Code snippet from The Coding Café - snippet-9.java

snippet-9.java

record TrendSignal(double price, String trend) {}

public static Gatherer<Double, ?, TrendSignal> trendDetection(int lookback) {
    return Gatherer.of(
        () -> new Object() { 
            List<Double> history = new ArrayList<>();  // 1️⃣ Price history buffer
        },
        (state, price, downstream) -> {
            state.history.add(price);                  // 2️⃣ Add to history
            if (state.history.size() >= lookback) {    // 3️⃣ Enough data?
                String trend = detectTrend(state.history);  // 4️⃣ Analyze trend
                downstream.push(new TrendSignal(price, trend));  // 5️⃣ Emit signal
                if (state.history.size() > lookback) {  // 6️⃣ Maintain window size
                    state.history.remove(0);
                }
            }
            return true;
        }
    );
}

private static String detectTrend(List<Double> prices) {
    double first = prices.get(0);
    double last = prices.get(prices.size() - 1);
    double change = (last - first) / first * 100;  // Percentage change
    
    if (change > 2) return "UPTREND ↑";       // 1️⃣ Significant rise
    else if (change < -2) return "DOWNTREND ↓"; // 2️⃣ Significant fall
    else return "SIDEWAYS →";                   // 3️⃣ Relatively flat
}

// Example output:
// Price: $100.50 → SIDEWAYS →
// Price: $102.30 → UPTREND ↑
// Price: $98.75 → DOWNTREND ↓