tui_panel_select/

wrapcache.rs

//! Shared line/wrap-structure cache backing both the Request JSON and
//! Response panels' rendering *and* their text selection.
//!
//! A panel's underlying text (an HTTP response body, a JSON request preview)
//! is split once into raw (unwrapped) lines and their wrapped-row extents —
//! not on every redraw — so scrolling/dragging a selection over an
//! "obscenely large" body costs only what's on screen, never the whole
//! body (see `rebuild_if_needed`/`visible_window`). The same structure also
//! converts between *screen* space (a wrapped row/col, valid only for the
//! current frame's scroll + panel width) and *logical* space (a raw line
//! index + character offset, stable across resizes/rewraps/rescrolls) —
//! which is what lets a selection survive a panel resize by staying on the
//! same characters instead of the same terminal coordinates.

use std::cell::RefCell;
use std::sync::Arc;

use ratatui::text::Line;

use crate::wrap::{wrap_line_window, wrapped_row_count};

/// A position in a panel's logical (unwrapped) text: which raw line
/// (0-based), and which character offset within it (0-based; may equal the
/// line's own length to mean "just past its last character"). Deliberately
/// never a screen/terminal coordinate, so it stays valid across rewraps.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct TextPos {
    pub line: usize,
    pub col: usize,
}

impl TextPos {
    pub fn new(line: usize, col: usize) -> Self {
        Self { line, col }
    }
}

/// Exclusive prefix sum of wrapped-row counts across a panel's raw lines:
/// `cum[i]` = total wrapped rows in lines `0..i`. `cum.len() == line_count +
/// 1`; `*cum.last()` is the grand total (0 for no lines at all). Also caches
/// each line's own character length (`lens`) — computed once here, from the
/// same pass that already has to walk every line to determine wrapped-row
/// counts — so `PanelWrap::line_char_len` never has to re-scan a line's
/// characters itself (an O(1) selection/highlight primitive, even for a
/// single enormous line).
struct LineRows {
    cum: Vec<u32>,
    lens: Vec<usize>,
}

impl LineRows {
    fn build(char_lens: impl Iterator<Item = usize>, width: usize) -> Self {
        let mut cum = vec![0u32];
        let mut lens = Vec::new();
        let mut total = 0u32;
        for len in char_lens {
            total += wrapped_row_count(len, width) as u32;
            cum.push(total);
            lens.push(len);
        }
        Self { cum, lens }
    }

    fn total_rows(&self) -> u32 {
        (*self.cum.last().unwrap_or(&0)).max(1)
    }

    fn line_count(&self) -> usize {
        self.cum.len().saturating_sub(1)
    }

    /// The raw line index and row-offset-within-that-line for absolute
    /// wrapped row `row`, found by binary search (not a linear scan) so
    /// locating a scroll position deep into a huge body stays cheap.
    fn locate(&self, row: u32) -> (usize, u32) {
        if self.cum.len() <= 1 {
            return (0, 0);
        }
        // First index whose cumulative count exceeds `row`; the line just
        // before it is the one containing `row`.
        let idx = self.cum.partition_point(|&c| c <= row);
        let line = idx.saturating_sub(1).min(self.cum.len() - 2);
        (line, row - self.cum[line])
    }
}

/// Cached line/wrap structure for one panel's text, rebuilt only when its
/// content or width actually changes (see [`PanelWrap::rebuild_if_needed`]).
pub struct PanelWrap {
    /// The exact text this cache was built from — kept alive so
    /// `line_ranges` (byte offsets into it) stay valid, and so a cheap
    /// `Arc::ptr_eq` can detect "the content hasn't changed" without ever
    /// comparing bytes.
    source: Arc<str>,
    /// Byte (start, end) of each raw line within `source` (split on '\n',
    /// stripping a trailing '\r', matching `str::lines()`).
    line_ranges: Vec<(usize, usize)>,
    rows: LineRows,
    width: usize,
    /// The last `visible_window` result, keyed by the `(scroll, height)` it
    /// was computed for. Most frames redraw with an unchanged scroll
    /// position, so this turns those into an O(1) clone of a handful of
    /// already-wrapped rows instead of re-wrapping anything — no per-frame
    /// work proportional to content size, no matter how large the body or
    /// how long an individual line is.
    last_window: RefCell<Option<(u16, u16, Vec<Line<'static>>)>>,
}

impl PanelWrap {
    /// Build fresh from `source` at `width` columns. O(source length) —
    /// call only when content/width has actually changed (see
    /// `rebuild_if_needed`), never unconditionally on every frame.
    pub fn build(source: Arc<str>, width: usize) -> Self {
        let mut line_ranges = Vec::new();
        let bytes = source.as_bytes();
        let mut start = 0usize;
        for (i, &b) in bytes.iter().enumerate() {
            if b == b'\n' {
                let mut end = i;
                if end > start && bytes[end - 1] == b'\r' {
                    end -= 1;
                }
                line_ranges.push((start, end));
                start = i + 1;
            }
        }
        if start < bytes.len() || line_ranges.is_empty() {
            line_ranges.push((start, bytes.len()));
        }
        let rows = LineRows::build(
            line_ranges
                .iter()
                .map(|&(s, e)| source[s..e].chars().count()),
            width,
        );
        Self {
            source,
            line_ranges,
            rows,
            width,
            last_window: RefCell::new(None),
        }
    }

    /// Rebuild only if `source`'s identity (by pointer — a new response/edit
    /// always produces a fresh allocation) or `width` differ from what's
    /// cached; otherwise this is a no-op, keeping repeated frames (drags,
    /// idle redraws) cheap regardless of how large the content is.
    pub fn rebuild_if_needed(cache: &mut Option<PanelWrap>, source: &Arc<str>, width: usize) {
        let stale = match cache {
            Some(c) => !Arc::ptr_eq(&c.source, source) || c.width != width,
            None => true,
        };
        if stale {
            *cache = Some(PanelWrap::build(Arc::clone(source), width));
        }
    }

    pub fn line_count(&self) -> usize {
        self.rows.line_count()
    }

    /// The exact, unmodified text this cache was built from — every line,
    /// with its original line endings, not just what's currently scrolled
    /// into view. Used for "copy the whole panel" (no selection needed).
    pub fn source(&self) -> &str {
        &self.source
    }

    pub fn line_text(&self, idx: usize) -> &str {
        let (s, e) = self.line_ranges[idx];
        &self.source[s..e]
    }

    pub fn line_char_len(&self, idx: usize) -> usize {
        self.rows.lens.get(idx).copied().unwrap_or(0)
    }

    pub fn total_rows(&self) -> u32 {
        self.rows.total_rows()
    }

    /// The exact wrapped rows visible in a `height`-row window starting at
    /// absolute wrapped-row `scroll` — the only rows actually wrapped, and
    /// only the portion of each raw line that window actually covers
    /// (`wrap_line_window`), regardless of the total content size or how
    /// long any single raw line is. Repeated calls with the same
    /// `(scroll, height)` (the common case across idle/unchanged frames)
    /// hit `last_window` and do no wrapping work at all.
    pub fn visible_window(&self, scroll: u16, height: u16) -> Vec<Line<'static>> {
        if height == 0 || self.line_count() == 0 {
            return Vec::new();
        }
        if let Some((cached_scroll, cached_height, cached)) = self.last_window.borrow().as_ref()
            && *cached_scroll == scroll
            && *cached_height == height
        {
            return cached.clone();
        }
        let (start_line, row_in_line) = self.rows.locate(scroll as u32);
        let height_usize = height as usize;
        let mut out: Vec<Line<'static>> = Vec::with_capacity(height_usize);
        let mut skip = row_in_line as usize;
        for idx in start_line..self.line_count() {
            if out.len() >= height_usize {
                break;
            }
            let budget = height_usize - out.len();
            out.extend(wrap_line_window(
                self.line_text(idx),
                self.width,
                skip,
                budget,
            ));
            skip = 0;
        }
        out.truncate(height_usize);
        *self.last_window.borrow_mut() = Some((scroll, height, out.clone()));
        out
    }

    /// Convert a logical [`TextPos`] into its absolute wrapped-row index and
    /// column-within-that-row — the reverse of [`Self::row_col_to_textpos`],
    /// used to project a (resize-invariant) selection back onto the current
    /// frame's screen space for highlighting or scroll-into-view.
    pub fn textpos_to_row_col(&self, pos: TextPos) -> (u32, usize) {
        if self.line_count() == 0 {
            return (0, 0);
        }
        let line = pos.line.min(self.line_count() - 1);
        let len = self.line_char_len(line);
        let col = pos.col.min(len);
        if self.width == 0 {
            return (self.rows.cum[line], col);
        }
        let rows_in_line = wrapped_row_count(len, self.width) as u32;
        let row_in_line = ((col / self.width) as u32).min(rows_in_line.saturating_sub(1));
        let col_in_row = col.saturating_sub(row_in_line as usize * self.width);
        (self.rows.cum[line] + row_in_line, col_in_row)
    }

    /// Convert an absolute wrapped-row index + column-in-row (screen space)
    /// into the logical [`TextPos`] it corresponds to — the reverse of
    /// [`Self::textpos_to_row_col`], used to map a mouse click/drag onto
    /// real content.
    pub fn row_col_to_textpos(&self, row: u32, col: usize) -> TextPos {
        if self.line_count() == 0 {
            return TextPos::new(0, 0);
        }
        let (line, row_in_line) = self.rows.locate(row);
        let len = self.line_char_len(line);
        let base = if self.width == 0 {
            0
        } else {
            row_in_line as usize * self.width
        };
        // `col` may be `usize::MAX` (callers use this to mean "clamp to the
        // end of the line", e.g. auto-scroll snapping the selection cursor
        // to a row's last character) — add with saturation so that intent
        // doesn't overflow before the `.min(len)` clamp gets a chance to
        // apply.
        TextPos::new(line, base.saturating_add(col).min(len))
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn wrap(text: &str, width: usize) -> PanelWrap {
        PanelWrap::build(Arc::from(text), width)
    }

    #[test]
    fn splits_lines_like_str_lines_including_trailing_newline_and_crlf() {
        let w = wrap("a\r\nb\nc", 10);
        assert_eq!(w.line_count(), 3);
        assert_eq!(w.line_text(0), "a");
        assert_eq!(w.line_text(1), "b");
        assert_eq!(w.line_text(2), "c");

        let w2 = wrap("a\nb\n", 10);
        assert_eq!(
            w2.line_count(),
            2,
            "no trailing empty line after a final \\n, matching str::lines()"
        );
    }

    #[test]
    fn empty_body_has_one_line_and_one_row() {
        let w = wrap("", 10);
        assert_eq!(w.line_count(), 1);
        assert_eq!(w.total_rows(), 1);
    }

    #[test]
    fn total_rows_accounts_for_wrapping_long_lines() {
        // "0123456789ABCDE" (15 chars) at width 10 -> 2 rows; "" -> 1 row.
        let w = wrap("0123456789ABCDE\n", 10);
        assert_eq!(w.total_rows(), 2);
    }

    #[test]
    fn row_col_and_textpos_roundtrip_for_a_wrapped_line() {
        let w = wrap("0123456789ABCDE", 10); // rows 0: "0123456789", row 1: "ABCDE"
        assert_eq!(w.row_col_to_textpos(0, 3), TextPos::new(0, 3));
        assert_eq!(w.row_col_to_textpos(1, 2), TextPos::new(0, 12));
        assert_eq!(w.textpos_to_row_col(TextPos::new(0, 3)), (0, 3));
        assert_eq!(w.textpos_to_row_col(TextPos::new(0, 12)), (1, 2));
        // A position exactly at the line's own length (cursor "past the end").
        assert_eq!(w.textpos_to_row_col(TextPos::new(0, 15)), (1, 5));
    }

    #[test]
    fn locate_binary_search_finds_the_right_line_for_a_huge_body() {
        let body: String = (0..100_000).map(|i| format!("line {i}\n")).collect();
        let w = wrap(&body, 20);
        // "line 50000" is 10 chars; at width 20 that's 1 row per line, so
        // wrapped-row 50_000 should land exactly on line 50_000, col 0.
        assert_eq!(w.row_col_to_textpos(50_000, 0), TextPos::new(50_000, 0));
    }

    #[test]
    fn visible_window_only_wraps_the_requested_rows() {
        let body: String = (0..1000).map(|i| format!("line {i}\n")).collect();
        let w = wrap(&body, 20);
        let rows = w.visible_window(500, 5);
        assert_eq!(rows.len(), 5);
        let text: Vec<String> = rows
            .iter()
            .map(|l| l.spans.iter().map(|s| s.content.as_ref()).collect())
            .collect();
        assert_eq!(
            text,
            vec!["line 500", "line 501", "line 502", "line 503", "line 504"]
        );
    }

    /// A single raw line with no newlines at all (e.g. a huge base64 blob or
    /// minified JSON payload) must still produce a correct, small window
    /// regardless of where the scroll offset falls inside it — and must do
    /// so without ever wrapping the whole line (this used to cost O(line
    /// length) per redraw and grind the app to a halt; see also the timing
    /// regression test below).
    #[test]
    fn visible_window_is_correct_for_a_single_enormous_unbroken_line() {
        let body: String = "abcdefghij".repeat(200_000); // 2,000,000 chars, one line
        let w = wrap(&body, 10);

        let top = w.visible_window(0, 3);
        assert_eq!(top.len(), 3);
        let row0: String = top[0].spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(row0, "abcdefghij", "row 0 is chars [0, 10)");
        let row2: String = top[2].spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(
            row2, "abcdefghij",
            "row 2 (chars [20, 30)) lands mid-repeat but still aligned"
        );

        // Deep into the line: row 50_000 covers chars [500_000, 500_010).
        let mid = w.visible_window(50_000, 2);
        assert_eq!(mid.len(), 2);
        let mid_row: String = mid[0].spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(mid_row, "abcdefghij");

        // Repeated calls with the same (scroll, height) hit the cache and
        // must return identical content.
        let again = w.visible_window(50_000, 2);
        let again_text: Vec<String> = again
            .iter()
            .map(|l| l.spans.iter().map(|s| s.content.as_ref()).collect())
            .collect();
        let mid_text: Vec<String> = mid
            .iter()
            .map(|l| l.spans.iter().map(|s| s.content.as_ref()).collect())
            .collect();
        assert_eq!(again_text, mid_text);
    }

    /// Regression test for the reported "obscenely large response makes the
    /// whole app grind to a halt" bug: a single multi-megabyte unwrapped
    /// line used to cost O(line length) on *every single redraw* (both in
    /// `visible_window`'s per-line `wrap_line` call and in
    /// `PanelWrap::line_char_len`'s repeated `.chars().count()`), which
    /// alone took >100ms per frame for a 5MB line. This asserts many
    /// repeated redraws of such a line stay fast, with a bound generous
    /// enough not to flake on slow CI hardware while still catching an
    /// accidental return to O(line length)-per-frame behaviour.
    #[test]
    fn visible_window_stays_fast_across_many_redraws_of_a_single_huge_line() {
        use std::time::{Duration, Instant};
        let body: String = "x".repeat(5_000_000);
        let w = wrap(&body, 78);

        let start = Instant::now();
        for _ in 0..200 {
            let rows = w.visible_window(0, 30);
            assert_eq!(
                rows.len(),
                30,
                "the first 30 wrapped rows of a 5,000,000-char line at width 78"
            );
        }
        let elapsed = start.elapsed();
        assert!(
            elapsed < Duration::from_secs(2),
            "200 redraws of a single 5MB line took {elapsed:?} — expected a small fraction of a second"
        );
    }

    #[test]
    fn rebuild_if_needed_skips_rebuilding_on_an_unchanged_pointer_and_width() {
        let source: Arc<str> = Arc::from("hello\nworld");
        let mut cache: Option<PanelWrap> = None;
        PanelWrap::rebuild_if_needed(&mut cache, &source, 10);
        let first_ptr = cache.as_ref().unwrap().source.as_ptr();
        // Same Arc, same width -> must not rebuild (same backing pointer).
        PanelWrap::rebuild_if_needed(&mut cache, &source, 10);
        assert_eq!(cache.as_ref().unwrap().source.as_ptr(), first_ptr);
        // Width changed -> must rebuild.
        PanelWrap::rebuild_if_needed(&mut cache, &source, 20);
        assert_eq!(cache.as_ref().unwrap().width, 20);
        // A genuinely new Arc (even with equal content) -> must rebuild too,
        // since a new response/edit always allocates fresh.
        let source2: Arc<str> = Arc::from("hello\nworld");
        PanelWrap::rebuild_if_needed(&mut cache, &source2, 20);
        assert!(Arc::ptr_eq(&cache.as_ref().unwrap().source, &source2));
    }
}