After all this time, I FINALLY implemented the calculator!

src/calc.c

@@ -1,11 +1,110 @@
 #include <stdio.h>
-#include <stdlib.h>
+#include <string.h>
+
+int evalExpr(char* expr, int* res);
+int parseTerm(char** ptr);
+int parseFactor(char** ptr);
+int parseNumber(char** ptr);
+
+// TODO: Parsing doesn't account for addition/subtraction inside brackets, fix that
 
 __attribute__ ((section("entry")))
 void main() {
-    print("Hello, World!\n");
-    char in[32];
-    read(in);
-    print(in);
+    printf("Welcome to calc.\n");   
+    char input[256];
+
+    while (1) {
+        printf("> ");
+        read(input);
+        *strchr(input, '\n') = '\0';
+
+        // Remove whitespace
+        char* old = input;
+        char* new = input;
+        while (*old != '\0') {
+            if (*old == ' ') old++;
+            *new++ = *old++;
+        }
+        *new = '\0';
+
+        if (strcmp(input, "exit") == 0) {
+            break;
+        }
+
+        int res;
+        if (evalExpr(input, &res)) {
+            printf("%d\n", res);
+        } else {
+            printf("?\n");
+        }
+    }
+
     return;
 }
+
+int evalExpr(char* expr, int* res) {
+    char* ptr = expr;
+    *res = parseTerm(&ptr);
+
+    while (*ptr != '\0') {
+        if (*ptr == '+') {
+            ptr++;
+            *res += parseTerm(&ptr);
+        } else if (*ptr == '-') {
+            ptr++;
+            *res -= parseTerm(&ptr);
+        } else {
+            return 0; // Invalid
+        }
+    }
+
+    return 1; // Success
+}
+
+int parseTerm(char** ptr) {
+    int res = parseFactor(ptr);
+
+    while (**ptr == '*' || **ptr == '/') {
+        if (**ptr == '*') {
+            (*ptr)++;
+            res *= parseFactor(ptr);
+        } else if (**ptr == '/') {
+            (*ptr)++;
+            int div = parseFactor(ptr);
+            if (div != 0) {
+                res /= div;
+            } else {
+                return 0;
+            }
+        }
+    }
+
+    return res;
+}
+
+int parseFactor(char** ptr) {
+    if (**ptr == '(') {
+        (*ptr)++;
+        int res = parseTerm(ptr);
+
+        if (**ptr == ')') {
+            (*ptr)++;
+        } else {
+            printf("Error: Missing ')'");
+            return 0;
+        }
+        return res;
+    } else {
+        return parseNumber(ptr);
+    }
+}
+int parseNumber(char** ptr) { 
+    int res = 0;
+    while (**ptr >= '0' && **ptr <= '9') {
+        res *= 10;
+        res += *(*ptr)++ - '0';
+    }
+
+    return res;
+}
+

src/filetable.asm

@@ -18,7 +18,7 @@ filetable:
     db "term16n",0,0,0,    "fnt",0,56,4
     db "testfont",0,0,     "fnt",0,60,4
     db "test",0,0,0,0,0,0, "txt",0,64,1
-    db "calc",0,0,0,0,0,0, "bin",0,65,2
+    db "calc",0,0,0,0,0,0, "bin",0,65,6
 
 times 512-($-$$) db 0