refac(console): factor out Durations widget from task view (#408)

There are 2 widgets which display the poll times for a task in the
detail view. The poll times percentiles are always displayed and if
UTF-8 is available, then a sparkline histogram is also shown to the
right.

The logic for displaying these two widgets is quite long and is
currently interspersed within the `render` function for the task detail
view plus helper functions. Additionally, it is not easy to add a second
set of widgets showing the time between waking and being polled for a
task which is planned for #409.

This change factors out that logic into separate widgets.

There was already a separate widget `MiniHistogram`. Some of the logic
that was previously in the task detail view has been moved here.

A new widget `Percentiles` has been added to encapsulate the logic for
preparing and displaying the percentiles.

A top level `Durations` widget occupies the entire width of the task
detail view and control the horizontal layout of the `Percentiles` and
`MiniHistogram` widgets. The new widget will also supress the histogram
if there isn't at least enough room to display the legend at the bottom

tokio-console/src/view/durations.rs

@@ -0,0 +1,108 @@
+use std::cmp;
+
+use tui::{
+    layout::{self},
+    widgets::Widget,
+};
+
+use crate::{
+    state::histogram::DurationHistogram,
+    view::{self, mini_histogram::MiniHistogram, percentiles::Percentiles},
+};
+
+// This is calculated so that a legend like the below generally fits:
+// │0647.17µs  909.31µs │
+// This also gives at characters for the sparkline itself.
+const MIN_HISTOGRAM_BLOCK_WIDTH: u16 = 22;
+
+/// This is a tui-rs widget to visualize durations as a list of percentiles
+/// and if possible, a mini-histogram too.
+///
+/// This widget wraps the [`Percentiles`] and [`MiniHistogram`] widgets which
+/// are displayed side by side. The mini-histogram will only be displayed if
+///   a) UTF-8 support is enabled via [`Styles`]
+///   b) There is at least a minimum width (22 characters to display the full
+///      bottom legend) left after drawing the percentiles
+///
+/// This
+///
+/// [`Styles`]: crate::view::Styles
+pub(crate) struct Durations<'a> {
+    /// Widget style
+    styles: &'a view::Styles,
+    /// The histogram data to render
+    histogram: Option<&'a DurationHistogram>,
+    /// Title for percentiles block
+    percentiles_title: &'a str,
+    /// Title for histogram sparkline block
+    histogram_title: &'a str,
+}
+
+impl<'a> Widget for Durations<'a> {
+    fn render(self, area: tui::layout::Rect, buf: &mut tui::buffer::Buffer) {
+        // Only split the durations area in half if we're also drawing a
+        // sparkline. We require UTF-8 to draw the sparkline and also enough width.
+        let (percentiles_area, histogram_area) = if self.styles.utf8 {
+            let percentiles_width = cmp::max(self.percentiles_title.len() as u16, 13_u16) + 2;
+
+            // If there isn't enough width left after drawing the percentiles
+            // then we won't draw the sparkline at all.
+            if area.width < percentiles_width + MIN_HISTOGRAM_BLOCK_WIDTH {
+                (area, None)
+            } else {
+                let areas = layout::Layout::default()
+                    .direction(layout::Direction::Horizontal)
+                    .constraints(
+                        [
+                            layout::Constraint::Length(percentiles_width),
+                            layout::Constraint::Min(MIN_HISTOGRAM_BLOCK_WIDTH),
+                        ]
+                        .as_ref(),
+                    )
+                    .split(area);
+                (areas[0], Some(areas[1]))
+            }
+        } else {
+            (area, None)
+        };
+
+        let percentiles_widget = Percentiles::new(self.styles)
+            .title(self.percentiles_title)
+            .histogram(self.histogram);
+        percentiles_widget.render(percentiles_area, buf);
+
+        if let Some(histogram_area) = histogram_area {
+            let histogram_widget = MiniHistogram::default()
+                .block(self.styles.border_block().title(self.histogram_title))
+                .histogram(self.histogram)
+                .duration_precision(2);
+            histogram_widget.render(histogram_area, buf);
+        }
+    }
+}
+
+impl<'a> Durations<'a> {
+    pub(crate) fn new(styles: &'a view::Styles) -> Self {
+        Self {
+            styles,
+            histogram: None,
+            percentiles_title: "Percentiles",
+            histogram_title: "Histogram",
+        }
+    }
+
+    pub(crate) fn histogram(mut self, histogram: Option<&'a DurationHistogram>) -> Self {
+        self.histogram = histogram;
+        self
+    }
+
+    pub(crate) fn percentiles_title(mut self, title: &'a str) -> Self {
+        self.percentiles_title = title;
+        self
+    }
+
+    pub(crate) fn histogram_title(mut self, title: &'a str) -> Self {
+        self.histogram_title = title;
+        self
+    }
+}