fix: correct lnurl msat to sat min rounding

Author: piotr-iohk

app/src/main/java/to/bitkit/ext/Lnurl.kt

@@ -3,9 +3,26 @@ package to.bitkit.ext
 import com.synonym.bitkitcore.LnurlPayData
 import com.synonym.bitkitcore.LnurlWithdrawData
 
+private const val MSATS_PER_SAT: ULong = 1000u
+
+/**
+ * LNURL amounts are expressed in millisatoshis (msat).
+ *
+ * When converting a minimum bound to whole sats we must round up:
+ * `minSendable = 100500 msat` means the minimum payable amount is `101 sat` (not `100 sat`).
+ */
+private fun msatsToSatsCeil(msats: ULong): ULong {
+    val quotient = msats / MSATS_PER_SAT
+    val remainder = msats % MSATS_PER_SAT
+    return when (remainder) {
+        0uL -> quotient
+        else -> quotient + 1uL
+    }
+}
+
 fun LnurlPayData.commentAllowed(): Boolean = commentAllowed?.let { it > 0u } == true
-fun LnurlPayData.maxSendableSat(): ULong = maxSendable / 1000u
-fun LnurlPayData.minSendableSat(): ULong = minSendable / 1000u
+fun LnurlPayData.maxSendableSat(): ULong = maxSendable / MSATS_PER_SAT
+fun LnurlPayData.minSendableSat(): ULong = msatsToSatsCeil(minSendable)
 
-fun LnurlWithdrawData.minWithdrawableSat(): ULong = (minWithdrawable ?: 0u) / 1000u
-fun LnurlWithdrawData.maxWithdrawableSat(): ULong = maxWithdrawable / 1000u
+fun LnurlWithdrawData.minWithdrawableSat(): ULong = msatsToSatsCeil(minWithdrawable ?: 0u)
+fun LnurlWithdrawData.maxWithdrawableSat(): ULong = maxWithdrawable / MSATS_PER_SAT