replaces `#include filename` with the contents of the file

preprocess.mjs

#!/usr/bin/env node
// @ts-check

import { createReadStream } from "node:fs";
import { realpath } from "node:fs/promises";
import * as rl from "node:readline/promises";
import * as path from "node:path";

const fileToProcess = process.argv[2];
if (!fileToProcess) {
    console.error("No file specified");
    process.exit(1);
}

emitFile(await realpath(path.resolve(fileToProcess)), []);

/**
 * @param {import("fs").PathLike} file
 * @param {import("fs").PathLike[]} alreadyIncluded
 */
async function emitFile(file, alreadyIncluded) {
    const lines = rl.createInterface({
        input: createReadStream(file),
    });

    for await (const line of lines) {
        if (line.startsWith("#")) {
            const includeStatement = /^#include\s+([\w\./-]+)$/gi.exec(line);
            if (includeStatement) {
                const includedFile = includeStatement[1];

                let includedFileFullPath = path.resolve(path.dirname(file), includedFile);
                if (file === includedFileFullPath) {
                    console.error(`Circular dependency detected: ${file} -> ${includedFile}, skipping include`);
                    continue;
                }

                includedFileFullPath = await realpath(includedFileFullPath);
                if (file === includedFileFullPath || alreadyIncluded.includes(includedFileFullPath)) {
                    console.error(`Circular dependency detected: ${file} -> ${includedFile}, skipping include`);
                } else {
                    await emitFile(includedFileFullPath, [...alreadyIncluded, file]);
                }
                continue;
            }
        }
        process.stdout.write(line + "\n");
    }
}