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📜 WAYPOINT LESSON

Checkpoint — The Analytics Engine

⭐⭐ intermediate⏳ 25 min read📍 Lesson 112 of 265

Combine grouping, joining, and aggregation into one report with honest single-pass semantics.

The gate (mini-build)

A tiny sales analytics engine over in-memory rows:

  1. Revenue(IEnumerable<Sale> sales) → total revenue (decimal).
  2. BestCategory(IEnumerable<Sale> sales) → category name with the highest revenue (ties: earliest alphabetically).
  3. CustomerMatrix(IEnumerable<Sale> sales, IEnumerable<string> customers) → per-customer totals in the ORDER GIVEN (zero for customers with no sales), formatted "name=total".
  4. Run(IEnumerable<Sale> sales, IEnumerable<string> customers) → a report string combining all three, computed from ONE enumeration of sales (graded with a counting source — a second pull fails).

Sale record: (string Customer, string Category, decimal Amount).