refac: rewrite a lil in rust

wasm/betterseqta-wasm/src/engage.rs

@@ -0,0 +1,48 @@
+//! Engage hash routing (`getEngageRoutePage`).
+
+use percent_encoding::percent_decode_str;
+use std::borrow::Cow;
+use wasm_bindgen::prelude::*;
+
+/// Mirrors `getEngageRoutePage` using `window.location.hash` and `window.location.href` inputs.
+#[wasm_bindgen(js_name = parseEngageRoutePage)]
+pub fn parse_engage_route_page(hash: &str, full_href: &str) -> Option<String> {
+    let hash = hash.strip_prefix('#').unwrap_or(hash);
+    if !hash.is_empty() {
+        let qs: Cow<'_, str> = if hash.starts_with('?') {
+            Cow::Borrowed(hash)
+        } else {
+            Cow::Owned(format!("?{hash}"))
+        };
+        if let Some(seg) = parse_page_segment_from_query_string(qs.as_ref()) {
+            return Some(seg);
+        }
+    }
+    segment_from_href_split(full_href)
+}
+
+fn parse_page_segment_from_query_string(qs: &str) -> Option<String> {
+    let body = qs.strip_prefix('?').unwrap_or(qs);
+    for pair in body.split('&') {
+        let mut it = pair.splitn(2, '=');
+        let key = it.next()?;
+        if key != "page" {
+            continue;
+        }
+        let enc = it.next().unwrap_or("");
+        let decoded = percent_decode_str(enc).decode_utf8_lossy();
+        let page = decoded.as_ref();
+        if let Some(rest) = page.strip_prefix('/') {
+            let seg = rest.split('/').next().unwrap_or("");
+            if !seg.is_empty() {
+                return Some(seg.to_string());
+            }
+            return None;
+        }
+    }
+    None
+}
+
+fn segment_from_href_split(full_href: &str) -> Option<String> {
+    full_href.split('/').nth(4).map(std::string::ToString::to_string)
+}