a net amount with less fees --- or if the available coins are
+ * abnormal in structure, it may take more.
+ *
+ * @param keys exchange key data
+ * @param brut gross amount (actual cost including fees)
+ * @param[out] net net amount (effective amount)
+ * @return #GNUNET_OK on success, #GNUNET_NO if the
+ *   resulting @a net is zero (or lower)
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_keys_stefan_b2n (
+  const struct TALER_EXCHANGE_Keys *keys,
+  const struct TALER_Amount *brut,
+  struct TALER_Amount *net);
+
+
+/**
+ * Use STEFAN curve in @a keys to convert @a net to @a brut.  Computes the
+ * expected maximum (!) @a brut amount that should be needed in the wallet to
+ * transfer @a net amount to the target account.  Note that STEFAN curves by
+ * design over-estimate actual fees and a wallet may be able to achieve the
+ * same @a net amount with less fees --- or if the available coins are
+ * abnormal in structure, it may take more.
+ *
+ * @param keys exchange key data
+ * @param net net amount (effective amount)
+ * @param[out] brut gross amount (actual cost including fees)
+ * @return #GNUNET_OK on success, #GNUNET_NO if the
+ *   resulting @a brut is zero (only if @a net was zero)
+ */
+enum GNUNET_GenericReturnValue
+TALER_EXCHANGE_keys_stefan_n2b (
+  const struct TALER_EXCHANGE_Keys *keys,
+  const struct TALER_Amount *net,
+  struct TALER_Amount *brut);
+
+
+/**
+ * Round brutto or netto value computed via STEFAN
+ * curve to decimal places commonly used at the exchange.
+ *
+ * @param keys exchange keys response data
+ * @param[in,out] val value to round
+ */
+void
+TALER_EXCHANGE_keys_stefan_round (
+  const struct TALER_EXCHANGE_Keys *keys,
+  struct TALER_Amount *val);
+
+
 /**
  * Test if the given @a pub is a the current signing key from the exchange
  * according to @a keys.
-- 
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