Implement subtraction

README.md

@@ -0,0 +1 @@
+https://jvns.ca/blog/2023/01/13/examples-of-floating-point-problems/

src/lib.rs

@@ -8,6 +8,11 @@ struct Real {
     decimal_parts: HashMap<i32, i8> 
 }
 
+struct DecimalSubtraction {
+    carry: bool,
+    values: HashMap<i32, i8>
+}
+
 impl Real {
     pub fn new_int(int_part: i32) -> Real {
         Real { int_part: int_part, decimal_parts: HashMap::new() }
@@ -25,8 +30,10 @@ impl Real {
                 Real { int_part: s.parse::<i32>().unwrap(), decimal_parts: HashMap::new() }
             },
             Some(idx) => {
-                let int_part = s[0..idx].parse::<i32>().unwrap();
-                dbg!(int_part);
+                let int_part = match &s[0..1] {
+                    "-" => {-s[1..idx].parse::<i32>().unwrap()},
+                    _ => {s[0..idx].parse::<i32>().unwrap()}
+                };
                 let decimal_string = &s[idx+1..];
                 dbg!(decimal_string);
 
@@ -52,6 +59,48 @@ impl Real {
             }
         }
     }
+
+    // Cannot put this inside the `Sub impl` because Rust only allows the implementation of the named methods.
+    fn subtract_decimals(minuend: HashMap<i32, i8>, subtrahend: HashMap<i32, i8>) -> DecimalSubtraction {
+        // default to -1 because the 0th index is the first digit after the decimal place
+        let highest_index = *max(minuend.keys().max().unwrap_or(&-1), subtrahend.keys().max().unwrap_or(&-1));
+        if highest_index == -1 as i32 {
+            // i.e. if both decimals are empty
+            return DecimalSubtraction{carry: false, values: HashMap::new()}
+        }
+        
+        let mut subtracted_decimal_parts = HashMap::new();
+        let mut should_subtraction_carry_to_integers = false;
+
+        for idx in (0..=highest_index).rev() {
+            // `decimal_part` added, here, because if a previous (i.e. smaller-denomination) subtraction ended up negative,
+            // a `-1` will have been put in the `decimal_part` map.
+            let subtraction = subtracted_decimal_parts.get(&idx).unwrap_or(&0) + minuend.get(&idx).unwrap_or(&0) - subtrahend.get(&idx).unwrap_or(&0);
+            
+            match subtraction {
+                x if 0 < x && x <=9 => {
+                    subtracted_decimal_parts.insert(idx, subtraction);
+                },
+                x if x < 0 => {
+                    subtracted_decimal_parts.insert(idx, 10+subtraction);
+                    if idx > 0 {
+                        // TODO - hmmm. I should probably actually be using unsigned ints for the values.
+                        // This approach would still work, I'd just need a rule that "if there's _any_ value
+                        // in the decimal_part slot, subtract 1", rather than blindly adding the value (which
+                        // is always -1)
+                        subtracted_decimal_parts.insert(idx-1, -1);
+                    } else {
+                        should_subtraction_carry_to_integers = true;
+                    }
+                },
+                _ => {
+                    // x == 0 - so clear the idx-th value
+                    subtracted_decimal_parts.remove(&idx);
+                }
+            }
+        };
+        DecimalSubtraction { carry: should_subtraction_carry_to_integers, values: subtracted_decimal_parts }
+    }
 }
 
 impl Add for Real {
@@ -84,11 +133,46 @@ impl Sub for Real {
     type Output = Real;
 
     fn sub(self, other: Real) -> Real {
-        let int_part = self.int_part - other.int_part;
-        Real::new_int(int_part)
+        let int_subtraction = self.int_part - other.int_part;
+        match int_subtraction {
+            x if x > 0 => {
+                let decimal_subtraction = Real::subtract_decimals(self.decimal_parts, other.decimal_parts);
+                Real::new(int_subtraction - (if decimal_subtraction.carry {1} else {0}), decimal_subtraction.values)
+            },
+            x if x < 0 => {
+                let inverted = other - self;
+                Real::new(-inverted.int_part, inverted.decimal_parts)
+            },
+            x if x == 0 => {
+                // Since integer parts are equal, we have to check case-by-case which is larger
+                for idx in 0..max(self.decimal_parts.len(), other.decimal_parts.len()) {
+                    // ...just when I think I understand the borrow-checker, this horror-show of casting becomes necessary...
+                    if self.decimal_parts.get(&(idx as i32)).unwrap_or(&(0 as i8)) > other.decimal_parts.get(&(idx as i32)).unwrap_or(&(0 as i8)) {
+                        let decimal_subtraction = Real::subtract_decimals(self.decimal_parts, other.decimal_parts);
+                        // We know that `decimal_subtraction.carry` is false, because the `self.decimal_parts` is larger
+                        return Real::new(0, decimal_subtraction.values);
+                    }
+
+                    if self.decimal_parts.get(&(idx as i32)).unwrap_or(&(0 as i8)) < other.decimal_parts.get(&(idx as i32)).unwrap_or(&(0 as i8)) {
+                        return other - self;
+                    }
+
+                    // and, implicitly - if they are equal, then continue
+                }
+                // Compared all the digits and they're equal, so the original two numbers are equal
+                Real::new(0, HashMap::new())
+
+            },
+            _ => {panic!("int_subtraction is not greater than, lesser than, or equal to 0, but a secret fourth thing.")}
+        }
+        
     }
+
+        
+
 }
 
+
 impl PartialEq for Real {
     fn eq(&self, other: &Real) -> bool {
         // And also compare decimals
@@ -115,6 +199,7 @@ mod tests {
         assert_eq!(one + two, Real::new_int(3));
     }
 
+    // TODO - Can I separate these into "test suites" so that each can run as their own function (with, presumably, better reporting?)
     #[test]
     fn real_addition() {
         // Basic equality
@@ -159,9 +244,45 @@ mod tests {
     }
 
     #[test]
-    fn subtraction() {
+    fn int_subtraction() {
         let three = Real::new_int(3);
         let one = Real::new_int(1);
         assert_eq!(three - one, Real::new_int(2));
     }
+    #[test]
+    fn real_subtraction() {
+        // Basic case
+        assert_eq!(
+            Real::from_string("3.45") - Real::from_string("1.23"),
+            Real::from_string("2.22")
+        );
+
+        // Negative result
+        assert_eq!(
+            Real::from_string("1.23") - Real::from_string("3.45"),
+            Real::from_string("-2.22")
+        );
+
+        // Equality
+        assert_eq!(
+            Real::from_string("1.23") - Real::from_string("1.23"),
+            Real::from_string("0")
+        );
+
+        // Carry
+        assert_eq!(
+            Real::from_string("1.11") - Real::from_string("1.02"),
+            Real::from_string("0.09")
+        );
+
+        // Carry into integers
+        assert_eq!(
+            Real::from_string("1.0") - Real::from_string("0.1"),
+            Real::from_string("0.9")
+        )
+
+        // Multiple carry
+    }
+
+    // TODO - test for optimization, don't keep unnecessary 0's!
 }