detect division by 0

README.md

@@ -132,7 +132,8 @@ The keys on the original calculator map as follows:
 Hold `mode` and `0` at the same time to turn off. NOTE: There is no auto power off.
 
 # Bugs
-1. The calculator does not properly check for overflow.
+1. The calculator does not properly check for underflow or overflow.
+1. After division by 0, it's possible to negate `Error` to get an invalid number.
 1. There are probably more bugs waiting to be discovered.
 
 # Internals

src/calc.c

@@ -48,7 +48,11 @@ __xdata dec80* get_y(void){
 }
 
 static void do_binary_op(void (*f_ptr)(dec80*, const dec80*)){
-	f_ptr(&stack(STACK_Y), &stack(STACK_X));
+	if (decn_is_nan(&stack(STACK_Y)) || decn_is_nan(&stack(STACK_X))){
+		set_dec80_NaN(&stack(STACK_Y));
+	} else {
+		f_ptr(&stack(STACK_Y), &stack(STACK_X));
+	}
 	pop();
 }
 

src/decn/decn.c

@@ -339,15 +339,28 @@ static uint8_t decn_is_zero(const dec80* x){
 	return 1;
 }
 
+uint8_t decn_is_nan(const dec80* x){
+	uint8_t i;
+	if (x->exponent != DEC80_NAN_EXP){
+		return 0;
+	}
+	for (i = 0; i < DEC80_NUM_LSU; i++){
+		if (x->lsu[i] != 0xff){
+			return 0;
+		}
+	}
+	return 1;
+}
+
 #ifdef EXTRA_CHECKS
 void set_dec80_NaN(dec80* dest){
 	uint8_t i;
 
 	//set exponent to special val
-	dest->exponent = 0xff;
-	//clear nibbles
+	dest->exponent = DEC80_NAN_EXP;
+	//set all nibbles
 	for (i = 0; i < DEC80_NUM_LSU; i++){
-		dest->lsu[i] = 0;
+		dest->lsu[i] = 0xff;
 	}
 }
 #endif
@@ -713,6 +726,9 @@ void div_decn(dec80* acc, const dec80* x){
 #ifdef EXTRA_CHECKS
 	if (decn_is_zero(x)){
 		set_dec80_NaN(acc);
+#ifdef DESKTOP
+		printf("error division by 0\n");
+#endif
 		return;
 	}
 #endif
@@ -775,6 +791,20 @@ int8_t decn_to_str(char* buf, const dec80* x){
 	uint8_t use_sci = 0;
 	static __xdata dec80 tmp;
 
+	//handle corner case of NaN
+	if (decn_is_nan(x)){
+		buf[0] = 'E';
+		buf[1] = 'r';
+		buf[2] = 'r';
+		buf[3] = 'o';
+		buf[4] = 'r';
+		buf[5] = '\0';
+#ifdef DEBUG
+		printf ("  corner case NaN ");
+#endif
+		return 0;
+	}
+
 	//copy and normalize
 	copy_decn(&tmp, x);
 	remove_leading_zeros(&tmp);

src/decn/decn.h

@@ -19,6 +19,7 @@ extern "C" {
 //allow full range, but reserve -16384 for special numbers
 #define DEC80_MIN_EXP (-16383)
 #define DEC80_MAX_EXP   16383
+#define DEC80_NAN_EXP (-16383 - 1)
 
 //decimal80 unpacked into 80 bits
 // for computation
@@ -45,6 +46,7 @@ void build_dec80(dec80* dest, const char* signif_str, int16_t exponent);
 
 void set_dec80_zero(dec80* dest);
 void set_dec80_NaN(dec80* dest);
+uint8_t decn_is_nan(const dec80* x);
 
 void negate_decn(dec80* x);
 int8_t compare_decn(const dec80* a, const dec80* b); //a<b: -1, a==b: 0, a>b: 1