Avoid returning negative zeroes in v2s_f32_rounded

This ensures that values roundtrip correctly since -0.0 and 0.0
correspond to the same normalized value.

CHANGELOG.md

@@ -10,6 +10,14 @@ Since there is no stable release yet, the changes are organized per day in
 reverse chronological order. The main purpose of this document in its current
 state is to list breaking changes.
 
+## [2023-04-27]
+
+### Changed
+
+- The `v2s_f32_rounded()` formatter now avoids returning negative zero values
+  for roundtripping reasons since -0.0 and 0.0 correspond to the same normalized
+  value.
+
 ## [2023-04-24]
 
 ### Breaking changes

src/formatters.rs

@@ -13,9 +13,19 @@ use crate::util;
 // TODO: The v2s and s2v naming convention isn't ideal, but at least it's unambiguous. Is there a
 //       better way to name these functions? Should we just split this up into two modules?
 
-/// Round an `f32` value to always have a specific number of decimal digits.
+/// Round an `f32` value to always have a specific number of decimal digits. Avoids returning
+/// negative zero values to make sure string->value->string roundtrips work correctly. Otherwise
+/// `-0.001` rounded to two digits would result in `-0.00`.
 pub fn v2s_f32_rounded(digits: usize) -> Arc<dyn Fn(f32) -> String + Send + Sync> {
-    Arc::new(move |value| format!("{value:.digits$}"))
+    let rounding_multiplier = 10u32.pow(digits as u32) as f32;
+    Arc::new(move |value| {
+        // See above
+        if (value * rounding_multiplier).round() / rounding_multiplier == 0.0 {
+            format!("{:.digits$}", 0.0)
+        } else {
+            format!("{value:.digits$}")
+        }
+    })
 }
 
 /// Format a `[0, 1]` number as a percentage. Does not include the percent sign, you should specify
@@ -324,6 +334,18 @@ pub fn s2v_bool_bypass() -> Arc<dyn Fn(&str) -> Option<bool> + Send + Sync> {
 mod tests {
     use super::*;
 
+    /// The rounding function should never return strings containing negative zero values.
+    #[test]
+    fn v2s_f32_rounded_negative_zero() {
+        let v2s = v2s_f32_rounded(2);
+
+        assert_eq!("0.00", v2s(-0.001));
+
+        // Sanity check
+        assert_eq!("-0.01", v2s(-0.009));
+        assert_eq!("0.01", v2s(0.009));
+    }
+
     // More of these validators could use tests, but this one in particular is tricky and I noticed
     // an issue where it didn't roundtrip correctly
     #[test]