qml: refactor invoice amount checks, msat precision for lightning. add qeconfig unit tests for conversion methods.

diff --git a/electrum/gui/qml/qeconfig.py b/electrum/gui/qml/qeconfig.py
index caa1509..979565e 100644
--- a/electrum/gui/qml/qeconfig.py
+++ b/electrum/gui/qml/qeconfig.py
@@ -358,13 +358,6 @@ class QEConfig(AuthMixin, QObject):
         else:
             return self.config.format_amount(msats/1000, precision=precision)
 
-    # TODO delegate all this to config.py/util.py
-    def decimal_point(self):
-        return self.config.BTC_AMOUNTS_DECIMAL_POINT
-
-    def max_precision(self):
-        return self.decimal_point() + 0  # self.extra_precision
-
     @pyqtSlot(str, result=QEAmount)
     def unitsToSats(self, unitAmount):
         self._amount = QEAmount()
@@ -373,16 +366,11 @@ class QEConfig(AuthMixin, QObject):
         except Exception:
             return self._amount
 
-        # scale it to max allowed precision, make it an int
-        max_prec_amount = int(pow(10, self.max_precision()) * x)
-        # if the max precision is simply what unit conversion allows, just return
-        if self.max_precision() == self.decimal_point():
-            self._amount = QEAmount(amount_sat=max_prec_amount)
-            return self._amount
-        self._logger.debug('fallthrough')
-        # otherwise, scale it back to the expected unit
-        #amount = Decimal(max_prec_amount) / Decimal(pow(10, self.max_precision()-self.decimal_point()))
-        #return int(amount) #Decimal(amount) if not self.is_int else int(amount)
+        sat_max_precision = self.config.BTC_AMOUNTS_DECIMAL_POINT
+        msat_max_precision = self.config.BTC_AMOUNTS_DECIMAL_POINT + 3
+        sat_max_prec_amount = int(pow(10, sat_max_precision) * x)
+        msat_max_prec_amount = int(pow(10, msat_max_precision) * x)
+        self._amount = QEAmount(amount_sat=sat_max_prec_amount, amount_msat=msat_max_prec_amount)
         return self._amount
 
     @pyqtSlot('quint64', result=float)
diff --git a/electrum/gui/qml/qeinvoice.py b/electrum/gui/qml/qeinvoice.py
index acb80bb..bedc1c6 100644
--- a/electrum/gui/qml/qeinvoice.py
+++ b/electrum/gui/qml/qeinvoice.py
@@ -1,7 +1,7 @@
 import copy
 import threading
 from enum import IntEnum
-from typing import Optional, Dict, Any
+from typing import Optional, Dict, Any, Tuple
 from urllib.parse import urlparse
 
 from PyQt6.QtCore import pyqtProperty, pyqtSignal, pyqtSlot, QObject, pyqtEnum, QTimer
@@ -327,23 +327,10 @@ class QEInvoice(QObject, QtEventListener):
                 PR_UNKNOWN: _('Invoice has unknown status'),
             }[_status]
 
-        if self.invoiceType == QEInvoice.Type.LightningInvoice:
-            if status in [PR_UNPAID, PR_FAILED]:
-                if self.get_max_spendable_lightning() >= amount.satsInt:
-                    lnaddr = self._effectiveInvoice._lnaddr
-                    if lnaddr.amount and amount.satsInt < lnaddr.amount * COIN:
-                        self.userinfo = _('Cannot pay less than the amount specified in the invoice')
-                elif not self.address or self.get_max_spendable_onchain() < amount.satsInt:
-                    # TODO: for onchain: validate address? subtract fee?
-                    self.userinfo = _('Insufficient balance')
-            else:
-                self.userinfo = userinfo_for_invoice_status(status)
-        elif self.invoiceType == QEInvoice.Type.OnchainInvoice:
-            if status in [PR_UNPAID, PR_FAILED]:
-                if not ((amount.isMax and self.get_max_spendable_onchain() > 0) or (self.get_max_spendable_onchain() >= amount.satsInt)):
-                    self.userinfo = _('Insufficient balance')
-            else:
-                self.userinfo = userinfo_for_invoice_status(status)
+        if status in [PR_UNPAID, PR_FAILED]:
+            x, self.userinfo = self.check_can_pay_amount(amount)
+        else:
+            self.userinfo = userinfo_for_invoice_status(status)
 
     def determine_can_pay(self):
         self.canPay = False
@@ -364,24 +351,25 @@ class QEInvoice(QObject, QtEventListener):
         if amount.isEmpty and status == PR_UNPAID:  # unspecified amount
             return
 
+        if status in [PR_UNPAID, PR_FAILED]:
+            self.canPay, x = self.check_can_pay_amount(amount)
+
+    def check_can_pay_amount(self, amount: QEAmount) -> Tuple[bool, Optional[str]]:
+        assert self.status in [PR_UNPAID, PR_FAILED]
         if self.invoiceType == QEInvoice.Type.LightningInvoice:
-            if status in [PR_UNPAID, PR_FAILED]:
-                if self.get_max_spendable_lightning() >= amount.satsInt:
-                    lnaddr = self._effectiveInvoice._lnaddr
-                    if not (lnaddr.amount and amount.satsInt < lnaddr.amount * COIN):
-                        self.canPay = True
-                elif self.address and self.get_max_spendable_onchain() > amount.satsInt:
-                    # TODO: validate address?
-                    # TODO: subtract fee?
-                    self.canPay = True
+            if self.get_max_spendable_lightning() * 1000 >= amount.msatsInt:
+                lnaddr = self._effectiveInvoice._lnaddr
+                if lnaddr.amount and amount.msatsInt < lnaddr.amount * COIN * 1000:
+                    return False, _('Cannot pay less than the amount specified in the invoice')
+                else:
+                    return True, None
+            elif self.address and self.get_max_spendable_onchain() > amount.satsInt:
+                return True, None
         elif self.invoiceType == QEInvoice.Type.OnchainInvoice:
-            if status in [PR_UNPAID, PR_FAILED]:
-                if amount.isMax and self.get_max_spendable_onchain() > 0:
-                    # TODO: dust limit?
-                    self.canPay = True
-                elif self.get_max_spendable_onchain() >= amount.satsInt:
-                    # TODO: subtract fee?
-                    self.canPay = True
+            if (amount.isMax and self.get_max_spendable_onchain() > 0) or (self.get_max_spendable_onchain() >= amount.satsInt):
+                return True, None
+
+        return False, _('Insufficient balance')
 
     @pyqtSlot()
     def payLightningInvoice(self):
diff --git a/tests/test_qml_qeconfig.py b/tests/test_qml_qeconfig.py
new file mode 100644
index 0000000..d76c543
--- /dev/null
+++ b/tests/test_qml_qeconfig.py
@@ -0,0 +1,66 @@
+from electrum import SimpleConfig
+from electrum.gui.qml.qeconfig import QEConfig
+from tests.qt_util import QETestCase, qt_test
+
+
+class TestConfig(QETestCase):
+    @classmethod
+    def setUpClass(cls):
+        QEConfig(SimpleConfig())
+
+    def setUp(self):
+        super().setUp()
+        self.q: QEConfig = QEConfig.instance
+        # raise Exception()  # NOTE: exceptions in setUp() will block the test
+
+    @qt_test
+    def test_satstounits(self):
+        self.q.config.BTC_AMOUNTS_DECIMAL_POINT = 5
+        self.assertEqual(self.q.satsToUnits(100_000), 1.0)
+        self.assertEqual(self.q.satsToUnits(1), 0.00001)
+        self.assertEqual(self.q.satsToUnits(0.001), 0.00000001)
+
+    @qt_test
+    def test_unitstosats(self):
+        qa = self.q.unitsToSats('')
+        self.assertTrue(qa.isEmpty)
+        qa = self.q.unitsToSats('0')
+        self.assertTrue(qa.isEmpty)
+        qa = self.q.unitsToSats('0.000')
+        self.assertTrue(qa.isEmpty)
+
+        self.q.config.BTC_AMOUNTS_DECIMAL_POINT = 5
+
+        qa = self.q.unitsToSats('1')
+        self.assertFalse(qa.isEmpty)
+        self.assertEqual(qa.satsInt, 100_000)
+        self.assertEqual(qa.msatsInt, 100_000_000)
+
+        qa = self.q.unitsToSats('1.001')
+        self.assertFalse(qa.isEmpty)
+        self.assertEqual(qa.satsInt, 100_100)
+        self.assertEqual(qa.msatsInt, 100_100_000)
+
+        qa = self.q.unitsToSats('1.000001')
+        self.assertFalse(qa.isEmpty)
+        self.assertEqual(qa.satsInt, 100_000)
+        self.assertEqual(qa.msatsInt, 100_000_100)
+
+        self.q.config.BTC_AMOUNTS_DECIMAL_POINT = 0
+
+        qa = self.q.unitsToSats('1.001')
+        self.assertFalse(qa.isEmpty)
+        self.assertEqual(qa.satsInt, 1)
+        self.assertEqual(qa.msatsInt, 1001)
+
+        qa = self.q.unitsToSats('1.0001')  # outside msat precision
+        self.assertFalse(qa.isEmpty)
+        self.assertEqual(qa.satsInt, 1)
+        self.assertEqual(qa.msatsInt, 1000)
+
+        self.q.config.BTC_AMOUNTS_DECIMAL_POINT = 8
+
+        qa = self.q.unitsToSats('0.00000001001')
+        self.assertFalse(qa.isEmpty)
+        self.assertEqual(qa.satsInt, 1)
+        self.assertEqual(qa.msatsInt, 1001)