From 1c3c25035ce6398b31046375485ed0100e8ce247 Mon Sep 17 00:00:00 2001
From: MarekSpodBiedry <73122968+MarekSpodBiedry@users.noreply.github.com>
Date: Sat, 24 Oct 2026 07:51:26 +0200
Subject: [PATCH] List binary fractions in the Units precision drop-down for
 Architectural and Fractional (#1861) (#1952)

---
 crates/ui-egui/src/dialogs.rs | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/crates/ui-egui/src/dialogs.rs b/crates/ui-egui/src/dialogs.rs
index 58e9cf02..2a46d7ed 100644
--- a/crates/ui-egui/src/dialogs.rs
+++ b/crates/ui-egui/src/dialogs.rs
@@ -235,6 +235,17 @@ fn pick(ui: &mut egui::Ui, salt: &str, value: &mut i64, options: &[(i64, String)
     });
 }
 
+/// Length precision choices for a LUNITS format. Architectural (4) and Fractional (5) step through
+/// binary fractions of an inch (LUPREC n is 1/2^n, 0 is whole inches); the other formats show a decimal sample.
+fn length_precisions(lunits: i64) -> Vec<(i64, String)> {
+    use cadcraft_engine::units::format_distance;
+    if matches!(lunits, 4 | 5) {
+        (0..=8).map(|p| (p, if p == 0 { "1\"".to_string() } else { format!("1/{}\"", 1i64 << p) })).collect()
+    } else {
+        (0..=8).map(|p| (p, format_distance(1.5, lunits, p))).collect()
+    }
+}
+
 /// Drawing Units (UNITS): edits a copy of LUNITS/LUPREC/AUNITS/AUPREC/INSUNITS and applies it
 /// through the `units` command on OK.
 fn units(app: &mut CadApp, ctx: &egui::Context, open: &mut bool) {
@@ -260,7 +271,7 @@ fn units(app: &mut CadApp, ctx: &egui::Context, open: &mut bool) {
             pick(ui, "units_lunits", lu, &types(&LENGTH_TYPES));
             ui.end_row();
             ui.label("Precision");
-            pick(ui, "units_luprec", lp, &(0..=8).map(|p| (p, format_distance(1.5, *lu, p))).collect::<Vec<_>>());
+            pick(ui, "units_luprec", lp, &length_precisions(*lu));
             ui.end_row();
             ui.strong("Angle");
             ui.end_row();
@@ -433,6 +444,15 @@ mod tests {
 
     use crate::{CadApp, Services};
 
+    #[test]
+    fn fractional_length_precisions_list_binary_fractions() {
+        let labels = ["1\"", "1/2\"", "1/4\"", "1/8\"", "1/16\"", "1/32\"", "1/64\"", "1/128\"", "1/256\""];
+        let want: Vec<(i64, String)> = labels.iter().enumerate().map(|(k, l)| (k as i64, l.to_string())).collect();
+        assert_eq!(super::length_precisions(4), want);
+        assert_eq!(super::length_precisions(5), want);
+        assert_eq!(super::length_precisions(2).len(), 9);
+    }
+
     #[test]
     fn picking_a_block_starts_insert_with_its_name() {
         let mut app = CadApp::new(Session::new(), Services::default());