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use crate::Matrix;
use crate::row_index_mapped::{RowIndexMap, RowIndexMappedView};
/// A vertical row-mapping strategy that selects every `stride`-th row from an inner matrix,
/// starting at a fixed `offset`.
///
/// This enables vertical striding like selecting rows: `offset`, `offset + stride`, etc.
#[derive(Debug)]
pub struct VerticallyStridedRowIndexMap {
/// The number of rows in the resulting view.
height: usize,
/// The step size between selected rows in the inner matrix.
stride: usize,
/// The offset to start the stride from.
offset: usize,
}
pub type VerticallyStridedMatrixView<Inner> =
RowIndexMappedView<VerticallyStridedRowIndexMap, Inner>;
impl VerticallyStridedRowIndexMap {
/// Create a new vertically strided view over a matrix.
///
/// This selects rows in the inner matrix starting from `offset`, and then every `stride` rows after.
/// Any choice of `offset` is valid.
/// An `offset` at or past the inner height yields an empty view.
///
/// # Arguments
/// - `inner`: The inner matrix to view.
/// - `stride`: The number of rows between each selected row.
/// - `offset`: The inner row index of the first selected row.
///
/// # Panics
/// Panics if `stride` is zero.
pub fn new_view<T: Send + Sync + Clone, Inner: Matrix<T>>(
inner: Inner,
stride: usize,
offset: usize,
) -> VerticallyStridedMatrixView<Inner> {
// View row i maps to inner row offset + i * stride, valid while < h.
// Count of valid rows: ceil((h - offset) / stride), saturating to 0 when offset >= h.
let height = inner.height().saturating_sub(offset).div_ceil(stride);
RowIndexMappedView {
index_map: Self {
height,
stride,
offset,
},
inner,
}
}
}
impl RowIndexMap for VerticallyStridedRowIndexMap {
fn height(&self) -> usize {
self.height
}
fn map_row_index(&self, r: usize) -> usize {
r * self.stride + self.offset
}
}
#[cfg(test)]
mod tests {
use alloc::vec;
use super::*;
use crate::{Matrix, RowMajorMatrix};
fn sample_matrix() -> RowMajorMatrix<i32> {
// A 5x3 matrix:
// [10, 11, 12]
// [20, 21, 22]
// [30, 31, 32]
// [40, 41, 42]
// [50, 51, 52]
RowMajorMatrix::new(
vec![10, 11, 12, 20, 21, 22, 30, 31, 32, 40, 41, 42, 50, 51, 52],
3,
)
}
#[test]
fn test_vertically_strided_view_stride_1_offset_0() {
let matrix = sample_matrix();
let view = VerticallyStridedRowIndexMap::new_view(matrix, 1, 0);
assert_eq!(view.height(), 5);
assert_eq!(view.width(), 3);
assert_eq!(view.get(0, 0), Some(10));
assert_eq!(view.get(1, 1), Some(21));
unsafe {
assert_eq!(view.get_unchecked(4, 2), 52);
}
assert_eq!(view.get(5, 0), None); // out of bounds
assert_eq!(view.get(0, 3), None); // out of bounds
}
#[test]
fn test_vertically_strided_view_stride_2_offset_0() {
let matrix = sample_matrix();
let view = VerticallyStridedRowIndexMap::new_view(matrix, 2, 0);
assert_eq!(view.height(), 3);
assert_eq!(view.get(0, 0), Some(10)); // row 0
unsafe {
assert_eq!(view.get_unchecked(1, 1), 31); // row 2
assert_eq!(view.get_unchecked(2, 2), 52); // row 4
}
assert_eq!(view.get(0, 3), None); // out of bounds
}
#[test]
fn test_vertically_strided_view_stride_2_offset_1() {
let matrix = sample_matrix();
let view = VerticallyStridedRowIndexMap::new_view(matrix, 2, 1);
assert_eq!(view.height(), 2);
assert_eq!(view.get(0, 0), Some(20)); // row 1
unsafe {
assert_eq!(view.get_unchecked(1, 1), 41);
} // row 3
}
#[test]
fn test_vertically_strided_view_stride_3_offset_0() {
let matrix = sample_matrix();
let view = VerticallyStridedRowIndexMap::new_view(matrix, 3, 0);
assert_eq!(view.height(), 2);
assert_eq!(view.get(0, 0), Some(10)); // row 0
assert_eq!(view.get(1, 1), Some(41)); // row 3
}
#[test]
fn test_vertically_strided_view_stride_3_offset_1() {
let matrix = sample_matrix();
let view = VerticallyStridedRowIndexMap::new_view(matrix, 3, 1);
assert_eq!(view.height(), 2);
unsafe {
assert_eq!(view.get_unchecked(0, 0), 20); // row 1
assert_eq!(view.get_unchecked(1, 1), 51); // row 4
}
}
#[test]
fn test_vertically_strided_view_stride_3_offset_2() {
let matrix = sample_matrix();
let view = VerticallyStridedRowIndexMap::new_view(matrix, 3, 2);
assert_eq!(view.height(), 1);
assert_eq!(view.get(0, 2), Some(32)); // row 2
}
#[test]
fn test_vertically_strided_view_stride_greater_than_height() {
let matrix = sample_matrix();
let view = VerticallyStridedRowIndexMap::new_view(matrix, 10, 0);
assert_eq!(view.height(), 1);
assert_eq!(view.get(0, 0), Some(10)); // row 0
}
#[test]
fn test_vertically_strided_view_stride_greater_than_height_with_valid_offset() {
let matrix = sample_matrix(); // height = 5
let view = VerticallyStridedRowIndexMap::new_view(matrix, 10, 4);
// offset == 4 < height == 5 → view selects row 4
assert_eq!(view.height(), 1);
assert_eq!(view.get(0, 2), Some(52)); // row 4
}
#[test]
fn test_vertically_strided_view_stride_greater_than_height_with_offset_beyond_height() {
let matrix = sample_matrix(); // height = 5
let view = VerticallyStridedRowIndexMap::new_view(matrix, 10, 6);
// offset == 6 > height == 5 → no valid row
assert_eq!(view.height(), 0);
assert_eq!(view.get(0, 0), None); // out of bounds
}
#[test]
fn test_vertically_strided_view_offset_greater_than_stride() {
// Regression: with offset >= stride the old height formula over-reported,
// letting view rows map past the inner height.
//
// h = 5, stride = 2, offset = 3 → inner rows 3, 5, 7, ... → only row 3 in bounds
let matrix = sample_matrix();
let view = VerticallyStridedRowIndexMap::new_view(matrix, 2, 3);
assert_eq!(view.height(), 1);
// The single view row is inner row 3.
assert_eq!(view.get(0, 0), Some(40));
// View row 1 would be inner row 5, past the inner height → rejected.
assert_eq!(view.get(1, 0), None);
}
#[test]
fn test_vertically_strided_view_offset_equal_to_stride() {
// h = 6, stride = 2, offset = 2 → inner rows 2, 4, 6, ... → rows 2 and 4 in bounds.
let matrix = RowMajorMatrix::new(vec![10, 20, 30, 40, 50, 60], 1);
let view = VerticallyStridedRowIndexMap::new_view(matrix, 2, 2);
assert_eq!(view.height(), 2);
assert_eq!(view.get(0, 0), Some(30)); // inner row 2
assert_eq!(view.get(1, 0), Some(50)); // inner row 4
// View row 2 would be inner row 6, past the inner height → rejected.
assert_eq!(view.get(2, 0), None);
}
#[test]
fn test_vertically_strided_view_exhaustive_height_and_bounds() {
// Invariant: for any (stride, offset), the view height matches a brute-force count
// and every view row reads the expected inner row.
for stride in 1..=7usize {
// Sweep offsets covering offset < stride, stride <= offset < h, and offset >= h.
for offset in 0..=12usize {
let matrix = sample_matrix();
let h = matrix.height();
let view = VerticallyStridedRowIndexMap::new_view(matrix, stride, offset);
// Brute-force reference: count inner rows offset + i * stride < h.
let expected_height = (offset..h).step_by(stride).count();
assert_eq!(
view.height(),
expected_height,
"height mismatch for stride={stride}, offset={offset}"
);
// Every view row must read back the matching inner row,
// proving the mapped index stays in bounds.
for (i, inner_row) in (offset..h).step_by(stride).enumerate() {
assert_eq!(
view.get(i, 0),
Some(10 * (inner_row as i32 + 1)),
"wrong row for stride={stride}, offset={offset}, view row {i}"
);
}
// The first row past the reported height must be rejected.
assert_eq!(view.get(expected_height, 0), None);
}
}
}
}