literal: round-half-even tie for normal-range float/complex repr

youknowone · youknowone · commit 374d56202e23 · 2026-07-09T17:04:20.000+09:00
to_string's normal-magnitude branch and complex::component_to_string
returned Rust's shortest formatting directly, which can pick the odd-digit
neighbour of a rounding tie where repr() picks the even one (e.g. bits
0x42e26687db6b9b04 formats as 161852602146008.13 vs repr's ...08.12).
Route both through prefer_cpython_tie_repr, generalized to fixed-notation
strings. Add unit tests and float/complex repr snippet tests.

Assisted-by: Claude
diff --git a/crates/literal/src/complex.rs b/crates/literal/src/complex.rs
@@ -19,9 +19,9 @@ fn component_to_string(value: f64) -> String {
         if exponent < 16 && exponent > -5 {
             // Normal magnitude — Rust's default Display emits "1" for 1.0,
             // "1.5" for 1.5, "1000000000000000" for 1e15, etc.
-            value.to_string()
+            float::prefer_cpython_tie_repr(value.to_string(), value)
         } else {
-            alloc::format!("{significand}e{exponent:+#03}")
+            float::prefer_cpython_tie_repr(alloc::format!("{significand}e{exponent:+#03}"), value)
         }
     } else {
         // nan / inf / -inf — `format!("{x:e}")` produces e.g. "NaN" with no
diff --git a/crates/literal/src/float.rs b/crates/literal/src/float.rs
@@ -209,11 +209,13 @@ pub fn format_general(
     }
 }
 
-fn prefer_cpython_tie_repr(s: String, value: f64) -> String {
-    let Some(exponent_pos) = s.find('e') else {
-        return s;
-    };
-    let Some(digit_pos) = s[..exponent_pos].bytes().rposition(|b| b.is_ascii_digit()) else {
+pub(crate) fn prefer_cpython_tie_repr(s: String, value: f64) -> String {
+    // Rust's shortest float formatter can land on the odd-digit neighbour of a
+    // rounding tie where round-half-to-even (what `repr` uses) picks the even
+    // one. When the last significant digit is odd and its even neighbour still
+    // round-trips and is no further from the value, prefer the even neighbour.
+    let boundary = s.find('e').unwrap_or(s.len());
+    let Some(digit_pos) = s[..boundary].bytes().rposition(|b| b.is_ascii_digit()) else {
         return s;
     };
 
@@ -258,11 +260,11 @@ fn checked_pow_u128(base: u128, exp: u32) -> Option<u128> {
 }
 
 fn parse_decimal_rational(s: &str) -> Option<(u128, u32)> {
-    let exponent_pos = s.find('e')?;
-    let exponent = s[exponent_pos + 1..].parse::<i32>().ok()?;
-    let significand = s[..exponent_pos]
-        .strip_prefix('-')
-        .unwrap_or(&s[..exponent_pos]);
+    let (mantissa, exponent) = match s.find('e') {
+        Some(pos) => (&s[..pos], s[pos + 1..].parse::<i32>().ok()?),
+        None => (s, 0),
+    };
+    let significand = mantissa.strip_prefix('-').unwrap_or(mantissa);
     let dot_pos = significand.find('.');
     let frac_digits = dot_pos
         .map(|pos| significand.len().saturating_sub(pos + 1))
@@ -325,7 +327,7 @@ pub fn to_string(value: f64) -> String {
             if is_integer(value) {
                 format!("{value:.1?}")
             } else {
-                value.to_string()
+                prefer_cpython_tie_repr(value.to_string(), value)
             }
         } else {
             prefer_cpython_tie_repr(format!("{significand}e{exponent:+#03}"), value)
@@ -351,6 +353,21 @@ mod tests {
             "6.1005353927612305e-05"
         );
     }
+
+    #[test]
+    fn repr_normal_range_uses_cpython_tie_digit() {
+        // Rust's shortest formatter yields "161852602146008.13" for this
+        // value; round-half-to-even (what `repr` uses) picks "…08.12".
+        assert_eq!(
+            to_string(f64::from_bits(0x42e26687db6b9b04)),
+            "161852602146008.12"
+        );
+        // Non-tie values are left untouched.
+        assert_eq!(to_string(1.5), "1.5");
+        assert_eq!(to_string(0.1), "0.1");
+        assert_eq!(to_string(3.14), "3.14");
+        assert_eq!(to_string(100.0), "100.0");
+    }
 }
 
 pub fn from_hex(s: &str) -> Option<f64> {
diff --git a/extra_tests/snippets/builtin_complex.py b/extra_tests/snippets/builtin_complex.py
@@ -268,3 +268,10 @@ class complex_subclass(complex):
 assert repr(float("-inf") + 1j) == "(-inf+1j)"
 assert repr(complex(1, float("nan"))) == "(1+nanj)"
 assert repr(complex(1, float("inf"))) == "(1+infj)"
+
+# Round-half-to-even ties: Rust's shortest formatter can land on the
+# odd-digit neighbour where repr()'s tie-breaking picks the even one.
+assert repr(161852602146008.12 + 1j) == "(161852602146008.12+1j)"
+assert repr(-788830060729777.2 + 2j) == "(-788830060729777.2+2j)"
+assert repr(complex(0.0, 1959276370239205.2)) == "1959276370239205.2j"
+assert repr(complex(-1818262230632059.2, 0.0)) == "(-1818262230632059.2+0j)"
diff --git a/extra_tests/snippets/builtin_float.py b/extra_tests/snippets/builtin_float.py
@@ -549,3 +549,15 @@ def _check_msg(call, exc_type, expected_msg):
     lambda: INF.__int__(), OverflowError, "cannot convert float infinity to integer"
 )
 _check_msg(lambda: NAN.__floor__(), ValueError, "cannot convert float NaN to integer")
+
+# repr round-half-to-even ties: Rust's shortest formatter can land on the
+# odd-digit neighbour where repr()'s tie-breaking picks the even one.
+assert repr(161852602146008.12) == "161852602146008.12"
+assert repr(-788830060729777.2) == "-788830060729777.2"
+assert repr(1959276370239205.2) == "1959276370239205.2"
+assert repr(-1818262230632059.2) == "-1818262230632059.2"
+assert str(161852602146008.12) == "161852602146008.12"
+# non-tie values are unaffected
+assert repr(1.5) == "1.5"
+assert repr(0.1) == "0.1"
+assert repr(100.0) == "100.0"
