/* @flow */

import { randomBytes } from 'crypto';

// Returns a new random hex string of the given even size.

export function randomHexString(size: number): string {

if (size === 0) {

throw new Error('Zero-length randomHexString is useless.');

}

if (size % 2 !== 0) {

throw new Error('randomHexString size must be divisible by 2.')

}

return randomBytes(size/2).toString('hex');

}

// Returns a new random alphanumeric string of the given size.

//

// Note: to simplify implementation, the result has slight modulo bias,

// because chars length of 62 doesn't divide the number of all bytes

// (256) evenly. Such bias is acceptable for most cases when the output

// length is long enough and doesn't need to be uniform.

export function randomString(size: number): string {

if (size === 0) {

throw new Error('Zero-length randomString is useless.');

}

let chars = ('ABCDEFGHIJKLMNOPQRSTUVWXYZ' +

'abcdefghijklmnopqrstuvwxyz' +

'0123456789');

let objectId = '';

let bytes = randomBytes(size);

for (let i = 0; i < bytes.length; ++i) {

objectId += chars[bytes.readUInt8(i) % chars.length];

}

return objectId;

}

// Returns a new random alphanumeric string suitable for object ID.

export function newObjectId(): string {

//TODO: increase length to better protect against collisions.

return randomString(10);

}

// Returns a new random hex string suitable for secure tokens.

export function newToken(): string {

return randomHexString(32);

}