clean up performance.zig

src/analytics/performance.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const Date = @import("../models/date.zig").Date;
 const Candle = @import("../models/candle.zig").Candle;
 const Dividend = @import("../models/dividend.zig").Dividend;
+const portfolio = @import("../models/portfolio.zig");
 
 /// Minimum holding period (in years) before annualizing returns.
 /// Set below 1.0 to handle trading-day snap (e.g. a "1-year" lookback
@@ -89,11 +90,14 @@ fn totalReturnWithDividendsSnap(
     to: Date,
     start_dir: SearchDirection,
 ) ?PerformanceResult {
+    // When `from` predates all cached candles, the only safe shortcut is
+    // for stable-NAV funds: if the earliest candle we have is at $1, we
+    // can extrapolate backward and synthesize a $1 candle at `from`.
+    // Reuse `stableNavCandle` so the synthesized candle has the same
+    // shape as the one used elsewhere.
     const start = findNearestCandle(candles, from, start_dir) orelse
-        // Stable-NAV fund (e.g. money market): synthesize start candle at $1
-        // when candle history doesn't reach back far enough.
         if (candles.len > 0 and from.lessThan(candles[0].date) and candles[0].close == 1.0)
-            stableNavCandle(from)
+            portfolio.stableNavCandle(from)
         else
             return null;
     const end = findNearestCandle(candles, to, .backward) orelse return null;
@@ -111,7 +115,7 @@ fn totalReturnWithDividendsSnap(
         // For stable-NAV funds, dividends before candle history use $1.
         const price_candle = findNearestCandle(candles, div.ex_date, .backward) orelse
             if (start.close == 1.0)
-                stableNavCandle(div.ex_date)
+                portfolio.stableNavCandle(div.ex_date)
             else
                 continue;
         if (price_candle.close > 0) {
@@ -131,11 +135,9 @@ fn totalReturnWithDividendsSnap(
     };
 }
 
-/// Synthesize a candle with stable $1 NAV for a given date.
-/// Used for money market funds whose NAV is always $1.
-fn stableNavCandle(date: Date) Candle {
-    return .{ .date = date, .open = 1, .high = 1, .low = 1, .close = 1, .adj_close = 1, .volume = 0 };
-}
+// Stable-NAV candle synthesis lives in src/models/portfolio.zig
+// (`portfolio.stableNavCandle`) so every caller agrees on the shape
+// of a synthesized $1 candle. Performance-only heuristics stay here.
 
 /// Convenience: compute 1yr, 3yr, 5yr, 10yr trailing returns from adjusted close.
 /// Uses the last available date as the endpoint.

src/models/portfolio.zig

@@ -1,5 +1,14 @@
 const std = @import("std");
 const Date = @import("date.zig").Date;
+const Candle = @import("candle.zig").Candle;
+
+/// Synthesize a stable-NAV (= $1) candle for a given date. Used when
+/// historical price data for a money-market fund doesn't reach back as
+/// far as the period under analysis — the close is known to be $1 by
+/// construction, so we can extrapolate backward without inventing data.
+pub fn stableNavCandle(date: Date) Candle {
+    return .{ .date = date, .open = 1, .high = 1, .low = 1, .close = 1, .adj_close = 1, .volume = 0 };
+}
 
 /// Type of holding in a portfolio lot.
 pub const LotType = enum {
@@ -806,3 +815,13 @@ test "totalForAccount" {
     const total = portfolio.totalForAccount(allocator, "IRA", prices);
     try std.testing.expectApproxEqAbs(@as(f64, 44500.0), total, 0.01);
 }
+
+test "stableNavCandle: fills all fields at $1" {
+    const c = stableNavCandle(Date.fromYmd(2026, 4, 1));
+    try std.testing.expectEqual(@as(f64, 1), c.close);
+    try std.testing.expectEqual(@as(f64, 1), c.open);
+    try std.testing.expectEqual(@as(f64, 1), c.high);
+    try std.testing.expectEqual(@as(f64, 1), c.low);
+    try std.testing.expectEqual(@as(f64, 1), c.adj_close);
+    try std.testing.expectEqual(@as(u64, 0), c.volume);
+}