How to use the fast-check property-based testing library with Deno

fast-check.test.ts

/**
 * Running the fast-check property-based testing library in the Deno 
 * JavaScript/TYpeScript runtime.
 * See: https://github.com/dubzzz/fast-check
 * 
 * This file contains all the 'simple' examples from the fast-check
 * repo using Deno.test for the test functions and assertions 
 * from the Deno standard library. I also added some missing type
 * annotations.
 * 
 * Deno docs: https://deno.land/
 *
 * To run this test file: 
 *  deno test fast-check.test.ts
 */

import fc from 'https://cdn.skypack.dev/fast-check';
import * as _ from 'https://cdn.skypack.dev/lodash';
import {
    assert,
    assertEquals,
  } from "https://deno.land/std@0.90.0/testing/asserts.ts";

/********************** contains() *************************************/  
const contains = (text: string, pattern: string): boolean => text.indexOf(pattern) >= 0;

// string text always contains itself
Deno.test('should always contain itself', () => {
    fc.assert(fc.property(fc.string(), (text: string) => contains(text, text)));
})

// string a + b + c always contains b, whatever the values of a, b and c
Deno.test('should always contain its substrings', () => {
    fc.assert(fc.property(fc.string(), fc.string(), fc.string(), 
        (a: string, b: string, c: string) => {
      // Alternatively: no return statement and direct usage of expect or assert
      return contains(a+b+c, b);
    }));
});

/*********************** indexOf() ************************************/  
const indexOf = (text: string, pattern: string): number => {
    return text.indexOf(pattern);
};

Deno.test('should confirm b is a substring of a + b + c', () => {
    fc.assert(
      fc.property(fc.string(), fc.string(), fc.string(), 
        (a: string, b: string, c: string) => {
        return indexOf(a + b + c, b) !== -1;
      })
    );
});

Deno.test('should return the starting position of the pattern within text if any', () => {
    fc.assert(
      fc.property(fc.string(), fc.string(), fc.string(), 
        (a: string, b: string, c: string) => {
        const text = a + b + c;
        const pattern = b;
        const index = indexOf(text, pattern);
        return index === -1 || text.substr(index, pattern.length) === pattern;
      })
    );
});

/************************** fibronacci() *********************************/  
function fibonacci(n: number) {
    let a = 0n;
    let b = 1n;
    for (let i = 0; i !== n; ++i) {
      const previousA = a;
      a = b;
      b = previousA + b;
    }
    return a;
  }

// The complexity of the algorithm is O(n)
// As a consequence we limit the value of n to 1000
const MaxN = 1000;

Deno.test('should be equal to the sum of fibonacci(n-1) and fibonacci(n-2)', () => {
    fc.assert(
      fc.property(fc.integer(2, MaxN), (n: number) => {
        assert(fibonacci(n) === (fibonacci(n - 1) + fibonacci(n - 2)));
      })
    );
  });

  // The following properties are listed on the Wikipedia page:
  // https://fr.wikipedia.org/wiki/Suite_de_Fibonacci#Divisibilit%C3%A9_des_nombres_de_Fibonacci

Deno.test('should fulfill fibonacci(p)*fibonacci(q+1)+fibonacci(p-1)*fibonacci(q) = fibonacci(p+q)', () => {
    fc.assert(
      fc.property(fc.integer(1, MaxN), fc.integer(0, MaxN), (p: number, q: number) => {
        assertEquals(fibonacci(p + q), fibonacci(p) * fibonacci(q + 1) + fibonacci(p - 1) * fibonacci(q));
      })
    );
  });

Deno.test('should fulfill fibonacci(2p-1) = fibo²(p-1)+fibo²(p)', () => {
    // Special case of the property above
    fc.assert(
      fc.property(fc.integer(1, MaxN), (p: number) => {
        assertEquals(fibonacci(2 * p - 1), fibonacci(p - 1) * fibonacci(p - 1) + fibonacci(p) * fibonacci(p));
      })
    );
  });

Deno.test('should fulfill Catalan identity', () => {
    fc.assert(
      fc.property(fc.integer(0, MaxN), fc.integer(0, MaxN), (a: number, b: number) => {
        const [p, q] = a < b ? [b, a] : [a, b];
        const sign = (p - q) % 2 === 0 ? 1n : -1n; // (-1)^(p-q)
        assertEquals(fibonacci(p) * fibonacci(p) - fibonacci(p - q) * fibonacci(p + q), sign * fibonacci(q) * fibonacci(q));
      })
    );
  });

Deno.test('should fulfill Cassini identity', () => {
    fc.assert(
      fc.property(fc.integer(1, MaxN), fc.integer(0, MaxN), (p: number) => {
        const sign = p % 2 === 0 ? 1n : -1n; // (-1)^p
        assert(fibonacci(p + 1) * fibonacci(p - 1) - fibonacci(p) * fibonacci(p) === sign);
      })
    );
  });

Deno.test('should fibonacci(nk) divisible by fibonacci(n)', () => {
    fc.assert(
      fc.property(fc.integer(1, MaxN), fc.integer(0, 100), (n: number, k: number) => {
        assert(fibonacci(n * k) % fibonacci(n) === 0n);
      })
    );
  });

Deno.test('should fulfill gcd(fibonacci(a), fibonacci(b)) = fibonacci(gcd(a,b))', () => {
    fc.assert(
      fc.property(fc.integer(1, MaxN), fc.integer(1, MaxN), (a: number, b: number) => {
        const gcd = <T extends bigint | number>(a: T, b: T, zero: T): T => {
          a = a < zero ? (-a as T) : a;
          b = b < zero ? (-b as T) : b;
          if (b > a) {
            const temp = a;
            a = b;
            b = temp;
          }
          // eslint-disable-next-line no-constant-condition
          while (true) {
            if (b == zero) return a;
            a = (a % b) as T;
            if (a == zero) return b;
            b = (b % a) as T;
          }
        };
        assert(gcd(fibonacci(a), fibonacci(b), 0n) === fibonacci(gcd(a, b, 0)));
      })
    );
  });

/********************* sort() **************************************/  
const sortInternal = <T>(tab: T[], start: number, end: number, cmp: (a: T, b: T) => boolean): T[] => {
    if (end - start < 2) return tab;
  
    let pivot = start;
    for (let idx = start + 1; idx < end; ++idx) {
      if (!cmp(tab[start], tab[idx])) {
        let prev = tab[++pivot];
        tab[pivot] = tab[idx];
        tab[idx] = prev;
      }
    }
    let prev = tab[pivot];
    tab[pivot] = tab[start];
    tab[start] = prev;
  
    sortInternal(tab, start, pivot, cmp);
    sortInternal(tab, pivot + 1, end, cmp);
    return tab;
  };
  
  export const sort = <T>(tab: T[], compare?: (a: T, b: T) => boolean): T[] => {
    return sortInternal([...tab], 0, tab.length, compare || ((a, b) => a < b));
  };
  
Deno.test('should have the same length as source', () => {
    fc.assert(
      fc.property(fc.array(fc.integer()), (data: Array<number>) => {
        assert(sort(data).length === data.length);
      })
    );
  });

Deno.test('should have exactly the same number of occurences as source for each item', () => {
    fc.assert(
      fc.property(fc.array(fc.integer()), (data: Array<number>) => {
        const sorted = sort(data);
        assertEquals(_.groupBy(sorted), _.groupBy(data));
      })
    );
  });

Deno.test('should produce an ordered array', () => {
    fc.assert(
      fc.property(fc.array(fc.integer()), (data: Array<number>) => {
        const sorted = sort(data);
        for (let idx = 1; idx < sorted.length; ++idx) {
          assert(sorted[idx - 1] <= sorted[idx]);
        }
      })
    );
  });

Deno.test('should produce an ordered array with respect to a custom compare function', () => {
    fc.assert(
      fc.property(fc.array(fc.integer()), fc.compareBooleanFunc(), (data: Array<number>, 
        compare: (n1: number, n2: number) => boolean) => {

        const sorted = sort(data, compare);
        for (let idx = 1; idx < sorted.length; ++idx) {
          // compare(sorted[idx], sorted[idx - 1]):
          // = true : sorted[idx - 1]  > sorted[idx]
          // = false: sorted[idx - 1] <= sorted[idx]
          assert(compare(sorted[idx], sorted[idx - 1]) === false);
          // Meaning of compare:
          // a = b means in terms of ordering a and b are equivalent
          // a < b means in terms of ordering a comes before b
          // One important property is: a < b and b < c implies a < c
        }
      })
    );
  });