Initial Rust WSON serde implementation

src/text/float_fmt.rs

@@ -0,0 +1,90 @@
+/// Formats `f64` with Java `Double.toString` semantics as closely as Rust's
+/// shortest-roundtrip formatting allows.
+pub fn format_java_double(value: f64) -> String {
+    if value.is_nan() {
+        return "NaN".to_string();
+    }
+    if value == f64::INFINITY {
+        return "Infinity".to_string();
+    }
+    if value == f64::NEG_INFINITY {
+        return "-Infinity".to_string();
+    }
+    if value == 0.0 {
+        return if value.is_sign_negative() {
+            "-0.0".to_string()
+        } else {
+            "0.0".to_string()
+        };
+    }
+
+    let scientific = format!("{value:e}");
+    let (mantissa, exp) = scientific
+        .split_once('e')
+        .expect("lowerexp formatting always contains e");
+    let exponent: i32 = exp.parse().expect("lowerexp exponent is an integer");
+    let negative = mantissa.starts_with('-');
+    let mantissa = mantissa.trim_start_matches('-');
+    let digits: String = mantissa.chars().filter(|c| *c != '.').collect();
+
+    // Java uses plain decimal notation for 1e-3 <= abs(value) < 1e7.
+    if (-3..7).contains(&exponent) {
+        let point_pos = exponent + 1;
+        let mut out = String::new();
+        if negative {
+            out.push('-');
+        }
+        if point_pos <= 0 {
+            out.push_str("0.");
+            for _ in 0..(-point_pos) {
+                out.push('0');
+            }
+            out.push_str(&digits);
+        } else if point_pos as usize >= digits.len() {
+            out.push_str(&digits);
+            for _ in 0..(point_pos as usize - digits.len()) {
+                out.push('0');
+            }
+            out.push_str(".0");
+        } else {
+            let split = point_pos as usize;
+            out.push_str(&digits[..split]);
+            out.push('.');
+            out.push_str(&digits[split..]);
+        }
+        out
+    } else {
+        let mut out = String::new();
+        if negative {
+            out.push('-');
+        }
+        out.push(digits.as_bytes()[0] as char);
+        out.push('.');
+        if digits.len() == 1 {
+            out.push('0');
+        } else {
+            out.push_str(&digits[1..]);
+        }
+        out.push('E');
+        out.push_str(&exponent.to_string());
+        out
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::format_java_double;
+
+    #[test]
+    fn java_double_formatting() {
+        assert_eq!(format_java_double(1.0), "1.0");
+        assert_eq!(format_java_double(36.0), "36.0");
+        assert_eq!(format_java_double(0.001), "0.001");
+        assert_eq!(format_java_double(0.0001), "1.0E-4");
+        assert_eq!(format_java_double(9_999_999.0), "9999999.0");
+        assert_eq!(format_java_double(10_000_000.0), "1.0E7");
+        assert_eq!(format_java_double(1e22), "1.0E22");
+        assert_eq!(format_java_double(1e-7), "1.0E-7");
+        assert_eq!(format_java_double(-2.5), "-2.5");
+    }
+}