Add scheduled investment plans (3d-10)

1 files changed, 97 insertions, 0 deletions

diff --git a/src/FundLab.Domain/InvestmentPlan.fs b/src/FundLab.Domain/InvestmentPlan.fs
new file mode 100644
index 0000000..ecc3425
--- /dev/null
+++ b/src/FundLab.Domain/InvestmentPlan.fs
@@ -0,0 +1,97 @@
+namespace FundLab.Domain
+
+open System
+
+type InvestmentFrequency =
+    | Daily
+    | Weekly
+    | Monthly
+
+module InvestmentPlanPolicy =
+    let cashMaximum = 999999999999999999.99m
+
+    let validateAmount (amount: decimal) : Result<unit, string> =
+        if amount <= 0m then
+            Error "investment plan amount must be positive"
+        elif Decimal.Round(amount, 2) <> amount then
+            Error "investment plan amount exceeds cash precision"
+        elif amount > cashMaximum then
+            Error "investment plan amount exceeds database precision"
+        else
+            Ok()
+
+    let parseFrequency (text: string) : InvestmentFrequency option =
+        match text with
+        | null -> None
+        | "daily" -> Some InvestmentFrequency.Daily
+        | "weekly" -> Some InvestmentFrequency.Weekly
+        | "monthly" -> Some InvestmentFrequency.Monthly
+        | _ -> None
+
+    let frequencyText (frequency: InvestmentFrequency) : string =
+        match frequency with
+        | InvestmentFrequency.Daily -> "daily"
+        | InvestmentFrequency.Weekly -> "weekly"
+        | InvestmentFrequency.Monthly -> "monthly"
+
+    let private isWeekend (date: DateOnly) =
+        date.DayOfWeek = DayOfWeek.Saturday || date.DayOfWeek = DayOfWeek.Sunday
+
+    let private rollToWeekday (date: DateOnly) : DateOnly =
+        let mutable candidate = date
+
+        while isWeekend candidate do
+            candidate <- candidate.AddDays 1
+
+        candidate
+
+    let private addMonthsPreservingDay (anchor: DateOnly) (months: int) : DateOnly =
+        let targetMonthIndex = anchor.Year * 12 + (anchor.Month - 1) + months
+        let year = targetMonthIndex / 12
+        let month = targetMonthIndex % 12 + 1
+        let daysInTargetMonth = DateTime.DaysInMonth(year, month)
+        DateOnly(year, month, min anchor.Day daysInTargetMonth)
+
+    /// First scheduled run date on or after `fromDate`, rolled forward off weekends.
+    /// The anchor date is the plan's first scheduled run date. Calendar approximation:
+    /// only weekends are skipped; public holidays are not modeled yet (no fabrication).
+    let nextRunDate (frequency: InvestmentFrequency) (anchorDate: DateOnly) (fromDate: DateOnly) : DateOnly =
+        let baseDate = if fromDate < anchorDate then anchorDate else fromDate
+
+        match frequency with
+        | InvestmentFrequency.Daily -> rollToWeekday baseDate
+        | InvestmentFrequency.Weekly ->
+            let steps = max 0 ((baseDate.DayNumber - anchorDate.DayNumber + 6) / 7)
+            rollToWeekday (anchorDate.AddDays(steps * 7))
+        | InvestmentFrequency.Monthly ->
+            let monthsBetween = (baseDate.Year - anchorDate.Year) * 12 + (baseDate.Month - anchorDate.Month)
+
+            let candidateMonths =
+                if baseDate.Day <= anchorDate.Day || monthsBetween < 0 then
+                    monthsBetween
+                else
+                    monthsBetween + 1
+
+            rollToWeekday (addMonthsPreservingDay anchorDate (max 0 candidateMonths))
+
+    /// Due run dates from `dueFrom` through `endDate` inclusive, skipping weekends.
+    /// Empty when nothing is due in the window. This is the "calendar truncation":
+    /// a run never fires past `endDate`, and a repeat drive over the same window
+    /// yields the same dates.
+    let dueDates
+        (frequency: InvestmentFrequency)
+        (anchorDate: DateOnly)
+        (dueFrom: DateOnly)
+        (endDate: DateOnly)
+        : DateOnly list =
+        let rec loop (current: DateOnly) (acc: DateOnly list) =
+            if current > endDate then
+                List.rev acc
+            else
+                loop (nextRunDate frequency anchorDate (current.AddDays 1)) (current :: acc)
+
+        if dueFrom > endDate then
+            []
+        else
+            let start = nextRunDate frequency anchorDate dueFrom
+            if start > endDate then [] else loop start []