2 files changed, 85 insertions, 0 deletions
diff --git a/src/FundLab.Domain/FundLab.Domain.fsproj b/src/FundLab.Domain/FundLab.Domain.fsproj
index 9e46c00..a2c5c24 100644
--- a/src/FundLab.Domain/FundLab.Domain.fsproj
+++ b/src/FundLab.Domain/FundLab.Domain.fsproj
@@ -8,6 +8,7 @@
   <ItemGroup>
     <Compile Include="Domain.fs" />
     <Compile Include="Ledger.fs" />
+    <Compile Include="Redemption.fs" />
     <Compile Include="Performance.fs" />
   </ItemGroup>
 </Project>
diff --git a/src/FundLab.Domain/Redemption.fs b/src/FundLab.Domain/Redemption.fs
new file mode 100644
index 0000000..a3e9188
--- /dev/null
+++ b/src/FundLab.Domain/Redemption.fs
@@ -0,0 +1,84 @@
+namespace FundLab.Domain
+
+open System
+
+module RedemptionPolicy =
+    let unitsMaximum = 99999999999999999999.99999999m
+    let cashMaximum = 999999999999999999.99m
+
+    let roundDown (scale: int) (value: decimal) : decimal =
+        let factor = decimal (pown 10 scale)
+        Decimal.Truncate(value * factor) / factor
+
+    let validateRequest
+        (availableUnits: decimal)
+        (requestedUnits: decimal)
+        (feeAmount: decimal)
+        : Result<unit, string> =
+        if requestedUnits <= 0m then
+            Error "redemption units must be positive"
+        elif requestedUnits > unitsMaximum then
+            Error "redemption units exceed supported precision"
+        elif requestedUnits > availableUnits then
+            Error "requested units exceed available holdings"
+        elif feeAmount < 0m then
+            Error "redemption fee cannot be negative"
+        elif feeAmount > cashMaximum then
+            Error "redemption fee exceeds supported precision"
+        else
+            Ok ()
+
+    type RedemptionComputation =
+        {
+            GrossCash: decimal
+            Fee: decimal
+            Proceeds: decimal
+            RedeemedCost: decimal
+        }
+
+    let compute
+        (requestedUnits: decimal)
+        (unitNav: decimal)
+        (feeAmount: decimal)
+        (positionCost: decimal)
+        (positionUnits: decimal)
+        : Result<RedemptionComputation, string> =
+        if unitNav <= 0m then
+            Error "unit nav must be positive"
+        elif unitNav < 0.00000001m then
+            Error "unit nav is below database precision"
+        elif requestedUnits <= 0m then
+            Error "redemption units must be positive"
+        elif positionUnits <= 0m then
+            Error "position units must be positive"
+        elif requestedUnits > positionUnits then
+            Error "redemption units exceed position units"
+        elif feeAmount < 0m then
+            Error "redemption fee cannot be negative"
+        else
+            let gross = requestedUnits * unitNav
+            let grossRounded = roundDown 2 gross
+
+            if grossRounded <= 0m then
+                Error "redemption cash rounds to zero"
+            elif feeAmount > grossRounded then
+                Error "redemption fee exceeds redemption value"
+            else
+                let proceeds = grossRounded - feeAmount
+
+                if proceeds <= 0m then
+                    Error "redemption proceeds rounds to zero"
+                else
+                    let redeemedCost =
+                        if requestedUnits = positionUnits then
+                            positionCost
+                        else
+                            roundDown 2 (positionCost * requestedUnits / positionUnits)
+
+                    Ok
+                        {
+                            GrossCash = grossRounded
+                            Fee = feeAmount
+                            Proceeds = proceeds
+                            RedeemedCost = redeemedCost
+                        }