Measure actual width of displayed characters

src/widthdb.rs

@@ -0,0 +1,46 @@
+use std::collections::{HashMap, HashSet};
+use std::io::{self, Write};
+
+use crossterm::cursor::MoveTo;
+use crossterm::style::Print;
+use crossterm::terminal::{Clear, ClearType};
+use crossterm::QueueableCommand;
+use unicode_segmentation::UnicodeSegmentation;
+use unicode_width::UnicodeWidthStr;
+
+#[derive(Debug, Default)]
+pub struct WidthDB {
+    known: HashMap<String, u8>,
+    requested: HashSet<String>,
+}
+
+impl WidthDB {
+    pub fn width(&mut self, s: &str) -> u8 {
+        let mut total = 0;
+        for grapheme in s.graphemes(true) {
+            total += if let Some(width) = self.known.get(grapheme) {
+                *width
+            } else {
+                self.requested.insert(grapheme.to_string());
+                grapheme.width() as u8
+            };
+        }
+        total
+    }
+
+    pub fn measuring_required(&self) -> bool {
+        !self.requested.is_empty()
+    }
+
+    pub fn measure_widths(&mut self, out: &mut impl Write) -> io::Result<()> {
+        for grapheme in self.requested.drain() {
+            out.queue(Clear(ClearType::All))?
+                .queue(MoveTo(0, 0))?
+                .queue(Print(&grapheme))?;
+            out.flush()?;
+            let width = crossterm::cursor::position()?.0.max(1) as u8;
+            self.known.insert(grapheme, width);
+        }
+        Ok(())
+    }
+}