Preventing inaccurate lastmod with diffing

[ryaneghrari]

As far as I know everytime next-sitemap runs it will generate new lastmod values based on the current date for each page. This is not ideal as the page might not be changed. It's not a major deal as it seems that google ignores this field if it is inaccurate but for larger sites it could be an issue because it google may not index changed pages (as it doesn't know which have changed and which haven't)

A potential solution to this would be to use the configs transform function.

transform: async (config, path) => {
        //make a request to get `https://${config.siteUrl}/${}` (old contents)
        //compare that path to the contents of the generated page
        //if it matches do no update the lastModified date (get previousLastModValue from sitemap)
        //if it doesn't match update the lastModified date
        
        return {
            loc: path, // => this will be exported as http(s)://<config.siteUrl>/<path>
            changefreq: config.changefreq,
            priority: config.priority,
            lastmod: matches ? previousLastModValue : new Date().toISOString(),
            alternateRefs: config.alternateRefs ?? [],
        }
    },

Would be cool to have this by default