controllers/helpers/pagination: Generate prev_page for seek-based p…

…agination

src/controllers/helpers/pagination.rs

@@ -277,6 +277,35 @@ impl<T> Paginated<T> {
         Ok(Some(opts))
     }
 
+    pub(crate) fn prev_seek_params<S, F>(&self, f: F) -> AppResult<Option<IndexMap<String, String>>>
+    where
+        F: Fn(&T) -> S,
+        S: Serialize,
+    {
+        // When the data size is smaller than the page size, we would expect the prev page to be
+        // unavailable during backward pagination but available during forward pagination.
+        if self.options.is_explicit()
+            || self.records_and_total.is_empty()
+            || (self.records_and_total.len() < self.options.per_page as usize
+                && self.options.is_backward())
+        {
+            return Ok(None);
+        }
+
+        // We also like to return None for prev page when it's the first forward pagination.
+        let mut opts = IndexMap::new();
+        match self.options.page {
+            Page::Unspecified => return Ok(None),
+            Page::Seek(ref raw) if raw.is_empty() => return Ok(None),
+            Page::Seek(_) | Page::SeekBackward(_) => {
+                let seek = f(&self.records_and_total.first().unwrap().record);
+                opts.insert("seek".into(), format!("-{}", encode_seek(seek)?));
+            }
+            Page::Numeric(_) => unreachable!(),
+        };
+        Ok(Some(opts))
+    }
+
     pub(crate) fn iter(&self) -> impl Iterator<Item = &T> {
         self.records_and_total.iter().map(|row| &row.record)
     }

src/controllers/krate/search.rs

@@ -245,7 +245,8 @@ pub async fn list_crates(
             data.total(),
             data.next_seek_params(|last| seek.to_payload(last))?
                 .map(|p| req.query_with_params(p)),
-            None,
+            data.prev_seek_params(|first| seek.to_payload(first))?
+                .map(|p| req.query_with_params(p)),
             data.into_iter().collect::<Vec<_>>(),
         )
     } else {

src/tests/routes/crates/list.rs

@@ -1077,7 +1077,9 @@ async fn seek_based_pagination() -> anyhow::Result<()> {
             assert_eq!(resp.meta.total, 0);
         }
 
-        assert_eq!(resp.meta.prev_page, None);
+        if calls == 1 {
+            assert_eq!(resp.meta.prev_page, None);
+        }
     }
 
     assert_eq!(calls, 4);