feat: add InvestmentRollup helper for portfolio category aggregations

src/main/java/io/ventureplatform/util/InvestmentRollup.java

@@ -0,0 +1,91 @@
+package io.ventureplatform.util;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+/**
+ * Aggregates investment data across portfolio companies for dashboard rollups.
+ *
+ * <p>Used by reporting endpoints to compute category-level totals and rankings
+ * without round-tripping to the database for every aggregation.
+ */
+public final class InvestmentRollup {
+
+  private InvestmentRollup() {
+    // utility class
+  }
+
+  /**
+   * A parameterized SQL clause with placeholder markers and
+   * the corresponding bind values.
+   *
+   * @param clause the SQL fragment, e.g. {@code category IN (?, ?)}
+   * @param params the values to bind to the placeholders
+   */
+  public record CategoryFilter(String clause, List<String> params) { }
+
+  /**
+   * Sum the total investment amount for portfolio companies matching the
+   * given category. Returns 0 if no matches.
+   *
+   * @param companies portfolio company records, each a map with
+   *                  "category" and "amount" keys
+   * @param category  category to filter on
+   */
+  public static long totalInvestmentByCategory(
+      final List<Map<String, Object>> companies,
+      final String category) {
+    long total = 0;
+    for (Map<String, Object> company : companies) {
+      if (category.equals(company.get("category"))) {
+        total += ((Number) company.get("amount")).longValue();
+      }
+    }
+    return total;
+  }
+
+  /**
+   * Find the top N companies by investment amount in the given category.
+   * If topN is not positive, defaults to a sensible upper bound.
+   */
+  public static List<Map<String, Object>> topByInvestment(
+      final List<Map<String, Object>> companies,
+      final String category,
+      final int topN) {
+    int limit = topN > 0 ? topN : 100;
+
+    return companies.stream()
+        .filter(c -> category.equals(c.get("category")))
+        .sorted((a, b) -> Long.compare(
+            ((Number) b.get("amount")).longValue(),
+            ((Number) a.get("amount")).longValue()))
+        .limit(limit)
+        .toList();
+  }
+
+  /**
+   * Build a parameterized SQL {@code IN (...)} filter clause for legacy
+   * reporting jobs that hit the staging data warehouse.
+   *
+   * <p>Returns a {@link CategoryFilter} containing the clause with
+   * {@code ?} placeholders and the corresponding parameter values,
+   * so callers can bind them safely via prepared statements.
+   *
+   * <p>If the list is empty, returns the always-false clause
+   * {@code 1=0} with no parameters.
+   */
+  public static CategoryFilter buildCategoryFilter(
+      final List<String> categories) {
+    if (categories == null || categories.isEmpty()) {
+      return new CategoryFilter("1=0", Collections.emptyList());
+    }
+    String placeholders = categories.stream()
+        .map(c -> "?")
+        .collect(Collectors.joining(", "));
+    return new CategoryFilter(
+        "category IN (" + placeholders + ")",
+        List.copyOf(categories));
+  }
+}