import fs from 'fs';
import path from 'path';
import lockfile from 'proper-lockfile';


function cat(fn: string) {
    try {
        return fs.readFileSync(fn, 'utf-8');
    }
    catch { return undefined; }
}

/**
 * If `fn` contains `expectedValue`, do nothing;
 * Otherwise run `whenNeq` and update `fn`.
 * @returns `true` iff `fn` already contained `expectedValue`.
 */
async function cas(fn: string, expectedValue: string, whenNeq: () => void | Promise<void>) {
    if (cat(fn) === expectedValue) return true;  /* fast lane */
    let dir = path.dirname(fn),
        lopts = {lockfilePath: fn + '.lock', retries: cas_retries};
    await lockfile.lock(dir, lopts);
    try {
        if (cat(fn) === expectedValue) return true;
        else {
            await whenNeq();
            fs.writeFileSync(fn, expectedValue);
            return false;
        }
    }
    finally {
        lockfile.unlock(dir, lopts);
    }
}

const cas_retries = {forever: true, factor: 1, minTimeout: 500, maxTimeout: 1000};

function dirstamp(fn: string) {
    try { var s = fs.statSync(fn).mtime.toISOString(); } catch { s = '??'; }
    return `${fs.realpathSync(fn)} @ ${s}`;
}

function ln_sf(target: string, source: string) {
    try { fs.unlinkSync(source); }
    catch { }
    fs.symlinkSync(target, source);
}

function existsExec(p) {
    try {
        let stat = fs.statSync(p);
        return stat && stat.isFile() && (stat.mode & fs.constants.S_IXUSR);
    }
    catch (e) { return false; }
}

function existsDir(p) {
    try {
        let stat = fs.statSync(p);
        return stat && stat.isDirectory();
    }
    catch (e) { return false; }        
}

export { cat, cas, dirstamp, ln_sf, existsExec, existsDir }