add option to statically disable decimal mode (#47)

* add the decimal mode feature to `Cargo.toml`
* add tests for disabled decimal mode to github workflows
* don't run decimal mode tests if decimal mode disabled

Co-authored-by: Sam M W <you@example.com>

.github/workflows/ci.yaml

@@ -33,3 +33,8 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: test
+      - name: Run tests with disabled decimal mode
+        uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --no-default-features

Cargo.toml

@@ -54,3 +54,7 @@ skeptic = "0.13.7"
 
 [dev-dependencies]
 skeptic = "0.13.7"
+
+[features]
+decimal_mode = []
+default = ["decimal_mode"]

src/cpu.rs

@@ -546,12 +546,16 @@ impl CPU {
             0x00
         };
 
+        #[cfg(feature = "decimal_mode")]
         let result: i8 = if self.registers.status.contains(Status::PS_DECIMAL_MODE) {
             a_after.wrapping_add(bcd1).wrapping_add(bcd2)
         } else {
             a_after
         };
 
+        #[cfg(not(feature = "decimal_mode"))]
+        let result: i8 = a_after;
+
         let did_carry = (result as u8) < (a_before as u8) || (c_before == 1 && value == -1);
 
         let did_overflow = (a_before < 0 && value < 0 && a_after >= 0)
@@ -619,12 +623,16 @@ impl CPU {
             0x00
         };
 
+        #[cfg(feature = "decimal_mode")]
         let result: i8 = if self.registers.status.contains(Status::PS_DECIMAL_MODE) {
             a_after.wrapping_sub(bcd1).wrapping_sub(bcd2)
         } else {
             a_after
         };
 
+        #[cfg(not(feature = "decimal_mode"))]
+        let result: i8 = a_after;
+
         // The carry flag is set on unsigned overflow.
         let did_carry = (result as u8) > (a_before as u8);
 
@@ -794,7 +802,7 @@ mod tests {
     use super::*;
     use num::range_inclusive;
 
-    #[test]
+    #[cfg_attr(feature = "decimal_mode", test)]
     fn decimal_add_test() {
         let mut cpu = CPU::new();
         cpu.registers.status.or(Status::PS_DECIMAL_MODE);
@@ -821,7 +829,7 @@ mod tests {
         assert!(cpu.registers.status.contains(Status::PS_OVERFLOW));
     }
 
-    #[test]
+    #[cfg_attr(feature = "decimal_mode", test)]
     fn decimal_subtract_test() {
         let mut cpu = CPU::new();
         cpu.registers