fn nth_tuple(lists: &[Vec<(ItemId, String)>], i: usize) -> Vec<(ItemId, String)>Expand description
The i-th line of the Cartesian product of lists (outer category first),
by mixed-radix decoding of i across the list lengths — the inner (last)
category is the least-significant digit, matching axis_tuples’ ordering
(which increments the inner category fastest). Returns the bound
(ItemId, name) per category.
§Panics
i must be < product_len(lists), which implies every list is non-empty.
A list filtered to zero items means the axis has NO lines, so there is no
i-th line to decode; callers (grid_dims, render_grid, cursor_key)
render zero lines in that case rather than calling here. The check is a
hard assert! (not debug_assert!) because i % 0 is a divide-by-zero
crash in release builds.
This is what lets the grid virtualize rows: instead of holding every row
tuple in a Vec, we decode line i (and, for group outlining, line i-1)
only when that row is actually painted.