# prime-generator

Generates every prime from 0 up to a limit with the sieve of Eratosthenes and
writes them one per line to a text file. The default limit is 1,000,000,000.

*Originally written in Java. In 2026 it was rewritten in Go; the Java version is
on the [feature/java](https://github.com/tiennm99/prime-generator/tree/feature/java)
branch.*

## Requirements

- Go 1.25 or higher
- Disk space for the output — a full run to 1,000,000,000 writes 50,847,534
  primes, roughly 500 MB
- About 1 GB of RAM at the default limit, since the sieve holds one byte per
  number

## Usage

```sh
go run .
```

Or build a binary first:

```sh
go build -o prime-generator .
./prime-generator
```

### Flags

| Flag | Default | Purpose |
| --- | --- | --- |
| `-n` | `1000000000` | Upper limit, inclusive, to sieve |
| `-out` | `primes.txt` | Output file path |

```sh
# Primes below 100, written somewhere else
./prime-generator -n 100 -out small.txt
```

## Tests

```sh
go test ./...
```

The sieve is checked against an independent trial-division implementation, and
against the published prime counts at powers of ten up to 1,000,000 — so a bug
in the sieve is not mirrored by the thing checking it.

## Related

[crawl-prime](https://github.com/tiennm99/crawl-prime) reaches the same output
file from the other direction: it downloads a published prime list instead of
computing one.