Add precision support for fixnums (#437)

- [ ] Changelog updated / no changelog update needed

agb-fixnum/src/lib.rs

@@ -539,16 +539,44 @@ impl<I: FixedWidthUnsignedInteger, const N: usize> Display for Num<I, N> {
         //
         // But if you think of a negative number, you'd like it to be `negative number - non negative fraction`
         // So we have to add 1 to the integral bit, and take 1 - fractional bit
-        if fractional != I::zero() && integral < I::zero() {
+        let sign = if fractional != I::zero() && integral < I::zero() {
             integral = integral + I::one();
-            if integral == I::zero() {
-                // If the number is in the range (-1, 0), then we just bumped `integral` from -1 to 0,
-                // so we need to compensate for the missing negative sign.
-                write!(f, "-")?;
-            }
             fractional = (I::one() << N) - fractional;
+            -1
+        } else {
+            1
+        };
+
+        if let Some(precision) = f.precision() {
+            let precision_multiplier = I::from_as_i32(10_i32.pow(precision as u32));
+
+            let fractional_as_integer = fractional * precision_multiplier * I::ten();
+            let mut fractional_as_integer = fractional_as_integer >> N;
+
+            if fractional_as_integer % I::ten() >= I::from_as_i32(5) {
+                fractional_as_integer = fractional_as_integer + I::ten();
             }
 
+            let mut fraction_to_write = fractional_as_integer / I::ten();
+
+            if fraction_to_write >= precision_multiplier {
+                integral = integral + I::from_as_i32(sign);
+                fraction_to_write = fraction_to_write - precision_multiplier;
+            }
+
+            if sign == -1 && integral == I::zero() && fraction_to_write != I::zero() {
+                write!(f, "-")?;
+            }
+
+            write!(f, "{integral}")?;
+
+            if precision != 0 {
+                write!(f, ".{:#0width$}", fraction_to_write, width = precision)?;
+            }
+        } else {
+            if sign == -1 && integral == I::zero() {
+                write!(f, "-")?;
+            }
             write!(f, "{integral}")?;
 
             if fractional != I::zero() {
@@ -560,6 +588,7 @@ impl<I: FixedWidthUnsignedInteger, const N: usize> Display for Num<I, N> {
                 write!(f, "{}", (fractional & !mask) >> N)?;
                 fractional = fractional & mask;
             }
+        }
 
         Ok(())
     }
@@ -1069,6 +1098,51 @@ mod tests {
         assert_eq!(format!("{d}"), "-0.25");
     }
 
+    mod precision {
+        use super::*;
+
+        macro_rules! num_ {
+            ($n: literal) => {{
+                let a: Num<i32, 20> = num!($n);
+                a
+            }};
+        }
+
+        macro_rules! test_precision {
+            ($TestName: ident, $Number: literal, $Expected: literal) => {
+                test_precision! { $TestName, $Number, $Expected, 2 }
+            };
+            ($TestName: ident, $Number: literal, $Expected: literal, $Digits: literal) => {
+                #[test]
+                fn $TestName() {
+                    assert_eq!(
+                        format!("{:.width$}", num_!($Number), width = $Digits),
+                        $Expected
+                    );
+                }
+            };
+        }
+
+        test_precision!(positive_down, 1.2345678, "1.23");
+        test_precision!(positive_round_up, 1.237, "1.24");
+        test_precision!(negative_round_down, -1.237, "-1.24");
+
+        test_precision!(trailing_zero, 1.5, "1.50");
+        test_precision!(leading_zero, 1.05, "1.05");
+
+        test_precision!(positive_round_to_next_integer, 3.999, "4.00");
+        test_precision!(negative_round_to_next_integer, -3.999, "-4.00");
+
+        test_precision!(negative_round_to_1, -0.999, "-1.00");
+        test_precision!(positive_round_to_1, 0.999, "1.00");
+
+        test_precision!(positive_round_to_zero, 0.001, "0.00");
+        test_precision!(negative_round_to_zero, -0.001, "0.00");
+
+        test_precision!(zero_precision_negative, -0.001, "0", 0);
+        test_precision!(zero_precision_positive, 0.001, "0", 0);
+    }
+
     #[test]
     fn sqrt() {
         for x in 1..1024 {