How do I avoid scraping duplicate content with Scraper (Rust)?

When using Scraper, a Rust crate for web scraping, avoiding duplicate content involves keeping track of already-visited URLs or content hashes to ensure you don't process the same content more than once. This is particularly important when scraping sites with a large amount of content where some pages might be accessible through multiple URLs or when the site has a complex navigation structure that could lead you to the same page multiple times.

Here's a general approach to avoiding duplicate content while scraping with Scraper in Rust:

1. Use a HashSet to Store Visited URLs: Before you scrape a page, check if its URL is in a HashSet. If it is, you've already visited it; if not, add it to the set and proceed with scraping.

2. Content Hashing: For content-based duplication checking, you can hash the content of a page and store these hashes in a HashSet. Before scraping, hash the content and check if the hash is in the set.

Here's a basic example of how you might implement these strategies:

extern crate scraper;
extern crate reqwest;

use scraper::{Html, Selector};
use std::collections::HashSet;
use std::hash::{Hash, Hasher};
use std::collections::hash_map::DefaultHasher;

fn main() {
    let start_url = "http://example.com";
    let mut visited_urls = HashSet::new();
    let mut visited_hashes = HashSet::new();

    scrape_url(start_url, &mut visited_urls, &mut visited_hashes);
}

fn scrape_url(url: &str, visited_urls: &mut HashSet<String>, visited_hashes: &mut HashSet<u64>) {
    // Check if URL is already visited
    if visited_urls.contains(url) {
        println!("Skipping already visited URL: {}", url);
        return;
    }

    // Mark this URL as visited
    visited_urls.insert(url.to_string());

    // Fetch and parse the document
    if let Ok(resp) = reqwest::blocking::get(url) {
        if let Ok(text) = resp.text() {
            let document = Html::parse_document(&text);

            // Calculate content hash
            let mut hasher = DefaultHasher::new();
            text.hash(&mut hasher);
            let content_hash = hasher.finish();

            // Check if content hash is already visited
            if visited_hashes.contains(&content_hash) {
                println!("Skipping duplicate content for URL: {}", url);
                return;
            }

            // Mark this content hash as visited
            visited_hashes.insert(content_hash);

            // Process the document with Scraper
            // ...

            // Continue scraping other URLs
            // (This assumes you have a function to extract URLs from the document)
            for next_url in extract_urls(&document) {
                scrape_url(&next_url, visited_urls, visited_hashes);
            }
        }
    }
}

fn extract_urls(document: &Html) -> Vec<String> {
    // Function to extract URLs from the document
    // This is a placeholder function - you would need to implement URL extraction based on the document structure
    vec![]
}

In this example, visited_urls is a HashSet of strings, which are the URLs that have been visited. visited_hashes is a HashSet of u64 hashes representing the unique content that has been scraped. Before scraping a page, the program checks if the URL or content hash has been seen before. If either has, the page is skipped.

Remember that this is a very simplistic approach. In practice, you will need to handle edge cases and issues such as:

• Canonicalization of URLs: Ensuring that different URLs that point to the same content (e.g., http://example.com, http://example.com/index.html, http://www.example.com) are treated as the same.
• Query Parameters: Some URLs may have query parameters that don't change the content. You might want to strip or standardize these before checking for uniqueness.
• Fragment Identifiers: Similar to query parameters, fragment identifiers (the part of the URL after #) often do not change the content. You may wish to ignore them when checking for duplicates.
• Rate Limiting: To avoid being blocked by the target site, you should implement polite scraping practices, such as respecting robots.txt and rate limiting your requests.

When implementing a web scraper, always ensure you are complying with the website's terms of service and legal requirements. Unauthorized scraping can lead to legal consequences and ethical issues.