Open ahs.app links in the app.

[xingyzt]

Background

Android and iOS have an "app link" feature where apps can suggest URLs of specified domains to be opened natively.

• Android Deep Links
• iOS Universal Links

We should direct the apps to open ahs.app links in order to not confuse app users when there are two possible views, and to gain the performance of native apps.

Parsing the URLs

ahs.app hosts articles in the following URL formats:
https://ahs.app/ARTICLE_TITLE/SCRAMBLED_ARTICLE_ID
https://ahs.app/SCRAMBLED_ARTICLE_ID
for example:
https://ahs.app/Teen-De-Stress-Kit/qrfpevcgvir-zrzbenoyr-xhqh
https://ahs.app/rkpryyrag-fnzr-rpuvqan

Article IDs are scrambled by a simple symmetric ROT13 cipher, which substitutes each letter with another 13 steps down the alphabet.

There are no trailing slashes as they are automatically redirected by the web server.

Swift:

func rot13(id: String) -> String {
    return String(Array(id).map {
        $0.isLetter ? "NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm"["ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz".index(of:$0)!] : $0
    })
}

func id(url: String) -> String {
    return rot13(id: url.components(separatedBy: "/").last!)
}

Kotlin:

fun rot13(id: String): String = id.map {
    if(it.isLetter())
        "NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm"
    	.get("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz".indexOf(it))
   else it
}.joinToString("")

fun id(url: String): String = rot13(url.split('/').last())