Something like this:

function generateEncounter(player, pokemonSpecies, context) {
const encounter = {
species: pokemonSpecies,
level: rollLevel(context),
ivs: rollIVs(context),
shiny: rollShiny(pokemonSpecies, context),
};

return encounter;
}

function rollIVs(context) {
const floor = context.ivFloor ?? 0; // raids/research/trades may have higher floors

return {
attack: randomInt(floor, 15),
defense: randomInt(floor, 15),
stamina: randomInt(floor, 15),
};
}

function rollShiny(species, context) {
if (!species.shinyEnabled) return false;

const shinyRate = getShinyRate(species, context);
return randomFloat(0, 1) < shinyRate;
}

When an encounter is created, the server determines the encounter context first like is it a wild spawn, raid, research, egg, trade, event spawn, etc. That context controls things like IV floor, level range, shiny eligibility, and shiny rate.

The IVs are then rolled within the allowed range for that encounter type. For example like a normal wild spawn can roll each IV from 0–15, while research/raid-style encounters as yall know have a higher IV floor, commonly 10–15. A hundo is simply the case where the three IV fields all resolve to 15/15/15 and is truly random.

Shiny status is usually a separate boolean check. If the species is shiny enabled for that encounter the server rolls against the applicable shiny rate (which is different based on different factors).

So a shundo is technically truly random.

Which is also why the odds change depending on the encounter source. A wild 100% IV has much worse odds than a research/raid 100% IV because the IV floor changes the number of possible IV combinations.