centralize annualized return calc

TODO.md

@@ -11,10 +11,8 @@ Example: 500 shares of AMZN at $225, with 3 sold calls at $220 strike.
 ## Human review of analytics modules
 
 AI review complete; human review still needed for:
-- `src/analytics/performance.zig` — Morningstar-style trailing returns
 - `src/analytics/risk.zig` — Sharpe, volatility, max drawdown, portfolio summary
 - `src/analytics/indicators.zig` — SMA, Bollinger Bands, RSI
-- `src/models/classification.zig` — Sector/geo/asset-class metadata parsing
 
 Known issues from AI review:
 - `risk.zig` uses population variance (divides by n) instead of sample

src/analytics/performance.zig

@@ -3,6 +3,19 @@ const Date = @import("../models/date.zig").Date;
 const Candle = @import("../models/candle.zig").Candle;
 const Dividend = @import("../models/dividend.zig").Dividend;
 
+/// Minimum holding period (in years) before annualizing returns.
+/// Set below 1.0 to handle trading-day snap (e.g. a "1-year" lookback
+/// that lands on 362 days due to weekends).
+const min_annualize_years = 0.95;
+
+/// Compute CAGR from a total return over a given number of years.
+/// Returns null for periods shorter than `min_annualize_years` where
+/// extrapolating to a full year would be misleading.
+inline fn annualizedReturn(total: f64, years: f64) ?f64 {
+    if (years < min_annualize_years) return null;
+    return std.math.pow(f64, 1.0 + total, 1.0 / years) - 1.0;
+}
+
 /// Performance calculation results, Morningstar-style.
 pub const PerformanceResult = struct {
     /// Total return over the period (e.g., 0.25 = 25%)
@@ -40,10 +53,7 @@ fn totalReturnFromAdjCloseSnap(candles: []const Candle, from: Date, to: Date, st
 
     return .{
         .total_return = total,
-        .annualized_return = if (years >= 0.95)
-            std.math.pow(f64, 1.0 + total, 1.0 / years) - 1.0
-        else
-            null,
+        .annualized_return = annualizedReturn(total, years),
         .from = start.date,
         .to = end.date,
     };
@@ -104,10 +114,7 @@ fn totalReturnWithDividendsSnap(
 
     return .{
         .total_return = total,
-        .annualized_return = if (years >= 0.95)
-            std.math.pow(f64, 1.0 + total, 1.0 / years) - 1.0
-        else
-            null,
+        .annualized_return = annualizedReturn(total, years),
         .from = start.date,
         .to = end.date,
     };