refactor: deduplicate net/discount calculation in line item strategies

src/Inflop.VatSharp/Strategies/Calculation/FromSumOfGrossValuesStrategy.cs

@@ -50,8 +50,7 @@ public DocumentAmounts Calculate(IReadOnlyList<InvoiceLineItem> lineItems, IRoun
 
             foreach (var (item, originalIndex) in group)
             {
-                var itemGross = item.TotalGrossWith(discountBehavior, rounding).Round(rounding);
-                var itemDiscount = item.DiscountAmountNetWith(discountBehavior, rounding).Round(rounding);
+                var (itemGross, itemDiscount) = item.CalculateGrossAndNetDiscount(discountBehavior, rounding);
 
                 sumGross += itemGross;
                 sumDiscount += itemDiscount;

src/Inflop.VatSharp/Strategies/Calculation/FromSumOfNetValuesStrategy.cs

@@ -36,8 +36,7 @@ public DocumentAmounts Calculate(IReadOnlyList<InvoiceLineItem> lineItems, IRoun
 
             foreach (var (item, originalIndex) in group)
             {
-                var itemNet = item.TotalNetWith(discountBehavior, rounding).Round(rounding);
-                var itemDiscount = item.DiscountAmountNetWith(discountBehavior, rounding).Round(rounding);
+                var (itemNet, itemDiscount) = item.CalculateNetAndDiscount(discountBehavior, rounding);
 
                 sumNet += itemNet;
                 sumDiscount += itemDiscount;

src/Inflop.VatSharp/Strategies/Calculation/IVatCalculationStrategy.cs

@@ -18,9 +18,6 @@ internal interface IVatCalculationStrategy
 
     DocumentAmounts Calculate(IReadOnlyList<InvoiceLineItem> lineItems, IRoundingStrategy rounding, IAbsoluteDiscountBehavior discountBehavior);
 
-    DocumentAmounts Calculate(IReadOnlyList<InvoiceLineItem> lineItems, IRoundingStrategy rounding)
-        => Calculate(lineItems, rounding, FromTotalAbsoluteDiscountBehavior.Instance);
-
     /// <summary>
     /// Converts a per-rate FCY summary to base currency using this strategy's authoritative field.
     /// Each strategy knows which field drives the others, so the conversion mirrors that hierarchy.

src/Inflop.VatSharp/Strategies/Calculation/SumOfLineItemVatAmountsStrategy.cs

@@ -28,8 +28,7 @@ public DocumentAmounts Calculate(IReadOnlyList<InvoiceLineItem> lineItems, IRoun
         var itemAmounts = lineItems
             .Select(item =>
             {
-                var net = item.TotalNetWith(discountBehavior, rounding).Round(rounding);
-                var discount = item.DiscountAmountNetWith(discountBehavior, rounding).Round(rounding);
+                var (net, discount) = item.CalculateNetAndDiscount(discountBehavior, rounding);
                 return LineItemAmounts.FromNet(net, item.VatRate, rounding, discount);
             })
             .ToList();

src/Inflop.VatSharp/ValueObjects/InvoiceLineItem.cs

@@ -95,10 +95,34 @@ internal Money TotalGrossWith(IAbsoluteDiscountBehavior discountBehavior, IRound
             : VatRate.GrossFromNet(totalInInputType);
     }
 
+    /// <summary>
+    /// Returns rounded net and net-equivalent discount in a single call,
+    /// avoiding a redundant second invocation of <see cref="TotalNetWith"/>.
+    /// </summary>
+    internal (Money Net, Money Discount) CalculateNetAndDiscount(IAbsoluteDiscountBehavior discountBehavior, IRoundingStrategy rounding)
+    {
+        var net = TotalNetWith(discountBehavior, rounding);
+        var discount = Discount.HasValue ? TotalNetBeforeDiscount - net : Money.Zero;
+        return (net.Round(rounding), discount.Round(rounding));
+    }
+
+    /// <summary>
+    /// Returns rounded gross and net-equivalent discount in a single call,
+    /// avoiding a redundant second invocation of <see cref="TotalGrossWith"/> via
+    /// <see cref="DiscountAmountNetWith"/>.
+    /// </summary>
+    internal (Money Gross, Money NetDiscount) CalculateGrossAndNetDiscount(IAbsoluteDiscountBehavior discountBehavior, IRoundingStrategy rounding)
+    {
+        var gross = TotalGrossWith(discountBehavior, rounding);
+        var netDiscount = Discount.HasValue
+            ? TotalNetBeforeDiscount - VatRate.NetFromGross(gross)
+            : Money.Zero;
+        return (gross.Round(rounding), netDiscount.Round(rounding));
+    }
+
     internal LineItemAmounts Calculate(IRoundingStrategy rounding, IAbsoluteDiscountBehavior discountBehavior)
     {
-        var net = TotalNetWith(discountBehavior, rounding).Round(rounding);
-        var discount = DiscountAmountNetWith(discountBehavior, rounding).Round(rounding);
+        var (net, discount) = CalculateNetAndDiscount(discountBehavior, rounding);
         return LineItemAmounts.FromNet(net, VatRate, rounding, discount);
     }
 

src/Inflop.VatSharp/ValueObjects/Quantity.cs

@@ -28,7 +28,7 @@ public sealed record Quantity : IComparable<Quantity>
     public static Quantity Of(int value) => new(value);
 
     /// <summary>A quantity of exactly one.</summary>
-    public static Quantity One => new(1m);
+    public static readonly Quantity One = Of(1);
 
     /// <inheritdoc />
     public int CompareTo(Quantity? other)

tests/Inflop.VatSharp.Tests/Inflop.VatSharp.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net10.0</TargetFramework>
+    <TargetFrameworks>net8.0;net9.0;net10.0</TargetFrameworks>
     <IsPackable>false</IsPackable>
     <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
   </PropertyGroup>