LINQ grouping and aggregation

// Ten orders from four states, grouped and totalled. The orders are made up for this lesson.
Order[] orders =
[
    new(1, "KA", "Jul", 1499m), new(2, "MH", "Jul", 899m), new(3, "KA", "Jul", 3250m),
    new(4, "TN", "Aug", 2199m), new(5, "MH", "Aug", 1499m), new(6, "DL", "Aug", 1150m),
    new(7, "KA", "Aug", 899m), new(8, "TN", "Aug", 1499m), new(9, "MH", "Sep", 3250m),
    new(10, "KA", "Sep", 2199m),
];

// GroupBy: one group per state. A group is its key and its orders, in the order they came.
foreach (IGrouping<string, Order> g in orders.GroupBy(o => o.State))
{
    Console.WriteLine($"{g.Key}: {g.Count()} order{(g.Count() == 1 ? "" : "s")}, ids {string.Join(" ", g.Select(o => o.Id))}");
}

// Fold each group into numbers, and sort the states by what they sold.
var perState = orders
    .GroupBy(o => o.State)
    .Select(g => new
    {
        State = g.Key,
        Orders = g.Count(),
        Revenue = g.Sum(o => o.Amount),
        Average = Math.Round(g.Average(o => o.Amount), 2),
        Largest = g.Max(o => o.Amount),
    })
    .OrderByDescending(s => s.Revenue);
Console.WriteLine("State  Orders  Revenue  Average  Largest");
foreach (var s in perState)
{
    Console.WriteLine($"{s.State,-5}  {s.Orders,6}  {s.Revenue,7}  {s.Average,7}  {s.Largest,7}");
}

// The same grouping in query syntax: group ... by ... into names the group for the clauses after it.
var monthly =
    from o in orders
    group o by o.Month into g
    select $"{g.Key}: {g.Count()} orders, Rs {g.Sum(o => o.Amount)}";
Console.WriteLine(string.Join(" | ", monthly));

// The aggregates work on the whole sequence too.
Console.WriteLine($"All orders: Count {orders.Length}, Sum {orders.Sum(o => o.Amount)}, Min {orders.Min(o => o.Amount)}, Max {orders.Max(o => o.Amount)}, Average {orders.Average(o => o.Amount)}");

record Order(int Id, string State, string Month, decimal Amount);

KA: 4 orders, ids 1 3 7 10
MH: 3 orders, ids 2 5 9
TN: 2 orders, ids 4 8
DL: 1 order, ids 6
State  Orders  Revenue  Average  Largest
KA          4     7847  1961.75     3250
MH          3     5648  1882.67     3250
TN          2     3698     1849     2199
DL          1     1150     1150     1150
Jul: 3 orders, Rs 5648 | Aug: 5 orders, Rs 7246 | Sep: 2 orders, Rs 5449
All orders: Count 10, Sum 18343, Min 899, Max 3250, Average 1834.3

// Practice: write ByRate, TotalTax and TopCodes at the bottom so that every check passes, then run this file again.
// Eight lines of four invoices. Each line has its own tax rate (illustrative), and the tax of a line is worked out
// and rounded to two decimal places on the line, as Tax does. A negative line is a credit note for returned goods.
using System.Globalization;

Line[] lines =
[
    new("INV-101", "P100", 5, 240.00m), new("INV-101", "P200", 18, 45.50m),
    new("INV-102", "P300", 5, 610.25m), new("INV-102", "P400", 18, 99.90m),
    new("INV-103", "P100", 5, 120.00m), new("INV-103", "P500", 18, 210.00m),
    new("INV-104", "P200", 18, -45.50m), new("INV-104", "P600", 12, 550.00m),
];

var checks = new List<(string Name, Func<string> Run, string Expected)>
{
    ("one row per rate, lowest rate first", () => string.Join(" | ", ByRate(lines)),
        "5%: taxable 970.25, tax 48.51, lines 3 | 12%: taxable 550.00, tax 66.00, lines 1 | 18%: taxable 309.90, tax 55.78, lines 4"),
    ("total tax", () => Money(TotalTax(lines)), "170.29"),
    ("top 3 codes by tax", () => string.Join(" | ", TopCodes(lines, 3)), "P600 66.00 | P500 37.80 | P300 30.51"),
    ("a credit note cancels its sale", () => TopCodes(lines, 6).Last(), "P200 0.00"),
    ("no lines: no rows", () => string.Join(" | ", ByRate([])), ""),
    ("no lines: no tax", () => Money(TotalTax([])), "0.00"),
};

int passed = 0;
foreach (var (name, run, expected) in checks)
{
    string got;
    try
    {
        got = run();
    }
    catch (Exception e)
    {
        got = e.GetType().Name;
    }
    string verdict = got == expected ? "ok" : $"FAIL, got \"{got}\"";
    if (verdict == "ok")
    {
        passed++;
    }
    Console.WriteLine($"{name}: {verdict}");
}
Console.WriteLine($"{passed} of {checks.Count} checks pass");
return passed == checks.Count ? 0 : 1;

// The tax of one line, rounded to two decimal places, halves away from zero.
static decimal Tax(Line line) => Math.Round(line.Taxable * line.Rate / 100m, 2, MidpointRounding.AwayFromZero);

// Two decimal places with a point, the same on every computer.
static string Money(decimal amount) => amount.ToString("0.00", CultureInfo.InvariantCulture);

// "rate%: taxable T, tax X, lines N" for each rate, lowest rate first; T and X are sums over the rate's lines.
static IEnumerable<string> ByRate(IEnumerable<Line> lines) =>
    throw new NotImplementedException("Group the lines by Rate, order the groups by their key, and Sum each one twice.");

// The sum of the taxes of all lines.
static decimal TotalTax(IEnumerable<Line> lines) =>
    throw new NotImplementedException("Sum the lines with Tax as the selector.");

// "code X" for the n item codes with the most tax over all their lines; equal taxes by code.
static IEnumerable<string> TopCodes(IEnumerable<Line> lines, int n) =>
    throw new NotImplementedException("Group by Code, total each group's Tax, sort by it, highest first, then by code, and Take n.");

record Line(string Invoice, string Code, int Rate, decimal Taxable);

one row per rate, lowest rate first: FAIL, got "NotImplementedException"
total tax: FAIL, got "NotImplementedException"
top 3 codes by tax: FAIL, got "NotImplementedException"
a credit note cancels its sale: FAIL, got "NotImplementedException"
no lines: no rows: FAIL, got "NotImplementedException"
no lines: no tax: FAIL, got "NotImplementedException"
0 of 6 checks pass