Handle more URL types

This is a partial fix to #556, but it at least prevents crashing when an unknown
URL type is encountered. There is still some work to do, since the
markdown plugin doesn't properly interpret URLs the way we want.

User-related parsing still doesn't work because there's work needed in
the markdown layer, but once that's done, this should just work as
intended.

app/src/main/java/com/jerboa/Utils.kt

@@ -375,7 +375,49 @@ fun LazyListState.isScrolledToEnd(): Boolean {
     return out
 }
 
+/*
+ * Parses a "url" and returns a spec-compliant Url:
+ *
+ * - https://host/path - leave as-is
+ * - /c/community -> https://currentInstance/c/community
+ * - /c/community@instance -> https://instance/c/community
+ * - !community@instance -> https://instance/c/community
+ * - @user@instance -> https://instance/u/user
+ */
+fun parseUrl(url: String): String? {
+    if (url.startsWith("https://")) {
+        return url
+    } else if (url.startsWith("/c/")) {
+        if (url.count({ c -> c == '@' }) == 1) {
+            val (community, host) = url.split("@", limit = 2)
+            return "https://$host$community"
+        }
+        return "https://${API.currentInstance}$url"
+    } else if (url.startsWith("/u/")) {
+        if (url.count({ c -> c == '@' }) == 1) {
+            val (userPath, host) = url.split("@", limit = 2)
+            return "https://$host$userPath"
+        }
+        return "https://${API.currentInstance}$url"
+    } else if (url.startsWith("!")) {
+        if (url.count({ c -> c == '@' }) == 1) {
+            val (community, host) = url.substring(1).split("@", limit = 2)
+            return "https://$host/c/$community"
+        }
+        return "https://${API.currentInstance}/c/${url.substring(1)}"
+    } else if (url.startsWith("@")) {
+        if (url.count({ c -> c == '@' }) == 2) {
+            val (user, host) = url.substring(1).split("@", limit = 2)
+            return "https://$host/u/$user"
+        }
+        return "https://${API.currentInstance}/u/${url.substring(1)}"
+    }
+    return null
+}
+
 fun openLink(url: String, ctx: Context, useCustomTab: Boolean, usePrivateTab: Boolean) {
+    val url = parseUrl(url) ?: return
+
     if (useCustomTab) {
         val intent = CustomTabsIntent.Builder()
             .build().apply {

app/src/test/java/com/jerboa/UtilsKtTest.kt

@@ -0,0 +1,24 @@
+package com.jerboa
+
+import com.jerboa.api.API
+import org.junit.Assert.*
+import org.junit.Test
+import org.junit.runner.RunWith
+import java.net.URL
+
+class UtilsKtTest {
+    @Test
+    fun testParseUrl() {
+        val cases = mapOf(
+            "https://feddit.de" to "https://feddit.de",
+            "/c/community" to "https://${API.currentInstance}/c/community",
+            "/c/[email protected]" to "https://instance.ml/c/community",
+            "[email protected]" to "https://instance.ml/c/community",
+            "!community" to "https://${API.currentInstance}/c/community",
+            "/u/[email protected]" to "https://instance.ml/u/user",
+            "@[email protected]" to "https://instance.ml/u/user",
+        )
+
+        cases.forEach { (url, exp) -> assertEquals(exp, parseUrl(url)) }
+    }
+}