Support adjusting form invoice amount by multiplier (#5463)

Co-authored-by: d11n <mail@dennisreimann.de>

BTCPayServer.Tests/FormTests.cs

@@ -198,6 +198,65 @@ public class FormTests : UnitTestBase
         service.SetValues(form, new JObject { ["test"] = "hello" });
         obj = service.GetValues(form);
         Assert.Equal("hello", obj["test"].Value<string>());
+
+        var req = service.GenerateInvoiceParametersFromForm(form);
+        Assert.Null(req.Amount);
+        Assert.Null(req.Currency);
+        
+        form.Fields.Add(new Field
+        {
+            Name = $"{FormDataService.InvoiceParameterPrefix}amount",
+            Type = "number",
+            Value = "1"
+        });
+        req = service.GenerateInvoiceParametersFromForm(form);
+        Assert.Equal(1, req.Amount);
+        
+        form.Fields.Add(new Field
+        {
+            Name = $"{FormDataService.InvoiceParameterPrefix}amount_adjustment",
+            Type = "number",
+            Value = "1"
+        });
+        req = service.GenerateInvoiceParametersFromForm(form);
+        Assert.Equal(2, req.Amount);
+        form.Fields.Add(new Field
+        {
+            Name = $"{FormDataService.InvoiceParameterPrefix}amount_adjustment2",
+            Type = "number",
+            Value = "2"
+        });
+        form.Fields.Add(new Field
+        {
+            Name = $"{FormDataService.InvoiceParameterPrefix}currency",
+            Type = "text",
+            Value = "eur"
+        });
+        req = service.GenerateInvoiceParametersFromForm(form);
+        Assert.Equal("eur", req.Currency);
+        Assert.Equal(4, req.Amount);
+        
+
+        form.Fields.Add(new Field
+        {
+            Name = $"{FormDataService.InvoiceParameterPrefix}amount_multiply_adjustment",
+            Type = "number",
+            Value = "2"
+        });
+        
+        req = service.GenerateInvoiceParametersFromForm(form);
+        Assert.Equal(8, req.Amount);
+        
+        
+        form.Fields.Add(new Field
+        {
+            Name = $"{FormDataService.InvoiceParameterPrefix}amount_multiply_adjustment1",
+            Type = "number",
+            Value = "2"
+        });
+        
+        req = service.GenerateInvoiceParametersFromForm(form);
+        Assert.Equal(16, req.Amount);
     }
 
     private void Clear(Form form)

BTCPayServer/Forms/FormDataService.cs

@@ -153,25 +153,31 @@ public class FormDataService
     {
         var amtRaw = GetValue(form, $"{InvoiceParameterPrefix}amount");
         var amt = string.IsNullOrEmpty(amtRaw) ? (decimal?) null : decimal.Parse(amtRaw, CultureInfo.InvariantCulture);
-        var adjustmentAmount = 0m;
-        foreach (var adjustmentField in form.GetAllFields().Where(f => f.FullName.StartsWith($"{InvoiceParameterPrefix}amount_adjustment")))
+        foreach (var f in form.GetAllFields())
         {
-            if (!decimal.TryParse(GetValue(form, adjustmentField.Field), out var adjustment))
+            if (f.FullName.StartsWith($"{InvoiceParameterPrefix}amount_adjustment") && decimal.TryParse(GetValue(form, f.Field), out var adjustment))
             {
-                continue;
+                if (amt is null)
+                {
+                    amt = adjustment;
+                }
+                else
+                {
+                    amt += adjustment;
+                }
+            } 
+            if (f.FullName.StartsWith($"{InvoiceParameterPrefix}amount_multiply_adjustment") && decimal.TryParse(GetValue(form, f.Field), out var adjustmentM))
+            {
+                if (amt is not null)
+                {
+                    amt *= adjustmentM;
+                }
             }
-
-            adjustmentAmount += adjustment;
         }
-
-        if (amt is null && adjustmentAmount > 0)
+        
+        if(amt is not null)
         {
-            amt = adjustmentAmount;
-        }
-        else if(amt is not null)
-        {
-            amt += adjustmentAmount;
-            amt = Math.Max(0, amt!.Value);
+            amt = Math.Max(0, amt.Value);
         }
         return new CreateInvoiceRequest
         {

BTCPayServer/Views/UIForms/Modify.cshtml

@@ -21,6 +21,7 @@
         <option value="invoice_amount">Determine the generated invoice amount</option>
         <option value="invoice_currency">Determine the generated invoice currency</option>
         <option value="invoice_amount_adjustment">Adjusts the generated invoice amount — use as a prefix to have multiple adjustment fields</option>
+        <option value="invoice_amount_multiply_adjustment">Adjusts the generated invoice amount by multiplying with this value — use as a prefix to have multiple adjustment fields</option>
     </datalist>
     <template id="form-template-email">
         @FormDataService.StaticFormEmail
@@ -47,6 +48,7 @@
                 <div class="form-text text-info" v-if="field.name === 'invoice_currency'">The configured name means the value of this field will determine the invoice currency for public forms.</div>
                 <div class="form-text text-info" v-if="field.name === 'invoice_amount'">The configured name means the value of this field will determine the invoice amount for public forms.</div>
                 <div class="form-text text-info" v-if="field.name && field.name.startsWith('invoice_amount_adjustment')">The configured name means the value of this field will adjust the invoice amount for public forms and the point of sale app.</div>
+                <div class="form-text text-info" v-if="field.name && field.name.startsWith('invoice_amount_multiply_adjustment')">The configured name means the value of this field will adjust the invoice amount by multiplying it for public forms and the point of sale app.</div>
             </div>
             <div class="form-group" v-if="field.type === 'select'">
                 <h5 class="mt-2">Options</h5>