// Exercise 1: remove quadratic string building (easy)
//
// BuildReport(rows) must return one line per row: "<index>: <name> = <value:0.00>\n".
// The stub uses `report += ...` in a loop, which copies the whole string every time (O(n²) bytes allocated).
// Numbers use the invariant culture ("2.35", never "2,35").
// Rewrite it with a StringBuilder (pre-size it with a reasonable capacity, e.g. rows.Length * 32).
//
// The self-check builds 5,000 rows and measures allocated bytes: the += version allocates hundreds of MB.
//
// Run with: dotnet run (inside a console project).

using System;
using System.Text;

class Program
{
    static string BuildReport((string name, double value)[] rows)
    {
        string report = "";
        for (int i = 0; i < rows.Length; i++) report += FormattableString.Invariant($"{i}: {rows[i].name} = {rows[i].value:0.00}\n");
        return report;
    }

    static void Main()
    {
        int pass = 0, total = 0;
        void Check(string name, bool ok) { total++; if (ok) pass++; Console.WriteLine($"{(ok ? "PASS" : "FAIL")}: {name}"); }

        var small = new[] { ("a", 1.0), ("b", 2.345) };
        Check("format", BuildReport(small) == "0: a = 1.00\n1: b = 2.35\n" || BuildReport(small) == "0: a = 1.00\n1: b = 2.34\n");
        var rows = new (string, double)[5000];
        for (int i = 0; i < rows.Length; i++) rows[i] = ("item" + i, i * 1.5);
        BuildReport(new[] { ("x", 1.0) });
        long before = GC.GetAllocatedBytesForCurrentThread();
        string r = BuildReport(rows);
        long mb = (GC.GetAllocatedBytesForCurrentThread() - before) / 1_000_000;
        Check("all lines present", r.Split('\n').Length == 5001 && r.StartsWith("0: item0 = 0.00\n"));
        Check($"linear allocations ({mb} MB allocated)", mb < 20);

        Console.WriteLine($"--- {pass}/{total} checks passed ---");
    }
}