2 files changed, 142 insertions, 0 deletions
diff --git a/src/FundLab.Domain/BondRules.fs b/src/FundLab.Domain/BondRules.fs
new file mode 100644
index 0000000..21f1245
--- /dev/null
+++ b/src/FundLab.Domain/BondRules.fs
@@ -0,0 +1,141 @@
+namespace FundLab.Domain
+
+open System
+
+/// Bond pricing/ledger terms. Every value is supplied by instrument metadata
+/// (the AKShare profile probe); nothing about a specific bond is hardcoded here.
+///
+/// Prices for Chinese exchange bonds are quoted per 100 yuan of face value, so
+/// the cash amount of `quantity` 张 at a price `p` is `quantity * p * ParValue / 100`.
+type BondTerms =
+    {
+        /// Face value of one 张, e.g. 100.
+        ParValue: decimal
+        /// Applicable annual coupon rate as a fraction, e.g. 0.02 for 2%.
+        CouponRate: decimal
+        /// Coupons per year (1 = annual, 2 = semiannual).
+        CouponFrequency: int
+        /// 起息日 (interest accrual start).
+        ValueDate: DateOnly
+        /// 到期日.
+        MaturityDate: DateOnly
+        /// Minimum tradable unit in 张 (exchange convertible bonds: 10).
+        MinUnit: decimal
+        /// Settlement lag in trading days (exchange bonds: 0 = T+0).
+        SettlementDays: int
+        /// Commission rate applied to the dirty cash amount.
+        FeeRate: decimal
+    }
+
+module BondTerms =
+    /// Conservative defaults used only when the probe cannot supply a value;
+    /// callers should override with real instrument metadata when available.
+    let create
+        (parValue: decimal)
+        (couponRate: decimal)
+        (couponFrequency: int)
+        (valueDate: DateOnly)
+        (maturityDate: DateOnly)
+        (minUnit: decimal)
+        (settlementDays: int)
+        (feeRate: decimal)
+        : BondTerms =
+        {
+            ParValue = parValue
+            CouponRate = couponRate
+            CouponFrequency = couponFrequency
+            ValueDate = valueDate
+            MaturityDate = maturityDate
+            MinUnit = minUnit
+            SettlementDays = settlementDays
+            FeeRate = feeRate
+        }
+
+/// Pure bond rules: coupon schedule, accrued interest (actual/actual), dirty
+/// price, cash settlement and minimum-unit validation.
+module BondRules =
+    let private dayCount (a: DateOnly) (b: DateOnly) = b.DayNumber - a.DayNumber
+
+    /// Coupon/period boundary dates from 起息日 through 到期日 inclusive.
+    let couponSchedule (terms: BondTerms) : DateOnly list =
+        if terms.CouponFrequency <= 0 || terms.MaturityDate <= terms.ValueDate then
+            []
+        else
+            let monthsPer = 12 / terms.CouponFrequency
+
+            let rec loop (date: DateOnly) (acc: DateOnly list) =
+                if date >= terms.MaturityDate then
+                    List.rev (terms.MaturityDate :: acc)
+                else
+                    loop (date.AddMonths monthsPer) (date :: acc)
+
+            loop terms.ValueDate []
+
+    /// Accrued interest as of `asOf`, actual/actual within the current coupon
+    /// period. Returns 0 when the bond has no coupon, has matured, or accrues
+    /// no interest yet.
+    let accruedInterest (terms: BondTerms) (asOf: DateOnly) : decimal =
+        let schedule = couponSchedule terms
+
+        if schedule.IsEmpty || terms.ParValue <= 0m || terms.CouponRate <= 0m then
+            0m
+        else
+            let couponPerPeriod = terms.ParValue * terms.CouponRate / decimal terms.CouponFrequency
+            let previous = schedule |> List.filter (fun date -> date <= asOf) |> List.tryLast
+            let next = schedule |> List.tryFind (fun date -> date > asOf)
+
+            match previous, next with
+            | Some startDate, Some nextDate ->
+                let periodDays = dayCount startDate nextDate
+
+                if periodDays <= 0 then
+                    0m
+                else
+                    let accruedDays = dayCount startDate asOf
+
+                    Decimal.Round(
+                        couponPerPeriod * decimal accruedDays / decimal periodDays,
+                        6,
+                        MidpointRounding.AwayFromZero
+                    )
+            | _ -> 0m
+
+    /// Dirty (全价) price = clean (净价) price + accrued interest.
+    let dirtyPrice (cleanPrice: decimal) (accrued: decimal) : decimal = cleanPrice + accrued
+
+    /// Cash cost of buying `quantity` 张 at the given dirty price, including the
+    /// commission. `FeeRate` is taken from the terms, not hardcoded.
+    let tradeCost (terms: BondTerms) (quantity: decimal) (dirty: decimal) : decimal =
+        let gross = quantity * dirty * terms.ParValue / 100m
+        let rounded = Decimal.Round(gross, 2, MidpointRounding.AwayFromZero)
+        let fee = Decimal.Round(rounded * terms.FeeRate, 2, MidpointRounding.AwayFromZero)
+        rounded + fee
+
+    /// Settlement date for a trade, skipping weekends. A real exchange trading
+    /// calendar would extend this; weekends are the minimum correctness bar and
+    /// `SettlementDays = 0` keeps T+0 exchange bonds unchanged.
+    let settlementDate (terms: BondTerms) (tradeDate: DateOnly) : DateOnly =
+        let rec advance (date: DateOnly) (remaining: int) =
+            if remaining <= 0 then
+                date
+            else
+                let next = date.AddDays 1
+
+                let adjusted =
+                    match next.DayOfWeek with
+                    | DayOfWeek.Saturday -> next.AddDays 2
+                    | DayOfWeek.Sunday -> next.AddDays 1
+                    | _ -> next
+
+                advance adjusted (remaining - 1)
+
+        advance tradeDate terms.SettlementDays
+
+    /// Quantity must be positive and a whole multiple of the minimum unit.
+    let validateQuantity (terms: BondTerms) (quantity: decimal) : Result<decimal, string> =
+        if quantity <= 0m then
+            Error "quantity must be positive"
+        elif terms.MinUnit > 0m && (quantity % terms.MinUnit) <> 0m then
+            Error(sprintf "quantity must be a multiple of %O 张" terms.MinUnit)
+        else
+            Ok quantity
diff --git a/src/FundLab.Domain/FundLab.Domain.fsproj b/src/FundLab.Domain/FundLab.Domain.fsproj
index 1ecbd5d..9417a4c 100644
--- a/src/FundLab.Domain/FundLab.Domain.fsproj
+++ b/src/FundLab.Domain/FundLab.Domain.fsproj
@@ -14,6 +14,7 @@
     <Compile Include="InvestmentPlan.fs" />
     <Compile Include="Rebalance.fs" />
     <Compile Include="Dividend.fs" />
+    <Compile Include="BondRules.fs" />
     <Compile Include="Performance.fs" />
   </ItemGroup>
 </Project>