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//! Cell-coordinate drag selections and width-aware text extraction from
//! plain-text screen rows.
use unicode_width::UnicodeWidthChar;
/// An in-progress drag selection: a pair of 0-based `(row, col)` cells.
///
/// The anchor is the pressed cell and never moves; the head tracks the
/// pointer. Either may precede the other: [`Selection::extract`] normalizes.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Selection {
anchor: (u16, u16),
head: (u16, u16),
}
impl Selection {
/// Start a selection at the pressed cell: anchor and head coincide.
pub fn begin(row: u16, col: u16) -> Self {
Self {
anchor: (row, col),
head: (row, col),
}
}
/// Move the head to the dragged cell; the anchor stays at the press.
pub fn extend(&mut self, row: u16, col: u16) {
self.head = (row, col);
}
/// Whether the head and anchor occupy the same cell.
pub fn is_click(&self) -> bool {
self.anchor == self.head
}
/// Extract the selected text from `rows`, the rendered screen top-down.
///
/// Endpoints are ordered by row and column. The first row starts at the
/// first endpoint, intermediate rows are included in full, and the final
/// row includes the cell under the second endpoint. A boundary inside a
/// wide glyph includes the whole glyph. Trailing whitespace is removed
/// from each segment, and segments are joined with `\n`.
///
/// Rows below the screen clamp to its last row. Columns beyond a row select
/// no text, and an empty screen produces an empty string.
///
/// Each selected screen row occupies one joined line, even when its
/// selected span is empty.
pub fn extract(&self, rows: &[String]) -> String {
let Some(last) = rows.len().checked_sub(1) else {
return String::new();
};
let (start, end) = self.bounds(last);
// `bounds` clamps both rows to `last`, so indexing `rows` is in range.
(start.0..=end.0)
.map(|row| {
self.row_segment(row, &rows[row], last)
.map_or("", |(_, seg)| seg)
})
.collect::<Vec<_>>()
.join("\n")
}
/// Return the selected text on `row` and its starting display column.
///
/// Endpoints are ordered and clamped before wide glyphs are expanded and
/// trailing whitespace is removed. Returns `None` outside the selection
/// or when the selected span is empty after trimming.
pub fn row_segment<'a>(
&self,
row: usize,
text: &'a str,
last_row: usize,
) -> Option<(u16, &'a str)> {
let (start, end) = self.bounds(last_row);
if row < start.0 || row > end.0 {
return None;
}
let from = if row == start.0 { start.1 } else { 0 };
let to = (row == end.0).then_some(end.1);
let (col, seg) = segment_span(text, from, to)?;
let seg = seg.trim_end();
// The first selected glyph starts at or before a `u16` endpoint.
(!seg.is_empty()).then_some((col as u16, seg))
}
/// Clamp endpoints to the last row, then order them by row and column.
/// Clamping first handles endpoints that collapse onto the same row.
fn bounds(&self, last: usize) -> ((usize, usize), (usize, usize)) {
let clamp = |(row, col): (u16, u16)| ((row as usize).min(last), col as usize);
let (mut start, mut end) = (clamp(self.anchor), clamp(self.head));
if start > end {
std::mem::swap(&mut start, &mut end);
}
(start, end)
}
}
/// Return the text overlapping display cells `from..=to` and its starting
/// display column. `None` for `to` extends through the row. Wide glyphs are
/// included whole, and zero-width characters following a selected glyph are
/// included with it.
fn segment_span(row: &str, from: usize, to: Option<usize>) -> Option<(usize, &str)> {
// Exclusive right edge; `to` is the inclusive cell under the head.
let to = to.map_or(usize::MAX, |t| t.saturating_add(1));
let mut col = 0;
let mut start = None;
let mut end = 0;
let mut taken = false;
for (i, c) in row.char_indices() {
let w = c.width().unwrap_or(0);
if w == 0 {
// A zero-width tail extends the glyph it follows.
if taken {
end = i + c.len_utf8();
}
continue;
}
// The glyph spans cells [col, col + w); take it on any overlap.
taken = col < to && col + w > from;
if taken {
start.get_or_insert((i, col));
end = i + c.len_utf8();
}
col += w;
}
start.map(|(s, c)| (c, &row[s..end]))
}
#[cfg(test)]
mod tests {
use super::*;
fn screen(rows: &[&str]) -> Vec<String> {
rows.iter().map(|r| r.to_string()).collect()
}
/// A selection pressed at `a` and dragged to `b`, both `(row, col)`.
fn drag(a: (u16, u16), b: (u16, u16)) -> Selection {
let mut s = Selection::begin(a.0, a.1);
s.extend(b.0, b.1);
s
}
#[test]
fn forward_and_reversed_drags_extract_identical_text() {
let rows = screen(&["hello world"]);
let fwd = drag((0, 2), (0, 6)).extract(&rows);
let rev = drag((0, 6), (0, 2)).extract(&rows);
assert_eq!(fwd, "llo w");
assert_eq!(fwd, rev);
}
#[test]
fn multi_row_takes_middle_rows_whole() {
let rows = screen(&["first ", "middle ", "last "]);
let fwd = drag((0, 4), (2, 1)).extract(&rows);
let rev = drag((2, 1), (0, 4)).extract(&rows);
assert_eq!(fwd, "t\nmiddle\nla");
assert_eq!(fwd, rev);
}
#[test]
fn dragging_onto_a_cell_selects_it() {
let rows = screen(&["abcdef"]);
// The end column is inclusive: the head's cell is part of the text.
assert_eq!(drag((0, 1), (0, 3)).extract(&rows), "bcd");
// Extraction of a motionless click yields the cell under it; the
// caller gates on `is_click`, not on emptiness.
assert_eq!(drag((0, 0), (0, 0)).extract(&rows), "a");
}
#[test]
fn motionless_click_is_degenerate() {
let mut s = Selection::begin(3, 7);
assert!(s.is_click());
// A drag event that stays on the pressed cell is still a click.
s.extend(3, 7);
assert!(s.is_click());
s.extend(3, 8);
assert!(!s.is_click());
// Returning to the pressed cell reads as a click again.
s.extend(3, 7);
assert!(s.is_click());
}
#[test]
fn trailing_padding_is_trimmed_per_row() {
let rows = screen(&["top ", " ", "bottom"]);
// The all-space row contributes an empty segment but keeps its
// newline slot.
assert_eq!(drag((0, 0), (2, 5)).extract(&rows), "top\n\nbottom");
}
#[test]
fn column_inside_a_wide_glyph_takes_the_whole_glyph() {
// Cells: a=0, 日=1-2, 本=3-4, b=5.
let rows = screen(&["a日本b"]);
// Start edge inside 日.
assert_eq!(drag((0, 2), (0, 5)).extract(&rows), "日本b");
// End edge inside 本.
assert_eq!(drag((0, 0), (0, 3)).extract(&rows), "a日本");
// Both edges inside glyphs.
assert_eq!(drag((0, 2), (0, 3)).extract(&rows), "日本");
}
#[test]
fn wide_emoji_rounds_like_cjk() {
// Cells: 😀=0-1, z=2.
let rows = screen(&["😀z"]);
assert_eq!(drag((0, 1), (0, 2)).extract(&rows), "😀z");
assert_eq!(drag((0, 0), (0, 0)).extract(&rows), "😀");
}
#[test]
fn zero_width_marks_travel_with_their_glyph() {
// VS16 reports zero width on the char walk; it rides in ❤'s cell.
let rows = screen(&["x❤\u{fe0f}y"]);
assert_eq!(drag((0, 1), (0, 1)).extract(&rows), "❤\u{fe0f}");
assert_eq!(drag((0, 1), (0, 2)).extract(&rows), "❤\u{fe0f}y");
// A combining mark rides with its base.
let rows = screen(&["e\u{0301}f"]);
assert_eq!(drag((0, 0), (0, 0)).extract(&rows), "e\u{0301}");
}
#[test]
fn columns_past_the_row_clamp() {
let rows = screen(&["ab", "cd"]);
assert_eq!(drag((0, 40), (0, 90)).extract(&rows), "");
// A start column beyond row 0 selects nothing there; row 1 still
// yields, and the empty first segment keeps its newline slot.
assert_eq!(drag((0, 40), (1, 0)).extract(&rows), "\nc");
}
#[test]
fn rows_below_the_screen_land_on_the_bottom_row() {
let rows = screen(&["ab", "cd"]);
assert_eq!(drag((0, 0), (9, 0)).extract(&rows), "ab\nc");
// Both endpoints below the screen: the drag collapses onto the
// bottom row, and clamping inverts the endpoint order.
assert_eq!(drag((5, 1), (9, 0)).extract(&rows), "cd");
}
#[test]
fn empty_screen_extracts_nothing() {
assert_eq!(drag((0, 0), (3, 3)).extract(&[]), "");
}
#[test]
fn zero_length_rows_hold_their_line_slots() {
let rows = screen(&["a", "", "b"]);
assert_eq!(drag((0, 0), (2, 0)).extract(&rows), "a\n\nb");
assert_eq!(drag((1, 0), (1, 5)).extract(&rows), "");
}
#[test]
fn row_segment_starts_at_the_glyph_not_the_boundary() {
// Cells: a=0, 日=1-2, 本=3-4, b=5. A start boundary inside 日 rounds
// back to cell 1, the glyph's first cell: the repaint position.
let s = drag((0, 2), (0, 4));
assert_eq!(s.row_segment(0, "a日本b", 0), Some((1, "日本")));
assert_eq!(s.extract(&screen(&["a日本b"])), "日本");
}
#[test]
fn row_segment_middle_rows_span_from_column_zero() {
let s = drag((0, 3), (2, 1));
assert_eq!(s.row_segment(0, "aaaa", 2), Some((3, "a")));
assert_eq!(s.row_segment(1, "bbbb", 2), Some((0, "bbbb")));
assert_eq!(s.row_segment(2, "cccc", 2), Some((0, "cc")));
}
#[test]
fn row_segment_outside_the_selection_is_none() {
let s = drag((1, 0), (2, 1));
assert_eq!(s.row_segment(0, "above", 3), None);
assert_eq!(s.row_segment(3, "below", 3), None);
}
#[test]
fn clamping_below_the_screen_inverts_endpoint_order() {
// Both endpoints clamp to the bottom row before their columns are
// ordered; row 0 is outside the resulting selection.
let s = drag((5, 1), (9, 0));
assert_eq!(s.extract(&screen(&["ab", "cd"])), "cd");
assert_eq!(s.row_segment(0, "ab", 1), None);
assert_eq!(s.row_segment(1, "cd", 1), Some((0, "cd")));
}
#[test]
fn row_segment_trims_padding_to_none() {
let s = drag((0, 0), (2, 3));
assert_eq!(s.row_segment(1, " ", 2), None);
// A column range past the row's content is likewise empty.
assert_eq!(drag((0, 40), (0, 90)).row_segment(0, "ab", 0), None);
}
#[test]
fn normalization_is_row_major_not_column_major() {
// The anchor's column (4) is past the head's (1), but the head is on
// a later row: row order decides, not column order.
let rows = screen(&["abcde", "fghij"]);
let fwd = drag((0, 4), (1, 1)).extract(&rows);
let rev = drag((1, 1), (0, 4)).extract(&rows);
assert_eq!(fwd, "e\nfg");
assert_eq!(fwd, rev);
}
}