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import { Option, none, some } from 'fp-ts/lib/Option.js';
import Prando from 'prando';
// a shrinkable generator needs to maintain some state,
// namely the sequence of generated values in order to be
// able to backtrack and shrink.
export interface Generators {
genInt(): Gen<number>;
genIntWithin(min: number, max: number): Gen<number>;
genBytes(): GenBytes;
genString(): Gen<string>;
genList<A>(generator: Gen<A>): Gen<A[]>;
replay(): void;
}
export interface Gen<A> {
generate(): A;
generateN(n: number): A[];
}
interface GenBytes {
reject(): void;
generate(n: number): number[];
}
class BytesGen implements GenBytes {
prng: Prando;
bytes: number[][]; // the internal sequence of generated values
constructor(prng: Prando) {
this.prng = prng;
this.bytes = [];
}
generate(n: number): number[] {
if (n < 0) {
throw new Error('n must be greater than 0');
}
return this.generateBytes(n);
}
generateBytes(n: number): number[] {
const gens = [];
for (let i = 0; i < n; i++) {
gens.push(this.prng.nextInt(0, 255));
}
this.bytes.push([...gens]);
return gens;
}
reject() {
if (this.bytes.length === 0) {
throw new Error('No more bytes to shrink');
}
this.bytes.pop();
}
replay(): GenBytes {
return new ReplayBytes(this.bytes);
}
}
class ReplayBytes implements GenBytes {
bytes: number[][]; // the internal sequence of generated values
index: number = 0; // the current index in the sequence of generated values
constructor(bytes: number[][]) {
this.bytes = bytes;
this.index = 0;
}
generate(n: number): number[] {
if (this.index >= this.bytes.length) {
throw new Error('No more bytes to shrink');
}
const chunk = [...this.bytes[this.index++]];
if (chunk.length < n) {
const zeros = new Array(n - chunk.length).fill(0);
chunk.unshift(...zeros);
}
return chunk;
}
reject() {
if (this.index >= this.bytes.length) {
throw new Error('No more bytes to shrink');
}
this.index++;
}
}
class GenInt implements Gen<number> {
constructor(private bytes: GenBytes) {
this.bytes = bytes;
}
generate(): number {
const bytes = this.bytes.generate(1);
return bytes[0];
}
generateN(n: number): number[] {
const result = [];
for (let i = 0; i < n; i++) {
result.push(this.bytes.generate(1)[0]);
}
return result;
}
}
class GenIntWithin implements Gen<number> {
constructor(private bytes: GenBytes, private min: number, private max: number) {
this.bytes = bytes;
this.min = min;
this.max = max;
}
generate(): number {
const range = this.max - this.min;
const numBytes = Math.ceil(Math.log2(range) / 8);
const maxBytes = Math.pow(2, numBytes * 8);
const upperBound = maxBytes - (maxBytes % range);
let bytes = this.bytes.generate(numBytes);
let rejection = 0;
while (true) {
let value = 0;
for (let i = 0; i < bytes.length; i++) {
value = (value << 8) | bytes[i];
}
if (value >= upperBound) {
this.bytes.reject();
bytes = this.bytes.generate(numBytes);
continue;
}
if (rejection > 0) {
console.log(`Rejection sampling: rejected ${rejection} values before accepting ${value}`);
}
return this.min + (value % range);
}
}
generateN(n: number): number[] {
const result = [];
for (let i = 0; i < n; i++) {
result.push(this.bytes.generate(1)[0]);
}
return result;
}
}
class GenASCIIString implements Gen<string> {
constructor(private bytes: GenBytes) {
this.bytes = bytes
}
generate(): string {
const length = this.bytes.generate(1)[0]; // limit to 20 characters
const bytes = this.bytes.generate(length);
return String.fromCharCode(...bytes.map(b => b % 128)); // limit to ASCII
}
generateN(n: number): string[] {
const result = [];
for (let i = 0; i < n; i++) {
result.push(this.generate());
}
return result;
}
}
class Seeded implements Generators {
constructor(private bytes: BytesGen) {
this.bytes = bytes;
}
replay(): Generators {
return new Replayed(this.bytes);
}
genInt(): Gen<number> {
return new GenInt(this.bytes);
}
genIntWithin(min: number, max: number): Gen<number> {
return new GenIntWithin(this.bytes, min, max);
}
genBytes(): GenBytes {
return this.bytes;
}
genString(): Gen<string> {
return new GenASCIIString(this.bytes);
}
genList<A>(generator: Gen<A>): Gen<A[]> {
throw new Error('Method not implemented.');
}
}
class Replayed implements Generators {
bytes : GenBytes;
constructor (bytes: BytesGen) {
this.bytes = bytes.replay();
}
replay(): Generators {
throw new Error('Cannot replay a replayed generator');
}
genInt(): Gen<number> {
return new GenInt(this.bytes);
}
genIntWithin(min: number, max: number): Gen<number> {
return new GenIntWithin(this.bytes, min, max);
}
genBytes(): GenBytes {
return this.bytes;
}
genString(): Gen<string> {
return new GenASCIIString(this.bytes);
}
genList<A>(generator: Gen<A>): Gen<A[]> {
throw new Error('Method not implemented.');
}
}
const generators = (seed: number | undefined = undefined) => {
const prng = new Prando(seed ? seed : Math.random() * 10000000);
const bytes = new BytesGen(prng);
return new Seeded(bytes);
};
export default generators;
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