Context engineering for agents

// A small repository: file -> [tokens, files it imports, words it contains].
const REPO = new Map<string, [number, string[], string[]]>([
  ["api/routes.py", [1200, ["billing/refund.py", "auth/session.py"], ["refund", "login"]]],
  ["billing/refund.py", [900, ["billing/money.py", "billing/policy.py"], ["refund"]]],
  ["billing/money.py", [400, [], ["round", "cents"]]],
  ["billing/policy.py", [500, [], ["refund", "window"]]],
  ["billing/invoice.py", [1100, ["billing/money.py"], ["invoice"]]],
  ["auth/session.py", [800, [], ["login"]]],
  ["auth/oauth.py", [1500, [], ["login"]]],
  ["ui/checkout.tsx", [2600, [], ["checkout", "refund"]]],
  ["docs/CHANGELOG.md", [6000, [], ["refund", "invoice", "login"]]],
  ["tests/test_refund.py", [700, ["billing/refund.py"], ["refund"]]],
]);
const CULPRIT = "billing/money.py"; // bug report: "a refund of $19.99 comes back as $19.98" (a rounding bug)

const everything = () => [...REPO.keys()];
const keyword = (word: string) => [...REPO].filter(([, [, , words]]) => words.includes(word)).map(([f]) => f);

// Start from the failing test and follow imports, the way an agent uses search and an import graph.
function followImports(start: string): string[] {
  const seen: string[] = [], todo = [start];
  while (todo.length) {
    const f = todo.shift()!;
    if (!seen.includes(f)) {
      seen.push(f);
      todo.push(...REPO.get(f)![1]);
    }
  }
  return seen;
}

const runs: [string, string[]][] = [["load everything", everything()], ['keyword search "refund"', keyword("refund")],
  ["failing test + imports", followImports("tests/test_refund.py")]];
for (const [name, files] of runs) {
  const tokens = files.reduce((s, f) => s + REPO.get(f)![0], 0);
  const found = files.includes(CULPRIT) ? "includes" : "misses";
  console.log(`${name.padEnd(24)} ${String(files.length).padStart(2)} files ${tokens.toLocaleString("en-US").padStart(6)} tokens, ${found} the culprit`);
}

import java.util.*;

public class Main {
    record FileInfo(int tokens, List<String> imports, Set<String> words) {}

    // A small repository: file -> tokens, files it imports, words it contains.
    static final Map<String, FileInfo> REPO = new LinkedHashMap<>();
    static {
        REPO.put("api/routes.py", new FileInfo(1200, List.of("billing/refund.py", "auth/session.py"), Set.of("refund", "login")));
        REPO.put("billing/refund.py", new FileInfo(900, List.of("billing/money.py", "billing/policy.py"), Set.of("refund")));
        REPO.put("billing/money.py", new FileInfo(400, List.of(), Set.of("round", "cents")));
        REPO.put("billing/policy.py", new FileInfo(500, List.of(), Set.of("refund", "window")));
        REPO.put("billing/invoice.py", new FileInfo(1100, List.of("billing/money.py"), Set.of("invoice")));
        REPO.put("auth/session.py", new FileInfo(800, List.of(), Set.of("login")));
        REPO.put("auth/oauth.py", new FileInfo(1500, List.of(), Set.of("login")));
        REPO.put("ui/checkout.tsx", new FileInfo(2600, List.of(), Set.of("checkout", "refund")));
        REPO.put("docs/CHANGELOG.md", new FileInfo(6000, List.of(), Set.of("refund", "invoice", "login")));
        REPO.put("tests/test_refund.py", new FileInfo(700, List.of("billing/refund.py"), Set.of("refund")));
    }
    static final String CULPRIT = "billing/money.py"; // bug report: "a refund of $19.99 comes back as $19.98" (a rounding bug)

    static List<String> keyword(String word) {
        List<String> out = new ArrayList<>();
        REPO.forEach((f, info) -> { if (info.words().contains(word)) out.add(f); });
        return out;
    }

    // Start from the failing test and follow imports, the way an agent uses search and an import graph.
    static List<String> followImports(String start) {
        List<String> seen = new ArrayList<>();
        Deque<String> todo = new ArrayDeque<>(List.of(start));
        while (!todo.isEmpty()) {
            String f = todo.poll();
            if (!seen.contains(f)) { seen.add(f); todo.addAll(REPO.get(f).imports()); }
        }
        return seen;
    }

    public static void main(String[] args) {
        Map<String, List<String>> runs = new LinkedHashMap<>();
        runs.put("load everything", new ArrayList<>(REPO.keySet()));
        runs.put("keyword search \"refund\"", keyword("refund"));
        runs.put("failing test + imports", followImports("tests/test_refund.py"));
        runs.forEach((name, files) -> {
            int tokens = files.stream().mapToInt(f -> REPO.get(f).tokens()).sum();
            String found = files.contains(CULPRIT) ? "includes" : "misses";
            System.out.println(String.format(Locale.US, "%-24s %2d files %,6d tokens, %s the culprit", name, files.size(), tokens, found));
        });
    }
}