# HyperLogLog

> For the complete documentation index, see [llms.txt](https://effectkit.dev/llms.txt).

Look up the HyperLogLog commands for approximate distinct counts and their reply types.

Source: https://effectkit.dev/docs/redis/commands/hyperloglog

A HyperLogLog estimates how many distinct elements it has seen in a small, fixed amount of memory.

**Example** (Counting Distinct Visitors)

```ts twoslash
import { Effect } from "effect";

import { Redis } from "@effect-kit/redis";

const program = Effect.gen(function* () {
  const redis = yield* Redis;

  const changed = yield* redis.pfadd("uniques:mon", ["ada", "grace", "linus"]);
  //    ^?
  const estimate = yield* redis.pfcount(["uniques:mon"]);
  //    ^?

  yield* redis.pfmerge("uniques:week", ["uniques:mon", "uniques:tue"]);

  return { changed, estimate };
});
```

`pfadd` returns `true` when the estimate changed. `pfcount` takes a non-empty tuple of keys and returns the estimate for their union as a `bigint`.

## Methods [#methods]

| Method          | Command   | Reply type | Purpose                                               |
| --------------- | --------- | ---------- | ----------------------------------------------------- |
| `redis.pfadd`   | `PFADD`   | `boolean`  | Add elements; true if the estimate changed.           |
| `redis.pfcount` | `PFCOUNT` | `bigint`   | Return the estimated cardinality of one or more keys. |
| `redis.pfmerge` | `PFMERGE` | `void`     | Merge several keys into a destination key.            |