Initial Proof of Concept

src/Business.Calculations.Tests/Business.Calculations.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Fluent.Calculations.Primitives" Version="1.1.0-aplha.39" />
+    <PackageReference Include="FluentAssertions" Version="6.12.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.0" />
+    <PackageReference Include="xunit" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.collector" Version="6.0.0">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Business.Calculations\Business.Calculations.csproj" />
+  </ItemGroup>
+
+</Project>

src/Business.Calculations.Tests/GlobalUsings.cs

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+global using Xunit;

src/Business.Calculations.Tests/ProgressiveTests.cs

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+using Business.Calculations;
+using Fluent.Calculations.Primitives.BaseTypes;
+using FluentAssertions;
+
+namespace Fluent.Calculations.Business.Tests
+{
+    public class ProgressiveTests
+    {
+        [Fact]
+        public void Test()
+        {
+            var progressive = new Progressive<decimal, decimal>()
+                .UpTo(100).MultiplyBy(0.01m)
+                .UpTo(200).MultiplyBy(0.02m)
+                .UpTo(300).MultiplyBy(0.03m)
+                .ReminderMultiplier(0.04m);
+
+            var result = progressive.Calculate(400);
+
+            result.Amount.Should().Be(10);
+        }
+
+        [Fact]
+        public void Test2()
+        {
+            var progressive = new Progressive<Number, Number>()
+                .UpTo(100).MultiplyBy(0.01m)
+                .UpTo(200).MultiplyBy(0.02m)
+                .UpTo(300).MultiplyBy(0.03m)
+                .Capped();
+
+            var result = progressive.Calculate(400);
+
+            result.Amount.Primitive.Should().Be(6);
+        }
+    }
+}

src/Business.Calculations.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.8.34309.116
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Business.Calculations", "Business.Calculations\Business.Calculations.csproj", "{B6624F0A-69B5-4D2D-AA86-3C392B45DBBE}"
+EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Business.Calculations.Tests", "Business.Calculations.Tests\Business.Calculations.Tests.csproj", "{1D0EF362-7EEA-4926-A42F-1EF39F97977B}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{B6624F0A-69B5-4D2D-AA86-3C392B45DBBE}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{B6624F0A-69B5-4D2D-AA86-3C392B45DBBE}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{B6624F0A-69B5-4D2D-AA86-3C392B45DBBE}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{B6624F0A-69B5-4D2D-AA86-3C392B45DBBE}.Release|Any CPU.Build.0 = Release|Any CPU
+		{1D0EF362-7EEA-4926-A42F-1EF39F97977B}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{1D0EF362-7EEA-4926-A42F-1EF39F97977B}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{1D0EF362-7EEA-4926-A42F-1EF39F97977B}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{1D0EF362-7EEA-4926-A42F-1EF39F97977B}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {BBD91840-5837-4742-BDE6-82CDDA0B903B}
+	EndGlobalSection
+EndGlobal

src/Business.Calculations/Business.Calculations.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>disable</Nullable>
+  </PropertyGroup>
+
+</Project>

src/Business.Calculations/ProgressiveGeneric.cs

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+using System.Numerics;
+
+public class Progressive<T, TMultiplier>
+    where T :
+        ISubtractionOperators<T, T, T>,
+        IAdditionOperators<T, T, T>,
+        IMultiplyOperators<T, TMultiplier, T>,
+        IComparisonOperators<T, T, bool>,
+        new()
+    where TMultiplier : IMultiplyOperators<TMultiplier, T, T>
+{
+    private readonly List<Bracket> brackets = new List<Bracket>();
+
+    private static T Zero = new T();
+
+    public record Bracket
+    {
+        public T From { get; set; }
+
+        public T To { get; set; }
+
+        public TMultiplier Multiplier { get; set; }
+
+        public bool ToInfinity { get; internal set; }
+
+        internal T Size => To - From;
+
+        public T Calculate(T value) => value * Multiplier;
+    }
+
+    public RateBuilder UpTo(T toValue) => new BracketRateBuilder(brackets).UpTo(Enforce.GreaterThanZero(toValue));
+
+    public ToBuilder From(T fromValue) => new BracketRateBuilder(brackets).From(Enforce.GreaterThanOrZero(fromValue));
+
+    public Progressive<T, TMultiplier> AddBracket(T fromValue, T toValue, TMultiplier multiplier)
+    {
+        brackets.Add(new Bracket { From = fromValue, To = toValue, Multiplier = multiplier });
+        return this;
+    }
+
+    public class BracketRateBuilder
+    {
+        private List<Bracket> brackets;
+
+        public BracketRateBuilder(List<Bracket> brackets) => this.brackets = brackets;
+
+        public ToBuilder From(T fromValue) => new ToBuilder(brackets, fromValue);
+
+        public RateBuilder UpTo(T toValue) => From(Zero).UpTo(toValue);
+    }
+
+    public class FromBuilder
+    {
+        private List<Bracket> brackets;
+
+        public FromBuilder(List<Bracket> brackets) => this.brackets = brackets;
+
+        public ToBuilder From(T fromValue) => new ToBuilder(brackets, fromValue);
+
+        public RateBuilder UpTo(T toValue) => From(brackets.Last().To).UpTo(toValue);
+
+        public ResultBuilder ReminderMultiplier(TMultiplier multipier)
+        {
+            From(brackets.Last().To).UpToInfinity().MultiplyBy(multipier);
+
+            return new ResultBuilder(brackets);
+        }
+
+        public ResultBuilder Capped() => new ResultBuilder(brackets);
+    }
+
+    public class ResultBuilder
+    {
+        private List<Bracket> brackets;
+
+        public ResultBuilder(List<Bracket> brackets) => this.brackets = brackets;
+
+        public BracketCalculationResult Calculate(T value)
+        {
+            BracketsAccumulator seed = new()
+            {
+                Remaining = value
+            };
+
+            return brackets.Aggregate(seed, Allocate, Calculate);
+
+            static BracketsAccumulator Allocate(BracketsAccumulator accumulator, Bracket currentBracket)
+            {
+                T allocationAmmount = accumulator.Remaining < currentBracket.Size || currentBracket.ToInfinity ?
+                    accumulator.Remaining : currentBracket.Size;
+
+                accumulator.BracketsAllocation.Add(new BracketsAccumulator.BracketAllocation
+                {
+                    Bracket = currentBracket,
+                    Amount = allocationAmmount
+                });
+
+                accumulator.Remaining -= allocationAmmount;
+
+                return accumulator;
+            }
+
+            static BracketCalculationResult Calculate(BracketsAccumulator accumulator)
+            {
+                var breakdown = accumulator.BracketsAllocation
+                    .Select(allocation => new BracketResult
+                    {
+                        Amount = allocation.Bracket.Calculate(allocation.Amount),
+                        Bracket = allocation.Bracket
+                    }).ToArray();
+
+                return new BracketCalculationResult
+                {
+                    Amount = SumAmounts(breakdown),
+                    Breakdown = breakdown
+                };
+
+                static T SumAmounts(BracketResult[] values)
+                {
+                    T aggregate = Zero;
+
+                    foreach (BracketResult value in values)
+                        aggregate += value.Amount;
+
+                    return aggregate;
+                }
+            }
+        }
+    }
+
+    public class BracketsAccumulator
+    {
+        public T Remaining;
+
+        public List<BracketAllocation> BracketsAllocation = new();
+
+        public class BracketAllocation
+        {
+            public Bracket Bracket { get; set; }
+            public T Amount { get; set; }
+        }
+    }
+
+    public class BracketCalculationResult
+    {
+        public T Amount { get; set; }
+
+        public BracketResult[] Breakdown { get; set; }
+    }
+
+    public class BracketResult
+    {
+        public Bracket Bracket { get; set; }
+
+        public T Amount { get; set; }
+    }
+
+    public class ToBuilder
+    {
+        private readonly T fromValue;
+        private readonly List<Bracket> brackets;
+
+        public ToBuilder(List<Bracket> brackets, T fromValue)
+        {
+            this.fromValue = fromValue;
+            this.brackets = brackets;
+        }
+
+        internal RateBuilder UpTo(T toValue) => new RateBuilder(brackets, fromValue, toValue, false);
+
+        internal RateBuilder UpToInfinity() => new RateBuilder(brackets, fromValue, new T(), true);
+    }
+
+    public class RateBuilder
+    {
+        private readonly T fromValue;
+        private readonly T toValue;
+        private readonly List<Bracket> brackets;
+        private readonly bool toInfinity;
+
+        public RateBuilder(List<Bracket> brackets, T fromValue, T toValue, bool toInfinity)
+        {
+            this.fromValue = fromValue;
+            this.toValue = toValue;
+            this.toInfinity = toInfinity;
+            this.brackets = brackets;
+        }
+
+        public FromBuilder MultiplyBy(TMultiplier multiplier)
+        {
+            brackets.Add(new Bracket
+            {
+                From = fromValue,
+                To = toValue,
+                Multiplier = multiplier,
+                ToInfinity = toInfinity
+            });
+
+            return new FromBuilder(brackets);
+        }
+    }
+
+    private class Enforce
+    {
+        internal static T GreaterThanOrZero(T fromValue) => fromValue >= Zero ? fromValue :
+            throw new ArgumentOutOfRangeException(nameof(fromValue), "Value shoul be greater than or equal to 0.");
+
+        internal static T GreaterThanZero(T toValue) => toValue > Zero ? toValue :
+            throw new ArgumentOutOfRangeException(nameof(toValue), "Value shoul be greater than 0.");
+    }
+}