sparser data transfers

gpx-rs/src/api/dto.rs

@@ -522,6 +522,29 @@ impl TrackpointDetails {
     }
 }
 
+/// A trackpoint of the selection: what the statistics buffers do not hold for each trackpoint.
+/// Absent fields are unknown.
+#[derive(Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SelectionPoint {
+    /// ms since the epoch.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub time: Option<f64>,
+    /// Slope (%) and length (km) of the smoothed elevation segment of the trackpoint.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub slope_segment_slope: Option<f64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub slope_segment_distance: Option<f64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub surface: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub highway: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub sac_scale: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub mtb_scale: Option<String>,
+}
+
 // Routing
 
 /// A category of the trackpoints of a route, as in `RouteCategory` of the engine.

gpx-rs/src/api/queries.rs

@@ -154,8 +154,6 @@ buffers!(
     elevation_gain,
     elevation_loss,
     slope,
-    slope_segment_slope,
-    slope_segment_distance,
     lng,
     lat,
     ele
@@ -175,9 +173,48 @@ macro_rules! optional_buffers {
 optional_buffers!(moving_distance: f64, speed: f64, hr: f64, cad: f64, atemp: f64, power: f64);
 optional_buffers!(total_time: i64, moving_time: i64);
 
-/// Timestamps in milliseconds since the epoch (`NO_TIME` when missing), see `optional_buffers`.
-pub fn timestamps<H: History>(engine: &Engine<H>) -> Option<&[i64]> {
-    engine.statistics().time.as_deref()
+// The smoothed elevation segments of the selection, over which the slope is averaged, as
+// intervals of trackpoints: `slope_segment_starts` holds the index of the first trackpoint of each
+// segment, and `slope_segment_slopes` its slope (%). Their length is in `selection_point`.
+
+pub fn slope_segment_starts<H: History>(engine: &Engine<H>) -> &[u32] {
+    &engine.statistics().slope_segments.starts
+}
+
+pub fn slope_segment_slopes<H: History>(engine: &Engine<H>) -> &[f64] {
+    &engine.statistics().slope_segments.slopes
+}
+
+/// What the buffers do not hold for each trackpoint of the selection, for the one at `index`
+/// (numbered as in the buffers), `None` if there is none: for example the one that is hovered on
+/// the elevation profile.
+pub fn selection_point<H: History>(engine: &Engine<H>, index: usize) -> Option<SelectionPoint> {
+    let stats = engine.statistics();
+    if index >= stats.len() {
+        return None;
+    }
+    let categories = engine.categories();
+    let name = |intervals: &engine::Intervals, categories: &engine::Categories| {
+        let value = intervals.value_at(index);
+        let code = value.checked_sub(1)?;
+        categories.name(code).map(str::to_owned)
+    };
+    let segment = stats.slope_segments.at(index);
+    let finite = |value: f64| value.is_finite().then_some(value);
+    Some(SelectionPoint {
+        time: stats
+            .time
+            .as_ref()
+            .map(|time| time[index])
+            .filter(|time| *time != engine::NO_TIME)
+            .map(|time| time as f64),
+        slope_segment_slope: segment.and_then(|i| finite(stats.slope_segments.slopes[i])),
+        slope_segment_distance: segment.and_then(|i| finite(stats.slope_segments.distances[i])),
+        surface: name(&stats.surface, &categories.surface),
+        highway: name(&stats.highway, &categories.highway),
+        sac_scale: name(&stats.sac_scale, &categories.sac_scale),
+        mtb_scale: name(&stats.mtb_scale, &categories.mtb_scale),
+    })
 }
 
 // Surface, highway, SAC scale and MTB scale of the trackpoints, as intervals of trackpoints that

gpx-rs/src/engine/derived/statistics_buffer.rs

@@ -1,4 +1,4 @@
-use crate::{GlobalStatistics, Statistics, TrackSegment, sum_options};
+use crate::{GlobalStatistics, SlopeSegment, Statistics, TrackSegment, sum_options};
 
 /// Value of the timestamps of the trackpoints that have none.
 pub const NO_TIME: i64 = i64::MIN;
@@ -27,8 +27,8 @@ pub struct StatisticsBuffer {
     pub elevation_gain: Vec<f64>,
     pub elevation_loss: Vec<f64>,
     pub slope: Vec<f64>,
-    pub slope_segment_slope: Vec<f64>,
-    pub slope_segment_distance: Vec<f64>,
+    /// The smoothed elevation segments that the trackpoints belong to, see [`SlopeSegments`].
+    pub slope_segments: SlopeSegments,
     pub lng: Vec<f64>,
     pub lat: Vec<f64>,
     pub ele: Vec<f64>,
@@ -62,6 +62,18 @@ pub struct Intervals {
 }
 
 impl Intervals {
+    /// The value of the trackpoint at `index` (the one of the last interval past the trackpoints),
+    /// 0 (unknown) when there are no trackpoints.
+    pub fn value_at(&self, index: usize) -> u8 {
+        match self
+            .starts
+            .partition_point(|start| *start as usize <= index)
+        {
+            0 => 0,
+            next => self.values[next - 1],
+        }
+    }
+
     fn clear(&mut self) {
         self.starts.clear();
         self.values.clear();
@@ -76,6 +88,50 @@ impl Intervals {
     }
 }
 
+/// The smoothed elevation segments of the selection, over which the slope is averaged, as the
+/// intervals of trackpoints that belong to the same one: segment `i` starts at the trackpoint
+/// `starts[i]` and ends where the next one starts, or at the last trackpoint. It has a `slope` (%)
+/// and a length (`distance`, km). A segment never spans two segments of the selection. All are
+/// empty when there are no trackpoints.
+#[derive(Debug, Default, PartialEq)]
+pub struct SlopeSegments {
+    pub starts: Vec<u32>,
+    pub slopes: Vec<f64>,
+    pub distances: Vec<f64>,
+}
+
+impl SlopeSegments {
+    /// The index of the segment of the trackpoint at `index` (the last one past the trackpoints),
+    /// `None` when there are no trackpoints.
+    pub fn at(&self, index: usize) -> Option<usize> {
+        let next = self
+            .starts
+            .partition_point(|start| *start as usize <= index);
+        next.checked_sub(1)
+    }
+
+    fn clear(&mut self) {
+        self.starts.clear();
+        self.slopes.clear();
+        self.distances.clear();
+    }
+
+    /// Adds the trackpoint at `index`, the indices being pushed in order: it starts a segment if
+    /// it is the first of a segment of the selection (`first`), or if its segment is not the one
+    /// of the previous trackpoint.
+    fn push(&mut self, index: usize, segment: &SlopeSegment, first: bool) {
+        // compared as bits, so that a NaN slope (no distance) is the same as the previous one
+        let same = |values: &[f64], value: f64| {
+            values.last().map(|last| last.to_bits()) == Some(value.to_bits())
+        };
+        if first || !same(&self.slopes, segment.slope) || !same(&self.distances, segment.distance) {
+            self.starts.push(index as u32);
+            self.slopes.push(segment.slope);
+            self.distances.push(segment.distance);
+        }
+    }
+}
+
 /// 0 for an unknown value, else the code plus one. The categories hold at most 255 values, so it
 /// fits.
 fn unknown_or_next(code: Option<u8>) -> u8 {
@@ -117,8 +173,7 @@ impl StatisticsBuffer {
         self.elevation_gain.clear();
         self.elevation_loss.clear();
         self.slope.clear();
-        self.slope_segment_slope.clear();
-        self.slope_segment_distance.clear();
+        self.slope_segments.clear();
         self.lng.clear();
         self.lat.clear();
         self.ele.clear();
@@ -133,6 +188,7 @@ impl StatisticsBuffer {
         self.mtb_scale.clear();
 
         for (segment, stats) in selected {
+            let segment_start = self.total_distance.len();
             for (trkpt, trkpt_stats) in segment.iter().zip(stats.local.iter()) {
                 let index = self.total_distance.len();
                 let cumul_stats = &self.global;
@@ -162,10 +218,8 @@ impl StatisticsBuffer {
                 self.elevation_loss
                     .push(cumul_stats.elevation_loss + trkpt_stats.elevation_loss);
                 self.slope.push(trkpt_stats.slope);
-                self.slope_segment_slope
-                    .push(trkpt_stats.slope_segment.slope);
-                self.slope_segment_distance
-                    .push(trkpt_stats.slope_segment.distance);
+                self.slope_segments
+                    .push(index, &trkpt_stats.slope_segment, index == segment_start);
                 self.lng.push(trkpt.coordinates.lng);
                 self.lat.push(trkpt.coordinates.lat);
                 self.ele.push(trkpt.ele);
@@ -229,25 +283,12 @@ mod tests {
         (segment, stats)
     }
 
-    /// The value of the trackpoint at `index`, 0 before the first interval.
-    fn value_at(intervals: &Intervals, index: usize) -> u8 {
-        match intervals
-            .starts
-            .partition_point(|start| *start as usize <= index)
-        {
-            0 => 0,
-            next => intervals.values[next - 1],
-        }
-    }
-
     fn all_lengths(buffer: &StatisticsBuffer) -> Vec<usize> {
         vec![
             buffer.total_distance.len(),
             buffer.elevation_gain.len(),
             buffer.elevation_loss.len(),
             buffer.slope.len(),
-            buffer.slope_segment_slope.len(),
-            buffer.slope_segment_distance.len(),
             buffer.lng.len(),
             buffer.lat.len(),
             buffer.ele.len(),
@@ -276,7 +317,19 @@ mod tests {
         for (i, local) in s.local.iter().enumerate() {
             assert_eq!(buffer.total_distance[i], local.total_distance);
             assert_eq!(buffer.slope[i], local.slope);
+            let segment = buffer.slope_segments.at(i).unwrap();
+            assert_eq!(
+                buffer.slope_segments.slopes[segment].to_bits(),
+                local.slope_segment.slope.to_bits()
+            );
+            assert_eq!(
+                buffer.slope_segments.distances[segment],
+                local.slope_segment.distance
+            );
         }
+        // a segment per smoothed elevation segment, not per trackpoint
+        assert!(buffer.slope_segments.starts.len() < n);
+        assert_eq!(buffer.slope_segments.starts[0], 0);
         // the trackpoints
         for (i, trkpt) in segment.iter().enumerate() {
             assert_eq!(buffer.lng[i], trkpt.coordinates.lng);
@@ -454,7 +507,7 @@ mod tests {
         assert!(all_lengths(&buffer).iter().all(|len| *len == 5));
 
         // and they can be looked up
-        let values: Vec<u8> = (0..6).map(|i| value_at(&buffer.surface, i)).collect();
+        let values: Vec<u8> = (0..6).map(|i| buffer.surface.value_at(i)).collect();
         assert_eq!(values, [1, 1, 0, 2, 2, 2]);
     }
 
@@ -494,10 +547,10 @@ mod tests {
         buffer.update(&[(&segment, &s)]);
         let at = |i: usize| {
             (
-                value_at(&buffer.surface, i),
-                value_at(&buffer.highway, i),
-                value_at(&buffer.sac_scale, i),
-                value_at(&buffer.mtb_scale, i),
+                buffer.surface.value_at(i),
+                buffer.highway.value_at(i),
+                buffer.sac_scale.value_at(i),
+                buffer.mtb_scale.value_at(i),
             )
         };
         assert_eq!(at(0), (8, 255, 1, 4));
@@ -522,7 +575,7 @@ mod tests {
         assert_eq!(buffer.highway, Intervals::default());
         assert_eq!(buffer.sac_scale, Intervals::default());
         assert_eq!(buffer.mtb_scale, Intervals::default());
-        assert_eq!(value_at(&buffer.surface, 0), 0);
+        assert_eq!(buffer.surface.value_at(0), 0);
     }
 
     #[test]

tauri/src/bytes.rs

@@ -34,7 +34,3 @@ pub fn optional(array: Option<Vec<u8>>) -> Bytes {
 pub fn f64_bytes(values: &[f64]) -> Vec<u8> {
     values.iter().flat_map(|v| v.to_le_bytes()).collect()
 }
-
-pub fn i64_bytes(values: &[i64]) -> Vec<u8> {
-    values.iter().flat_map(|v| v.to_le_bytes()).collect()
-}

tauri/src/commands/statistics.rs

@@ -17,15 +17,7 @@ macro_rules! f64_getters {
     )*};
 }
 
-f64_getters!(
-    total_distance,
-    slope,
-    slope_segment_slope,
-    slope_segment_distance,
-    lng,
-    lat,
-    ele
-);
+f64_getters!(total_distance, slope, slope_segment_slopes, lng, lat, ele);
 
 macro_rules! optional_f64_getters {
     ($($name:ident),*) => {$(
@@ -38,10 +30,16 @@ macro_rules! optional_f64_getters {
 
 optional_f64_getters!(speed, hr, cad, atemp, power);
 
-/// Timestamps in milliseconds since the epoch (`NO_TIME` when missing), as `i64`.
+/// The index of the first trackpoint of each smoothed elevation segment of the selection.
 #[tauri::command(async)]
-pub fn timestamps(state: State<AppState>) -> Bytes {
-    optional(state.read(|e| api::timestamps(e).map(i64_bytes)))
+pub fn slope_segment_starts(state: State<AppState>) -> Bytes {
+    u32s(&state.read(|e| api::slope_segment_starts(e).to_vec()))
+}
+
+/// What the buffers do not hold for each trackpoint of the selection, for the one at `index`.
+#[tauri::command(async)]
+pub fn selection_point(state: State<AppState>, index: usize) -> Option<api::SelectionPoint> {
+    state.read(move |e| api::selection_point(e, index))
 }
 
 #[tauri::command(async)]

tauri/src/lib.rs

@@ -146,8 +146,8 @@ fn register_commands<R: tauri::Runtime>(builder: tauri::Builder<R>) -> tauri::Bu
         // statistics
         statistics::total_distance,
         statistics::slope,
-        statistics::slope_segment_slope,
-        statistics::slope_segment_distance,
+        statistics::slope_segment_starts,
+        statistics::slope_segment_slopes,
         statistics::lng,
         statistics::lat,
         statistics::ele,
@@ -156,7 +156,7 @@ fn register_commands<R: tauri::Runtime>(builder: tauri::Builder<R>) -> tauri::Bu
         statistics::cad,
         statistics::atemp,
         statistics::power,
-        statistics::timestamps,
+        statistics::selection_point,
         statistics::selection_statistics,
         statistics::slice_statistics,
         statistics::surface_starts,

tauri/src/tests.rs

@@ -142,6 +142,18 @@ fn reads_the_statistics_of_the_selection() {
     // optional arrays start with a byte telling whether they are there
     let hr = app.bytes("hr", json!({}));
     assert_eq!(hr[0], 0);
+    // the slope segments are intervals of trackpoints
+    let starts = app.bytes("slope_segment_starts", json!({}));
+    let slopes = f64s(&app.bytes("slope_segment_slopes", json!({})));
+    assert_eq!(starts.len() / 4, slopes.len());
+    assert!(!slopes.is_empty() && slopes.len() < distance.len());
+    let point = app.call("selection_point", json!({ "index": 1 }));
+    assert!(point["slopeSegmentDistance"].is_number());
+    assert!(point.get("time").is_none());
+    assert_eq!(
+        app.call("selection_point", json!({ "index": distance.len() })),
+        Value::Null
+    );
     assert!(
         app.call("slice_statistics", json!({ "start": 1, "end": 3 }))["totalDistance"].is_number()
     );

wasm/src/queries/statistics.rs

@@ -26,8 +26,6 @@ stats_getter!(total_distance, Float64Array);
 stats_getter!(elevation_gain, Float64Array);
 stats_getter!(elevation_loss, Float64Array);
 stats_getter!(slope, Float64Array);
-stats_getter!(slope_segment_slope, Float64Array);
-stats_getter!(slope_segment_distance, Float64Array);
 stats_getter!(lng, Float64Array);
 stats_getter!(lat, Float64Array);
 stats_getter!(ele, Float64Array);
@@ -49,14 +47,6 @@ optional_stats_getter!(moving_distance, Float64Array);
 optional_stats_getter!(total_time, BigInt64Array);
 optional_stats_getter!(moving_time, BigInt64Array);
 optional_stats_getter!(speed, Float64Array);
-// Timestamps in milliseconds since the epoch (`NO_TIME`, the smallest i64, when missing).
-optional_stats_getter!(timestamps, BigInt64Array);
-
-/// Value of the timestamps of the trackpoints that have none.
-#[wasm_bindgen]
-pub fn no_time() -> i64 {
-    gpx_engine::NO_TIME
-}
 
 optional_stats_getter!(hr, Float64Array);
 optional_stats_getter!(cad, Float64Array);
@@ -96,6 +86,19 @@ categories_getters!(highway_starts, highway_values, highways);
 categories_getters!(sac_scale_starts, sac_scale_values, sac_scales);
 categories_getters!(mtb_scale_starts, mtb_scale_values, mtb_scales);
 
+// The smoothed elevation segments of the selection, over which the slope is averaged, as
+// intervals of trackpoints: `slope_segment_starts` holds the index of the first trackpoint of each
+// segment, and `slope_segment_slopes` its slope (%). Their length is in `selection_point`.
+stats_getter!(slope_segment_starts, Uint32Array);
+stats_getter!(slope_segment_slopes, Float64Array);
+
+/// What the buffers do not hold for each trackpoint of the selection, for the one at `index`
+/// (numbered as in the buffers), `undefined` if there is none.
+#[wasm_bindgen]
+pub fn selection_point(index: usize) -> SelectionPoint {
+    optional_js(with_engine(|e| api::selection_point(e, index)).flatten()).unchecked_into()
+}
+
 /// For each trackpoint of the selection (numbered as in the statistics), the highest tolerance
 /// (m) of `reduce` that keeps it: infinite for the ends of the segments.
 #[wasm_bindgen]

wasm/src/tests.rs

@@ -4,7 +4,7 @@
 
 #![allow(clippy::needless_borrows_for_generic_args)]
 
-use js_sys::{Array, BigInt64Array, Float64Array, Object, Reflect};
+use js_sys::{Array, Float64Array, Object, Reflect};
 use wasm_bindgen::prelude::*;
 use wasm_bindgen_test::*;
 
@@ -144,7 +144,10 @@ fn test_loaded_file_has_statistics_and_coordinates() {
     assert_eq!(crate::queries::statistics::lng().length(), n);
     assert_eq!(crate::queries::statistics::ele().length(), n);
     // no timestamps in this file
-    assert!(crate::queries::statistics::timestamps().is_none());
+    let point = crate::queries::statistics::selection_point(0);
+    assert!(get(&point, "slopeSegmentDistance").as_f64().is_some());
+    assert!(get(&point, "time").is_undefined());
+    assert!(crate::queries::statistics::selection_point(n as usize).is_undefined());
 
     let structure = crate::queries::files::file_structure(&id);
     let segment = get(
@@ -165,8 +168,8 @@ fn test_loaded_file_has_statistics_and_coordinates() {
 #[wasm_bindgen_test]
 fn test_timestamps_and_categories() {
     started_with(&[(WITH_TIME, "time")]);
-    let times: BigInt64Array = crate::queries::statistics::timestamps().unwrap();
-    assert!(times.length() > 0);
+    let point = crate::queries::statistics::selection_point(0);
+    assert!(get(&point, "time").as_f64().unwrap() > 0.0);
     assert!(crate::queries::statistics::total_time().is_some());
 
     started_with(&[(WITH_SURFACE, "surface")]);
@@ -179,6 +182,14 @@ fn test_timestamps_and_categories() {
         crate::queries::statistics::surface_starts().length(),
         values.len() as u32
     );
+    // the name of the surface of a trackpoint that has one
+    let starts = crate::queries::statistics::surface_starts().to_vec();
+    let known = values.iter().position(|v| *v > 0).unwrap();
+    let point = crate::queries::statistics::selection_point(starts[known] as usize);
+    assert_eq!(
+        get(&point, "surface").as_string(),
+        Some(surfaces[values[known] as usize - 1].clone())
+    );
 }
 
 #[wasm_bindgen_test]

wasm/src/ts.rs

@@ -64,6 +64,21 @@ export interface TrackpointDetails {
     /** ms since epoch */
     time?: number;
 }
+/**
+ * A trackpoint of the selection: what the statistics buffers do not hold for each trackpoint.
+ * Absent fields are unknown.
+ */
+export interface SelectionPoint {
+    /** ms since epoch */
+    time?: number;
+    /** Slope (%) and length (km) of the smoothed elevation segment of the trackpoint. */
+    slopeSegmentSlope?: number;
+    slopeSegmentDistance?: number;
+    surface?: string;
+    highway?: string;
+    sacScale?: string;
+    mtbScale?: string;
+}
 /** What was copied or cut, to be pasted: the kind of the elements and their ids. */
 export interface Clipboard {
     type: 'files' | 'tracks' | 'segments' | 'waypoints';
@@ -161,6 +176,8 @@ extern "C" {
     pub type WaypointDetails;
     #[wasm_bindgen(typescript_type = "TrackpointDetails | undefined")]
     pub type TrackpointDetails;
+    #[wasm_bindgen(typescript_type = "SelectionPoint | undefined")]
+    pub type SelectionPoint;
     #[wasm_bindgen(typescript_type = "MoveTarget")]
     pub type MoveTarget;
 

website/src/lib/components/elevation-profile/elevation-profile.ts

@@ -24,14 +24,15 @@ import { get, type Readable, type Writable } from 'svelte/store';
 import type { Coordinates } from '$lib/geo/coordinates';
 import {
     engine,
-    NO_TIME,
+    EMPTY_SELECTION_STATISTICS,
+    type SelectionPoint,
     type SelectionStatistics,
     type StatisticsMetric,
     type StatisticsRequest,
 } from '$lib/engine';
 import type { SlicedStatistics } from '$lib/state/selection-statistics';
 import { mode } from 'mode-watcher';
-import { categoryValues } from '$lib/engine/trackpoint-categories';
+import { categoryValue, intervalAt } from '$lib/engine/trackpoint-categories';
 import { getHighwayColor, getSlopeColor, getSurfaceColor } from '$lib/data/colors';
 
 const { distanceUnits, velocityUnits, temperatureUnits } = settings;
@@ -44,20 +45,13 @@ function optional(value: number, convert: (value: number) => number = (v) => v)
     return Number.isNaN(value) ? undefined : convert(value);
 }
 
+/**
+ * A point of the chart. The rest of the data of the trackpoint is only read when it is hovered
+ * (see `SelectionStatistics.point`), and the categories of the fill from their intervals.
+ */
 interface ElevationProfilePoint {
     x: number;
     y: number;
-    time?: Date;
-    slope: {
-        at: number;
-        segment: number;
-        length: number;
-    };
-    surface?: string;
-    highway?: string;
-    sacScale?: string;
-    mtbScale?: string;
-    coordinates: Coordinates;
     index: number;
 }
 
@@ -69,6 +63,14 @@ export class ElevationProfile {
     private _panning = false;
 
     private _statistics: Readable<SelectionStatistics>;
+    /** The statistics that the chart shows, which the fills read for each of its segments. */
+    private _data: SelectionStatistics = EMPTY_SELECTION_STATISTICS;
+    /**
+     * The hovered trackpoint, and its data once it is read from the engine (`undefined` if the
+     * statistics changed in the meantime).
+     */
+    private _hovered: { index: number; loaded: boolean; point?: SelectionPoint } | null = null;
+    private _loadingHovered = false;
     private _slicedStatistics: Writable<SlicedStatistics | undefined>;
     private _hoveredPoint: Writable<Coordinates | null>;
     private _additionalDatasets: Readable<string[]>;
@@ -200,7 +202,7 @@ export class ElevationProfile {
                                 if (this._dragging) {
                                     this._hoveredPoint.set(null);
                                 } else {
-                                    this._hoveredPoint.set(point.coordinates);
+                                    this.hover(point.index);
                                 }
                                 return `${i18n._('quantities.elevation')}: ${getElevationWithUnits(point.y, false)}`;
                             } else if (context.datasetIndex === 1) {
@@ -219,28 +221,40 @@ export class ElevationProfile {
                             let context = contexts.filter((context) => context.datasetIndex === 0);
                             if (context.length === 0) return;
                             let point = context[0].raw as ElevationProfilePoint;
-                            let slope = {
-                                at: point.slope.at.toFixed(1),
-                                segment: point.slope.segment.toFixed(1),
-                                length: getDistanceWithUnits(point.slope.length),
-                            };
-                            let surface = point.surface ?? 'unknown';
-                            let highway = point.highway ?? 'unknown';
-                            let sacScale = point.sacScale;
-                            let mtbScale = point.mtbScale;
+                            // the rest of the data is there once it is read (see `hover`)
+                            const hovered = this._hovered;
+                            const details =
+                                hovered?.index === point.index ? hovered.point : undefined;
+                            const elevationFill = get(this._elevationFill);
+                            let slope = `${this._data.slope[point.index]?.toFixed(1)} %`;
+                            if (
+                                elevationFill === 'slope' &&
+                                details?.slopeSegmentSlope !== undefined &&
+                                details.slopeSegmentDistance !== undefined
+                            ) {
+                                slope += ` (${getDistanceWithUnits(details.slopeSegmentDistance)} @${details.slopeSegmentSlope.toFixed(1)} %)`;
+                            }
 
                             let labels = [
                                 `    ${i18n._('quantities.distance')}: ${getDistanceWithUnits(point.x, false)}`,
-                                `    ${i18n._('quantities.slope')}: ${slope.at} %${get(this._elevationFill) === 'slope' ? ` (${slope.length} @${slope.segment} %)` : ''}`,
+                                `    ${i18n._('quantities.slope')}: ${slope}`,
                             ];
 
-                            if (get(this._elevationFill) === 'surface') {
+                            if (!details) {
+                                return labels;
+                            }
+                            let surface = details.surface ?? 'unknown';
+                            let highway = details.highway ?? 'unknown';
+                            let sacScale = details.sacScale;
+                            let mtbScale = details.mtbScale;
+
+                            if (elevationFill === 'surface') {
                                 labels.push(
                                     `    ${i18n._('quantities.surface')}: ${i18n._(`toolbar.routing.surface.${surface}`)}`
                                 );
                             }
 
-                            if (get(this._elevationFill) === 'highway') {
+                            if (elevationFill === 'highway') {
                                 labels.push(
                                     `    ${i18n._('quantities.highway')}: ${i18n._(`toolbar.routing.highway.${highway}`)}${
                                         sacScale
@@ -255,9 +269,9 @@ export class ElevationProfile {
                                 }
                             }
 
-                            if (point.time) {
+                            if (details.time !== undefined) {
                                 labels.push(
-                                    `    ${i18n._('quantities.time')}: ${i18n.df.format(point.time)}`
+                                    `    ${i18n._('quantities.time')}: ${i18n.df.format(new Date(details.time))}`
                                 );
                             }
 
@@ -339,6 +353,7 @@ export class ElevationProfile {
                     events: ['mouseout'],
                     afterEvent: (chart: Chart, args: { event: ChartEvent }) => {
                         if (args.event.type === 'mouseout') {
+                            this._hovered = null;
                             this._hoveredPoint.set(null);
                         }
                     },
@@ -430,6 +445,9 @@ export class ElevationProfile {
             return;
         }
         const data = get(this._statistics);
+        this._data = data;
+        // the indices of the trackpoints are the ones of other statistics
+        this._hovered = null;
         const units = {
             distance: get(distanceUnits),
             velocity: get(velocityUnits),
@@ -438,28 +456,11 @@ export class ElevationProfile {
 
         const datasets: Array<Array<any>> = [[], [], [], [], [], []];
         const { global } = data;
-        const surfaces = categoryValues(data.surface, data.length);
-        const highways = categoryValues(data.highway, data.length);
-        const sacScales = categoryValues(data.sacScale, data.length);
-        const mtbScales = categoryValues(data.mtbScale, data.length);
         for (let index = 0; index < data.length; index++) {
             const x = getConvertedDistance(data.totalDistance[index], units.distance);
-            const ele = data.ele[index];
-            const timestamp = data.timestamps?.[index] ?? NO_TIME;
             datasets[0].push({
                 x,
-                y: getConvertedElevation(ele, units.distance),
-                time: timestamp === NO_TIME ? undefined : new Date(Number(timestamp)),
-                slope: {
-                    at: data.slope[index],
-                    segment: data.slopeSegmentSlope?.[index] ?? NaN,
-                    length: data.slopeSegmentDistance?.[index] ?? NaN,
-                },
-                surface: surfaces[index],
-                highway: highways[index],
-                sacScale: sacScales[index],
-                mtbScale: mtbScales[index],
-                coordinates: { lat: data.lat[index], lng: data.lng[index] },
+                y: getConvertedElevation(data.ele[index], units.distance),
                 index,
             });
             if (data.speed && (global.totalTime ?? 0) > 0) {
@@ -643,21 +644,71 @@ export class ElevationProfile {
         }
     }
 
-    slopeFillCallback(context: ScriptableLineSegmentContext & { p0: { raw: any } }) {
-        const point = context.p0.raw as ElevationProfilePoint;
-        return getSlopeColor(point.slope.segment);
+    /**
+     * Shows the hovered trackpoint on the map, and reads the rest of its data from the engine for
+     * the tooltip. One trackpoint is read at a time, the last one hovered.
+     */
+    hover(index: number) {
+        this._hoveredPoint.set({ lat: this._data.lat[index], lng: this._data.lng[index] });
+        if (this._hovered?.index === index) {
+            return;
+        }
+        this._hovered = { index, loaded: false };
+        this.loadHovered();
     }
 
-    surfaceFillCallback(context: ScriptableLineSegmentContext & { p0: { raw: any } }) {
-        const point = context.p0.raw as ElevationProfilePoint;
-        return getSurfaceColor(point.surface ?? '');
+    private async loadHovered() {
+        if (this._loadingHovered) {
+            return; // the one being read is followed by the last one hovered
+        }
+        this._loadingHovered = true;
+        try {
+            while (this._hovered && !this._hovered.loaded) {
+                const hovered = this._hovered;
+                const point = await this._data.point(hovered.index);
+                if (this._hovered !== hovered) {
+                    continue; // another trackpoint is hovered, or none
+                }
+                hovered.loaded = true;
+                hovered.point = point;
+                const chart = this._chart;
+                if (chart?.tooltip && chart.tooltip.getActiveElements().length > 0) {
+                    // the tooltip asks its callbacks again (the method is not in the types)
+                    (chart.tooltip as unknown as { update(changed: boolean): void }).update(true);
+                    chart.draw();
+                }
+            }
+        } finally {
+            this._loadingHovered = false;
+        }
     }
 
-    highwayFillCallback(context: ScriptableLineSegmentContext & { p0: { raw: any } }) {
-        const point = context.p0.raw as ElevationProfilePoint;
-        return getHighwayColor(point.highway ?? '', point.sacScale, point.mtbScale);
+    /** The trackpoint that a segment of the chart starts at. */
+    private static segmentStart(context: ScriptableLineSegmentContext & { p0: { raw: any } }) {
+        return (context.p0.raw as ElevationProfilePoint).index;
     }
 
+    slopeFillCallback = (context: ScriptableLineSegmentContext & { p0: { raw: any } }) => {
+        const segments = this._data.slopeSegments;
+        const index = ElevationProfile.segmentStart(context);
+        const segment = segments ? intervalAt(segments.starts, index) : -1;
+        return getSlopeColor(segment >= 0 ? segments!.slopes[segment] : NaN);
+    };
+
+    surfaceFillCallback = (context: ScriptableLineSegmentContext & { p0: { raw: any } }) => {
+        const index = ElevationProfile.segmentStart(context);
+        return getSurfaceColor(categoryValue(this._data.surface, index) ?? '');
+    };
+
+    highwayFillCallback = (context: ScriptableLineSegmentContext & { p0: { raw: any } }) => {
+        const index = ElevationProfile.segmentStart(context);
+        return getHighwayColor(
+            categoryValue(this._data.highway, index) ?? '',
+            categoryValue(this._data.sacScale, index),
+            categoryValue(this._data.mtbScale, index)
+        );
+    };
+
     destroy() {
         this._request.release();
         if (this._chart) {

website/src/lib/components/ui/slider/slider.svelte

@@ -26,7 +26,11 @@ get along, so we shut typescript up by casting `value` to `never`.
 	)}
 	{...restProps}
 >
-	{#snippet children({ thumbItems })}
+	<!--
+	The thumbs are the values: the snippet does not read the items that bits-ui gives it, which
+	it computes again at each change of the value, with one tick per step of the slider.
+	-->
+	{#snippet children()}
 		<span
 			data-slot="slider-track"
 			data-orientation={orientation}
@@ -41,10 +45,10 @@ get along, so we shut typescript up by casting `value` to `never`.
 				)}
 			/>
 		</span>
-		{#each thumbItems as thumb (thumb)}
+		{#each Array.isArray(value) ? value : [value] as _, index (index)}
 			<SliderPrimitive.Thumb
 				data-slot="slider-thumb"
-				index={thumb.index}
+				{index}
 				class="border-ring ring-ring/50 relative size-4 rounded-full border bg-white transition-[color,box-shadow] after:absolute after:-inset-2 hover:ring-3 focus-visible:ring-3 focus-visible:outline-hidden active:ring-3 block shrink-0 select-none disabled:pointer-events-none disabled:opacity-50"
 			/>
 		{/each}

website/src/lib/data/example/example-data.ts

@@ -174,61 +174,22 @@ export const exampleData = {
         1.5840695473562374, 0.1214109154598013, -4.214883885805801, -4.182735032776085,
         -5.523076373263165, -10.644434966065772,
     ],
-    slopeSegmentSlope: [
-        7.077037465314024, 7.077037465314024, 7.077037465314024, 7.077037465314024,
-        0.5753520337401837, 0.5753520337401837, 0.5753520337401837, 0.5753520337401837,
-        0.5753520337401837, 5.143330470981779, -5.289467529413177, 3.1409086334150653,
-        3.1409086334150653, -0.553863318953831, -0.553863318953831, -0.553863318953831,
-        -0.553863318953831, -0.553863318953831, -0.553863318953831, -0.553863318953831,
-        -0.553863318953831, -0.553863318953831, -0.553863318953831, -0.553863318953831,
-        -0.553863318953831, -0.553863318953831, -0.553863318953831, -0.553863318953831,
-        -0.553863318953831, -0.553863318953831, 6.771412441716311, 6.771412441716311,
-        6.771412441716311, 6.771412441716311, 6.771412441716311, 6.771412441716311,
-        6.771412441716311, 6.771412441716311, 6.771412441716311, 6.771412441716311,
-        6.771412441716311, 6.771412441716311, 1.246024238407464, 1.246024238407464,
-        5.604351892140715, 5.604351892140715, 5.604351892140715, 2.2941711127147597,
-        2.2941711127147597, 2.2941711127147597, 2.2941711127147597, 2.2941711127147597,
-        2.2941711127147597, 10.861290836693483, 10.861290836693483, 10.861290836693483,
-        10.861290836693483, -4.414487617055606, -4.414487617055606, 6.94339810974512,
-        -3.539036752039, -3.539036752039, -3.539036752039, -13.990509017835178, -13.990509017835178,
-        -13.990509017835178, -13.990509017835178, -2.033792785924166, -2.033792785924166,
-        -2.033792785924166, -2.033792785924166, -2.033792785924166, -2.033792785924166,
-        -2.033792785924166, -2.033792785924166, -2.033792785924166, -2.033792785924166,
-        -2.033792785924166, -2.033792785924166, -2.033792785924166, -2.033792785924166,
-        -2.033792785924166, -2.033792785924166, 0.45298474033593794, 0.45298474033593794,
-        0.45298474033593794, 0.45298474033593794, 0.45298474033593794, 0.45298474033593794,
-        0.45298474033593794, 0.45298474033593794, -7.692745586787598, -7.692745586787598,
-        -0.4142590757867528, -0.4142590757867528, -0.4142590757867528, -0.4142590757867528,
-        -0.4142590757867528, -5.154482373206842, -5.154482373206842, -5.154482373206842,
-        -5.154482373206842,
+    slopeSegmentStarts: [
+        0, 4, 9, 10, 11, 13, 30, 42, 44, 47, 53, 57, 59, 60, 63, 67, 83, 91, 93, 98,
     ],
-    slopeSegmentDistance: [
-        1.0682436029275066, 1.0682436029275066, 1.0682436029275066, 1.0682436029275066,
-        1.8771116406407027, 1.8771116406407027, 1.8771116406407027, 1.8771116406407027,
-        1.8771116406407027, 0.6571617396684242, 0.38189099162956897, 1.2607816597626882,
-        1.2607816597626882, 4.622078972183025, 4.622078972183025, 4.622078972183025,
-        4.622078972183025, 4.622078972183025, 4.622078972183025, 4.622078972183025,
-        4.622078972183025, 4.622078972183025, 4.622078972183025, 4.622078972183025,
-        4.622078972183025, 4.622078972183025, 4.622078972183025, 4.622078972183025,
-        4.622078972183025, 4.622078972183025, 4.040515953724009, 4.040515953724009,
-        4.040515953724009, 4.040515953724009, 4.040515953724009, 4.040515953724009,
-        4.040515953724009, 4.040515953724009, 4.040515953724009, 4.040515953724009,
-        4.040515953724009, 4.040515953724009, 0.802552606262374, 0.802552606262374,
-        3.3545359680865605, 3.3545359680865605, 3.3545359680865605, 2.9030092668715675,
-        2.9030092668715675, 2.9030092668715675, 2.9030092668715675, 2.9030092668715675,
-        2.9030092668715675, 0.8857142437895185, 0.8857142437895185, 0.8857142437895185,
-        0.8857142437895185, 0.8200272181110968, 0.8200272181110968, 0.5818476682663238,
-        1.3280449820963618, 1.3280449820963618, 1.3280449820963618, 1.255136605652762,
-        1.255136605652762, 1.255136605652762, 1.255136605652762, 4.7694141050816405,
-        4.7694141050816405, 4.7694141050816405, 4.7694141050816405, 4.7694141050816405,
-        4.7694141050816405, 4.7694141050816405, 4.7694141050816405, 4.7694141050816405,
-        4.7694141050816405, 4.7694141050816405, 4.7694141050816405, 4.7694141050816405,
-        4.7694141050816405, 4.7694141050816405, 4.7694141050816405, 1.7660639062735584,
-        1.7660639062735584, 1.7660639062735584, 1.7660639062735584, 1.7660639062735584,
-        1.7660639062735584, 1.7660639062735584, 1.7660639062735584, 0.610965220021356,
-        0.610965220021356, 1.738042790330141, 1.738042790330141, 1.738042790330141,
-        1.738042790330141, 1.738042790330141, 1.2493980061849328, 1.2493980061849328,
-        1.2493980061849328, 1.2493980061849328,
+    slopeSegmentSlopes: [
+        7.077037465314024, 0.5753520337401837, 5.143330470981779, -5.289467529413177,
+        3.1409086334150653, -0.553863318953831, 6.771412441716311, 1.246024238407464,
+        5.604351892140715, 2.2941711127147597, 10.861290836693483, -4.414487617055606,
+        6.94339810974512, -3.539036752039, -13.990509017835178, -2.033792785924166,
+        0.45298474033593794, -7.692745586787598, -0.4142590757867528, -5.154482373206842,
+    ],
+    slopeSegmentDistances: [
+        1.0682436029275066, 1.8771116406407027, 0.6571617396684242, 0.38189099162956897,
+        1.2607816597626882, 4.622078972183025, 4.040515953724009, 0.802552606262374,
+        3.3545359680865605, 2.9030092668715675, 0.8857142437895185, 0.8200272181110968,
+        0.5818476682663238, 1.3280449820963618, 1.255136605652762, 4.7694141050816405,
+        1.7660639062735584, 0.610965220021356, 1.738042790330141, 1.2493980061849328,
     ],
     lng: [
         -66.658624, -66.658127, -66.658472, -66.660451, -66.650629, -66.646183, -66.646388,

website/src/lib/data/example/index.ts

@@ -4,7 +4,8 @@
  * and the documentation. They are the ones that the engine computes for `example.gpx`, kept as
  * they are (see `example-data.ts`) so that the example does not need the engine.
  */
-import type { GlobalStatistics, SelectionStatistics } from '$lib/engine';
+import type { GlobalStatistics, SelectionPoint, SelectionStatistics } from '$lib/engine';
+import { intervalAt } from '$lib/engine/trackpoint-categories';
 import { exampleData as data } from '$lib/data/example/example-data';
 
 const length = data.lng.length;
@@ -46,6 +47,21 @@ async function slice(start: number, end: number): Promise<GlobalStatistics | und
     };
 }
 
+const slopeSegmentStarts = Uint32Array.from(data.slopeSegmentStarts);
+
+/** The data of the trackpoint at `index`, as the engine gives it (the example has no OSM data). */
+async function point(index: number): Promise<SelectionPoint | undefined> {
+    if (!(index >= 0 && index < length)) {
+        return undefined;
+    }
+    const segment = intervalAt(slopeSegmentStarts, index);
+    return {
+        time: data.timestamps[index],
+        slopeSegmentSlope: data.slopeSegmentSlopes[segment],
+        slopeSegmentDistance: data.slopeSegmentDistances[segment],
+    };
+}
+
 export const exampleStatistics: SelectionStatistics = {
     global: data.global,
     length,
@@ -54,10 +70,12 @@ export const exampleStatistics: SelectionStatistics = {
     lng: Float64Array.from(data.lng),
     lat: Float64Array.from(data.lat),
     ele: Float64Array.from(data.ele),
-    timestamps: BigInt64Array.from(data.timestamps, (time) => BigInt(time)),
     speed: Float64Array.from(data.speed),
     atemp: Float64Array.from(data.atemp),
-    slopeSegmentSlope: Float64Array.from(data.slopeSegmentSlope),
-    slopeSegmentDistance: Float64Array.from(data.slopeSegmentDistance),
+    slopeSegments: {
+        starts: slopeSegmentStarts,
+        slopes: Float64Array.from(data.slopeSegmentSlopes),
+    },
     slice,
+    point,
 };

website/src/lib/engine/backend/tauri.ts

@@ -182,14 +182,14 @@ export async function createTauriBackend(): Promise<Backend> {
         lng: () => f64('lng'),
         lat: () => f64('lat'),
         ele: () => f64('ele'),
-        timestamps: () => optionalArray('timestamps', BigInt64Array),
         speed: () => f64Optional('speed'),
         hr: () => f64Optional('hr'),
         cad: () => f64Optional('cad'),
         atemp: () => f64Optional('atemp'),
         power: () => f64Optional('power'),
-        slope_segment_slope: () => f64('slope_segment_slope'),
-        slope_segment_distance: () => f64('slope_segment_distance'),
+        slope_segment_starts: () => u32('slope_segment_starts'),
+        slope_segment_slopes: () => f64('slope_segment_slopes'),
+        selection_point: async (index) => (await invoke('selection_point', { index })) ?? undefined,
         surface_starts: () => u32('surface_starts'),
         surface_values: () => u8('surface_values'),
         surfaces: () => invoke('surfaces'),

website/src/lib/engine/backend/types.ts

@@ -6,6 +6,7 @@ import type {
     Outcome,
     RouteAttributes,
     Selection,
+    SelectionPoint,
     StorageOpened,
     TrackpointDetails,
     WaypointDetails,
@@ -277,14 +278,14 @@ export interface Backend {
     lng(): Promise<Float64Array>;
     lat(): Promise<Float64Array>;
     ele(): Promise<Float64Array>;
-    timestamps(): Promise<BigInt64Array | undefined>;
     speed(): Promise<Float64Array | undefined>;
     hr(): Promise<Float64Array | undefined>;
     cad(): Promise<Float64Array | undefined>;
     atemp(): Promise<Float64Array | undefined>;
     power(): Promise<Float64Array | undefined>;
-    slope_segment_slope(): Promise<Float64Array>;
-    slope_segment_distance(): Promise<Float64Array>;
+    slope_segment_starts(): Promise<Uint32Array>;
+    slope_segment_slopes(): Promise<Float64Array>;
+    selection_point(index: number): Promise<SelectionPoint | undefined>;
     surface_starts(): Promise<Uint32Array>;
     surface_values(): Promise<Uint8Array>;
     surfaces(): Promise<string[]>;

website/src/lib/engine/backend/wasm.ts

@@ -135,14 +135,14 @@ export async function createWasmBackend(): Promise<Backend> {
         lng: lift(w.lng),
         lat: lift(w.lat),
         ele: lift(w.ele),
-        timestamps: lift(w.timestamps),
         speed: lift(w.speed),
         hr: lift(w.hr),
         cad: lift(w.cad),
         atemp: lift(w.atemp),
         power: lift(w.power),
-        slope_segment_slope: lift(w.slope_segment_slope),
-        slope_segment_distance: lift(w.slope_segment_distance),
+        slope_segment_starts: lift(w.slope_segment_starts),
+        slope_segment_slopes: lift(w.slope_segment_slopes),
+        selection_point: lift(w.selection_point),
         surface_starts: lift(w.surface_starts),
         surface_values: lift(w.surface_values),
         surfaces: lift(w.surfaces),

website/src/lib/engine/engine.test.ts

@@ -9,7 +9,6 @@ import {
     ALL_EXPORT_OPTIONS,
     engine,
     FileStateCollectionObserver,
-    NO_TIME,
     type ExportOptions,
     type FileState,
 } from '$lib/engine';
@@ -195,7 +194,7 @@ describe('statistics', () => {
         assert.equal(statistics.totalDistance[0], 0);
         assert.ok(statistics.global.totalDistance > 0);
         assert.equal(statistics.global.totalDistance, file.statistics.totalDistance);
-        assert.equal(statistics.timestamps, undefined);
+        assert.equal((await statistics.point(0))!.time, undefined);
         assert.ok(statistics.global.bounds!.west <= statistics.global.bounds!.east);
 
         await engine.select([]);
@@ -206,9 +205,7 @@ describe('statistics', () => {
     it('has timestamps when the file has some', async () => {
         await load('with_time');
         const statistics = get(engine.statistics);
-        assert.ok(statistics.timestamps);
-        assert.equal(statistics.timestamps!.length, statistics.length);
-        assert.ok(statistics.timestamps!.some((t) => t !== NO_TIME));
+        assert.ok((await statistics.point(0))!.time! > 0);
         assert.ok(statistics.global.totalTime! > 0);
         assert.ok(statistics.global.movingSpeed === undefined || statistics.global.movingSpeed > 0);
     });
@@ -218,8 +215,13 @@ describe('statistics', () => {
         assert.equal(get(engine.statistics).hr, undefined);
 
         const request = engine.requestStatistics();
-        await request.set(['hr', 'anchors']);
+        await request.set(['hr', 'anchors', 'slopeSegment']);
         const statistics = get(engine.statistics);
+        assert.equal(statistics.slopeSegments!.starts[0], 0);
+        assert.equal(
+            statistics.slopeSegments!.starts.length,
+            statistics.slopeSegments!.slopes.length
+        );
         assert.equal(statistics.hr!.length, statistics.length);
         assert.ok(statistics.hr!.some((hr) => !Number.isNaN(hr)));
         assert.equal(statistics.cad, undefined);
@@ -248,6 +250,18 @@ describe('statistics', () => {
         await engine.select([]);
         assert.equal(await statistics.slice(2, 5), undefined);
     });
+
+    it('gives the data of a trackpoint of the selection', async () => {
+        await load('simple');
+        const statistics = get(engine.statistics);
+        const point = (await statistics.point(3))!;
+        assert.ok(point.slopeSegmentDistance! > 0);
+        assert.equal(await statistics.point(statistics.length), undefined);
+
+        // only for the selection it was read for
+        await engine.select([]);
+        assert.equal(await statistics.point(3), undefined);
+    });
 });
 
 describe('file state', () => {

website/src/lib/engine/statistics.ts

@@ -46,15 +46,16 @@ export class StatisticsReader {
     private async read(backend: Backend): Promise<SelectionStatistics> {
         const version = ++this._version;
         this._loaded.clear();
-        const [global, totalDistance, slope, lng, lat, ele, timestamps] = await Promise.all([
+        const [global, totalDistance, slope, lng, lat, ele] = await Promise.all([
             backend.selection_statistics(),
             backend.total_distance(),
             backend.slope(),
             backend.lng(),
             backend.lat(),
             backend.ele(),
-            backend.timestamps(),
         ]);
+        const current = <T>(read: () => Promise<T | undefined>) =>
+            this.enqueue(async () => (version === this._version ? read() : undefined));
         const statistics: SelectionStatistics = {
             global: global ?? EMPTY_STATISTICS,
             length: totalDistance.length,
@@ -63,11 +64,8 @@ export class StatisticsReader {
             lng,
             lat,
             ele,
-            timestamps,
-            slice: (start, end) =>
-                this.enqueue(async () =>
-                    version === this._version ? backend.slice_statistics(start, end) : undefined
-                ),
+            slice: (start, end) => current(() => backend.slice_statistics(start, end)),
+            point: (index) => current(() => backend.selection_point(index)),
         };
         await this.readMetrics(backend, statistics, this.requestedMetrics());
         return statistics;
@@ -114,13 +112,14 @@ export class StatisticsReader {
                     case 'power':
                         statistics.power = await backend.power();
                         break;
-                    case 'slopeSegment':
-                        [statistics.slopeSegmentSlope, statistics.slopeSegmentDistance] =
-                            await Promise.all([
-                                backend.slope_segment_slope(),
-                                backend.slope_segment_distance(),
-                            ]);
+                    case 'slopeSegment': {
+                        const [starts, slopes] = await Promise.all([
+                            backend.slope_segment_starts(),
+                            backend.slope_segment_slopes(),
+                        ]);
+                        statistics.slopeSegments = { starts, slopes };
                         break;
+                    }
                     case 'surface':
                         statistics.surface = await categories(
                             backend.surface_starts(),

website/src/lib/engine/trackpoint-categories.test.ts

@@ -2,7 +2,9 @@ import { describe, it } from 'node:test';
 import assert from 'node:assert/strict';
 import {
     categoryIntervals,
+    categoryValue,
     categoryValues,
+    intervalAt,
     type CategoryIntervals,
 } from '$lib/engine/trackpoint-categories';
 
@@ -68,3 +70,37 @@ describe('categoryIntervals', () => {
         }
     });
 });
+
+describe('intervalAt', () => {
+    const starts = new Uint32Array([0, 2, 3, 6]);
+
+    it('is the interval that the trackpoint is in', () => {
+        const intervals = [0, 1, 2, 3, 4, 5, 6, 9].map((index) => intervalAt(starts, index));
+        assert.deepEqual(intervals, [0, 0, 1, 2, 2, 2, 3, 3]);
+    });
+
+    it('is -1 when there is no interval', () => {
+        assert.equal(intervalAt(new Uint32Array(), 0), -1);
+    });
+});
+
+describe('categoryValue', () => {
+    const intervals: CategoryIntervals = {
+        starts: new Uint32Array([0, 2, 3, 6]),
+        values: new Uint8Array([1, 0, 2, 0]),
+        names: ['asphalt', 'gravel'],
+    };
+
+    it('is the value of each trackpoint, as categoryValues', () => {
+        const values = categoryValues(intervals, 7);
+        values.forEach((value, index) => assert.equal(categoryValue(intervals, index), value));
+    });
+
+    it('is unknown without intervals', () => {
+        assert.equal(categoryValue(undefined, 3), undefined);
+        assert.equal(
+            categoryValue({ starts: new Uint32Array(), values: new Uint8Array(), names: [] }, 0),
+            undefined
+        );
+    });
+});

website/src/lib/engine/trackpoint-categories.ts

@@ -36,6 +36,34 @@ export function categoryValues(
     return result;
 }
 
+/** The index of the interval of `starts` (sorted) that the trackpoint `index` is in, -1 if none. */
+export function intervalAt(starts: Uint32Array, index: number): number {
+    let low = 0;
+    let high = starts.length;
+    // the first interval that starts after the trackpoint
+    while (low < high) {
+        const middle = (low + high) >>> 1;
+        if (starts[middle] <= index) {
+            low = middle + 1;
+        } else {
+            high = middle;
+        }
+    }
+    return low - 1;
+}
+
+/** The value of the trackpoint `index` for a category, `undefined` when it is unknown. */
+export function categoryValue(
+    intervals: CategoryIntervals | undefined,
+    index: number
+): string | undefined {
+    if (!intervals) {
+        return undefined;
+    }
+    const value = intervals.values[intervalAt(intervals.starts, index)];
+    return value ? intervals.names[value - 1] : undefined;
+}
+
 /**
  * The intervals of the trackpoints that share a value, for a value per trackpoint (`undefined`
  * when unknown): the inverse of `categoryValues`. The names are in the order of their first

website/src/lib/engine/types.ts

@@ -2,7 +2,7 @@ import type { Readable } from 'svelte/store';
 import type { Feature, FeatureCollection, LineString, Point } from 'geojson';
 import type { Visibility } from '$lib/engine/file-visibility';
 import type { CategoryIntervals } from '$lib/engine/trackpoint-categories';
-import type { FileStructure, GlobalStatistics } from 'gpx-wasm';
+import type { FileStructure, GlobalStatistics, SelectionPoint } from 'gpx-wasm';
 
 export type { FileStatus, FolderEntry, UnsavedChoice, WorkspaceState } from './backend/types';
 
@@ -12,6 +12,7 @@ export type {
     TrackpointDetails,
     MoveTarget,
     GlobalStatistics,
+    SelectionPoint,
     FileStructure,
     FilesUpdate,
     Outcome,
@@ -136,12 +137,6 @@ export const ALL_EXPORT_OPTIONS: ExportOptions = {
     asRoute: false,
 };
 
-/**
- * Timestamp of the trackpoints that have none, in `SelectionStatistics.timestamps` (the smallest
- * 64-bit integer, like `NO_TIME` in the engine).
- */
-export const NO_TIME = -(2n ** 63n);
-
 /**
  * Statistics of the selection that are only read from the engine when something asks for them
  * (see `Engine.requestStatistics`). A metric can bring several arrays.
@@ -152,7 +147,7 @@ export type StatisticsMetric =
     | 'cad'
     | 'atemp'
     | 'power'
-    /** `slopeSegmentSlope` and `slopeSegmentDistance`. */
+    /** `slopeSegments`. */
     | 'slopeSegment'
     | 'surface'
     /** `highway`, `sacScale` and `mtbScale`. */
@@ -185,17 +180,29 @@ export type SelectionAnchors = {
     revision: number;
 };
 
+/**
+ * The smoothed elevation segments of the selection, over which the slope is averaged, as the
+ * intervals of trackpoints that belong to the same one: segment `i` starts at the trackpoint
+ * `starts[i]` and lasts until the next start (or the last trackpoint), with a slope of
+ * `slopes[i]` (%). Their length is in `SelectionStatistics.point`.
+ */
+export type SlopeSegments = {
+    starts: Uint32Array;
+    slopes: Float64Array;
+};
+
 /**
  * The statistics of the selection: its global statistics, and the values at each of its
- * trackpoints (all the selected segments, one after the other). The arrays are copies.
+ * trackpoints (all the selected segments, one after the other). The arrays are copies. What is
+ * only needed for one trackpoint at a time (its time, its OSM data...) is read with `point`.
  *
  * Distances are cumulative over the whole selection. Distances are in km, speeds in km/h, times in
  * ms (timestamps since the epoch, as 64-bit integers), elevations in m and slopes in %.
  *
  * The values of the trackpoints that the selection does not have at all (for example the heart
  * rates, when no trackpoint has any) are `undefined`, and so are the ones that were not requested
- * (see `StatisticsMetric`). Otherwise they have one entry per trackpoint: missing timestamps are
- * `NO_TIME`, missing measures are NaN.
+ * (see `StatisticsMetric`). Otherwise they have one entry per trackpoint, missing measures are
+ * NaN.
  */
 export type SelectionStatistics = {
     global: GlobalStatistics;
@@ -207,7 +214,6 @@ export type SelectionStatistics = {
     lng: Float64Array;
     lat: Float64Array;
     ele: Float64Array;
-    timestamps?: BigInt64Array;
 
     // the following need to be requested
     speed?: Float64Array;
@@ -215,9 +221,7 @@ export type SelectionStatistics = {
     cad?: Float64Array;
     atemp?: Float64Array;
     power?: Float64Array;
-    /** Slope and length (km) of the smoothed elevation segment the trackpoint belongs to. */
-    slopeSegmentSlope?: Float64Array;
-    slopeSegmentDistance?: Float64Array;
+    slopeSegments?: SlopeSegments;
     surface?: CategoryIntervals;
     highway?: CategoryIntervals;
     /** SAC hiking scale. */
@@ -232,6 +236,12 @@ export type SelectionStatistics = {
      * in the selection, or if the selection is not the current one anymore.
      */
     slice(start: number, end: number): Promise<GlobalStatistics | undefined>;
+    /**
+     * All the data of the trackpoint at `index`, for example the one hovered on the elevation
+     * profile. `undefined` if it is not in the selection, or if the selection is not the current
+     * one anymore.
+     */
+    point(index: number): Promise<SelectionPoint | undefined>;
 };
 
 export const EMPTY_SELECTION_STATISTICS: SelectionStatistics = {
@@ -243,6 +253,7 @@ export const EMPTY_SELECTION_STATISTICS: SelectionStatistics = {
     lat: new Float64Array(),
     ele: new Float64Array(),
     slice: async () => undefined,
+    point: async () => undefined,
 };
 
 export type StatisticsRequest = {

website/src/lib/scripts/example-statistics.ts

@@ -48,8 +48,11 @@ export async function computeExampleData() {
     engine.load_files(new Uint8Array(data), Uint32Array.from([data.length]), ['example']);
 
     // numbers are enough for the times, which are milliseconds
-    const numbers = (values: Values) =>
+    const numbers = (values: Values | Uint32Array) =>
         values && Array.from(values as ArrayLike<number | bigint>, (value) => Number(value));
+    const length = engine.total_distance().length;
+    const points = Array.from({ length }, (_, index) => engine.selection_point(index)!);
+    const slopeSegmentStarts = Array.from(engine.slope_segment_starts());
     return {
         global: engine.selection_statistics(),
         totalDistance: numbers(engine.total_distance()),
@@ -59,12 +62,16 @@ export async function computeExampleData() {
         elevationGain: numbers(engine.elevation_gain()),
         elevationLoss: numbers(engine.elevation_loss()),
         slope: numbers(engine.slope()),
-        slopeSegmentSlope: numbers(engine.slope_segment_slope()),
-        slopeSegmentDistance: numbers(engine.slope_segment_distance()),
+        slopeSegmentStarts,
+        slopeSegmentSlopes: numbers(engine.slope_segment_slopes()),
+        // the length of each segment, which is the one of its trackpoints
+        slopeSegmentDistances: slopeSegmentStarts.map(
+            (start) => points[start].slopeSegmentDistance!
+        ),
         lng: numbers(engine.lng()),
         lat: numbers(engine.lat()),
         ele: numbers(engine.ele()),
-        timestamps: numbers(engine.timestamps()),
+        timestamps: points.map((point) => point.time!),
         speed: numbers(engine.speed()),
         atemp: numbers(engine.atemp()),
     };