fix: F56 Display trait new algorithm to display 12 sig figs

Author: StevenLove

builtins/src/types/numbers.rs

@@ -96,6 +96,52 @@ mod tests {
         )
     }
 
+    #[test]
+    fn test_f56_formatting() {
+        // F56 should have a maximum of 12 decimal digits displayed
+        // And the 12th digit should be rounded using the 13th digit
+        // If we input an f64 with the 13th digit of 5
+        // The nearest F56 representation might have the 13th digit become 4 or 6
+        // So be wary that rounding may not occur as expected
+        // 3.999_999_999_995 rounds down instead of rounding up to 4
+        let map_from_value_to_expected_string = [
+            // (3.999_999_999_995, "4"), // bad rounding
+            (399.999_999_999_58527_f64, "400"),
+            (399.999_999_999_585_f64, "400"),
+            (399.999_999_999_58_f64, "400"),
+            (399.999_999_999_6_f64, "400"),
+            (399.999_999_999_f64, "399.999999999"),
+            (
+                0.000_000_012_312_312_412_412_312_3_f64,
+                "0.0000000123123124124",
+            ),
+            (0.000_000_012_312_312_452_57_f64, "0.0000000123123124526"),
+            (399_999_999.999_58_f64, "400000000"),
+            (
+                399_999_999_999_600_000_000_000_000_000_000_0_f64,
+                "4000000000000000000000000000000000",
+            ),
+            (
+                399_999_999_999_600_000_000_000_000_000_000_0_f64,
+                "4000000000000000000000000000000000",
+            ),
+            // special values
+            (f64::NAN, "NaN"),
+            (f64::INFINITY, "inf"),
+            (f64::NEG_INFINITY, "-inf"),
+            (0.0, "0"),
+            (-0.0, "-0"),
+            (f64::MIN_POSITIVE, "0"),
+            (F56::MIN_POSITIVE.into(), "0.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000002983336292479999"),
+        ];
+        for (f, expected) in map_from_value_to_expected_string.iter() {
+            // println!("{:?}", f);
+            let f56 = F56::from(*f);
+            let formatted = format!("{:}", f56);
+            assert_eq!(formatted, *expected);
+        }
+    }
+
     #[test]
     fn test_i32_conversions_rust_to_value() {
         let mut vm = new_slosh_vm();

slosh/benches/all.rs

@@ -92,7 +92,8 @@ fn run_float_script(n: usize, m: f32, expected: f64) {
     let last = run_reader(&mut reader);
     match last {
         Ok(Value::Float(f)) => {
-            assert_eq!(f64::from(f), expected);
+            assert_eq!(f.to_string(), expected.to_string());
+            // assert_eq!(f64::from(f), expected);
         }
         _ => {
             panic!("Not a float");

slosh/benches/bench.slosh

@@ -4,13 +4,13 @@
 
 (def pu (eval-pol 100 0.5))
 (prn "(eval-pol 100 0.5) =>\n" pu)
-(prn "passed: " (equal? pu 399.99999999958527))
+(prn "passed: " (equal? pu 400))
 
 (def pu (eval-pol 1000 0.05))
 (prn "(eval-pol 1000 0.05) =>\n" pu)
-(prn "passed: " (equal? pu 2105.2631578924484))
+(prn "passed: " (equal? pu 2105.26315789))
 
 (def pu (eval-pol 10000 0.2))
 (prn "(eval-pol 10000 0.2) =>\n" pu)
-(prn "passed: " (equal? pu 24999.999999998603))
+(prn "passed: " (equal? pu 2500))
 

vm/src/value.rs

@@ -152,17 +152,18 @@ impl Display for F56 {
         // F56 only has 12, meaning that .0023 appears as .0022999999999999687
         // if F56 knew to only print 12 digits then it would be fine,
         // but when going through f64, it thinks it has 15 perfect digits.
+        let as_f64 = f64::from(*self);
 
-        // We can set the precision to 12 but that increases numbers like 1.0 to 1.0000000000000
-        // So let's do that, and remove the trailing zeros and decimal points added on
-        let first_pass = format!("{:.*}", F56::DIGITS, f64::from(*self));
-        // remove trailing zeros and trailing decimal point
-        let second_pass = if first_pass.contains('.') {
-            first_pass.trim_end_matches('0').trim_end_matches('.')
-        } else {
-            &first_pass
-        };
-        write!(f, "{}", second_pass)
+        // Handle special cases by printing the f64 value
+        if as_f64.is_nan() || as_f64.is_infinite() || as_f64 == 0.0 {
+            return write!(f, "{}", as_f64);
+        }
+
+        // round to a max of F56::DIGITS sig figs
+        let exponent_value = as_f64.log10().floor() as i32; // the number after 'e' in scientific notation
+        let scale = 10f64.powi(exponent_value - F56::DIGITS as i32 + 1);
+        let rounded = (as_f64 / scale).round() * scale;
+        write!(f, "{}", rounded)
     }
 }
 impl From<f64> for F56 {