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//! Assembling a row window from one or more server pages.
use ciborium::Value as CborValue;
use indexmap::IndexMap;
use vantage_dataset::traits::Result;
use vantage_expressions::Expression;
use vantage_types::Record;
use super::{Order, PaginationMode, RestApi};
impl RestApi {
/// The rows of `window`, with the envelope total when `total_key` is
/// configured and the body carries it. Unlike [`Self::fetch_total`] this never errors on a
/// missing total: the rows are the point here, and a caller that needs a
/// definitive count can still ask for one.
///
/// A window comes back short only when the server ran out: callers take a
/// short window as the end of the set.
pub(super) async fn fetch_windowed<'a>(
&self,
table_name: &str,
id_field: Option<&str>,
row_fields: &[String],
window: Option<(i64, i64)>,
order: Order<'_>,
conditions: impl IntoIterator<Item = &'a Expression<CborValue>>,
) -> Result<(IndexMap<String, Record<CborValue>>, Option<i64>)> {
// A non-paginating endpoint returns the whole list on the first
// window; a later one would re-deliver the same rows and a scrolling
// grid would never see the end. Windows past the start are empty.
if self.no_pagination && window.is_some_and(|(offset, _)| offset > 0) {
return Ok((IndexMap::new(), None));
}
let conditions: Vec<&Expression<CborValue>> = conditions.into_iter().collect();
let fetch =
|window| self.fetch_page(table_name, id_field, row_fields, window, order, &conditions);
let Some((offset, limit)) = window.filter(|_| !self.no_pagination) else {
let page = fetch(window).await?;
return Ok((page.rows, page.total));
};
let (offset, limit) = (offset.max(0), limit.max(1));
let start = match self.pagination.mode {
PaginationMode::Skip => offset,
PaginationMode::Page => offset - offset % limit,
};
let first = fetch(Some((start, limit))).await?;
if start == offset && !first.lossy {
return Ok((first.rows, first.total));
}
// Assemble the window page by page until it is full or the server
// runs out. When client filters dropped rows, a row's place in the
// result is only known by walking from the start.
let (mut server_offset, mut skip) = if first.lossy {
(0, offset)
} else {
(start, offset - start)
};
let mut page = (server_offset == start).then_some(first);
let mut rows = IndexMap::new();
let mut total = None;
let mut lossy = false;
loop {
let current = match page.take() {
Some(p) => p,
None => fetch(Some((server_offset, limit))).await?,
};
total = total.or(current.total);
lossy |= current.lossy;
for (id, row) in current.rows {
if skip > 0 {
skip -= 1;
} else if (rows.len() as i64) < limit {
// An id repeated across pages (rows shifting between
// fetches) keeps the later row and doesn't end the window.
rows.insert(id, row);
}
}
if rows.len() as i64 >= limit || (current.server_rows as i64) < limit {
break;
}
server_offset += limit;
}
Ok((rows, if lossy { None } else { total }))
}
}