Proptest: Strategy to generate vectors of vectors - rust

Question

I want to generate DAGs with proptest. The algorithm that I pick would be this. I've written the plain algorithm below -- but I need help transforming this to a proptest strategy.
What would a strategy need to look like that did the same as the below code but without using a random number generator? (It goes without saying that random number generators are bad for property-based testing.)
Standard code without proptest strategy:
(https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=2de4a757a96d123bf83b5157e0633d33)
use rand::Rng;
fn main() {
println!("{:?}", random_vec_of_vec());
}
fn random_vec_of_vec() -> Vec<Vec<u16>> {
const N: u16 = 30;
const K: usize = 3;
let mut rng = rand::thread_rng();
let length: u16 = rng.gen_range(0, N);
let mut outer = vec![];
for index in 1..length {
let mut inner = vec![0u16; rng.gen_range(0, K)];
for e in &mut inner {
*e = rng.gen_range(0, index);
}
// De-duplicate elements. Particularly a problem with `index < K`.
inner.sort();
inner.dedup();
outer.push(inner);
}
outer
}
Previous work
I tried using the vec function, but I would need to nest two vec functions. And, the inner vec function could only generate values up to the index in the outer vector.
use proptest::collection::vec;
// INDEX should be the value of the position of the inner vector
// in the outer vector. How could the be found?
let strategy = vec(vec(1..INDEX, 0..K), 0..N);
The index method is not helpful because the right size would still not be known.

Answer 1

One way to go about this, is to replace each rng.gen_range() call with a strategy. Nested strategies must then be connected with prop_flat_map.
In the below code, I replaced my pattern
let length = rng.gen_range(0, N); for i in 1..length { .. }, with a new function vec_from_length(length: usize), which returns a Strategy.
#[cfg(test)]
mod tests {
use super::*;
use proptest::collection::hash_set;
use proptest::prelude::*;
use std::collections::HashSet;
proptest! {
#[test]
fn meaningless_test(v in vec_of_vec()) {
let s = sum(&v); // sum of the sum of all vectors.
prop_assert!(s < 15);
}
}
fn vec_of_vec() -> impl Strategy<Value = Vec<Vec<u16>>> {
const N: u16 = 10;
let length = 0..N;
length.prop_flat_map(vec_from_length).prop_map(convert)
}
fn vec_from_length(length: u16) -> impl Strategy<Value = Vec<HashSet<u16>>> {
const K: usize = 5;
let mut result = vec![];
for index in 1..length {
// Using a hash_set instead of vec because the elements should be unique.
let inner = hash_set(0..index, 0..K);
result.push(inner);
}
result
}
/// Convert Vec<HashSet<T>> to Vec<Vec<T>>
fn convert(input: Vec<HashSet<u16>>) -> Vec<Vec<u16>> {
let mut output = vec![];
for inner in input {
output.push(inner.into_iter().collect())
}
output
}
}
One more thing: An impl Strategy<Value=Vec<T>> can be generated from either the vec function (a strategy of vector) or from a vector of strategies! In the above code, I do this through having result be pushed with hash_set(..) which is a Strategy. The type is thus something like Vec<Strategy<T>> not Strategy<Vec<T>> (pedantic: Strategy is not a type, maybe).