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main.c
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main.c
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#include <stdio.h>
#include <stdlib.h>
#define STACK_MAX 256
typedef enum {
OBJ_INT,
OBJ_PAIR
} ObjectType;
typedef struct sObject {
ObjectType type;
unsigned char marked;
/* The next object in the linked list of heap allocated objects. */
struct sObject* next;
union {
/* OBJ_INT */
int value;
/* OBJ_PAIR */
struct {
struct sObject* head;
struct sObject* tail;
};
};
} Object;
typedef struct {
Object* stack[STACK_MAX];
int stackSize;
/* The first object in the linked list of all objects on the heap. */
Object* firstObject;
/* The total number of currently allocated objects. */
int numObjects;
/* The number of objects required to trigger a GC. */
int maxObjects;
} VM;
void assert(int condition, const char* message) {
if (!condition) {
printf("%s\n", message);
exit(1);
}
}
VM* newVM() {
VM* vm = malloc(sizeof(VM));
vm->stackSize = 0;
vm->firstObject = NULL;
vm->numObjects = 0;
vm->maxObjects = 8;
return vm;
}
void push(VM* vm, Object* value) {
assert(vm->stackSize < STACK_MAX, "Stack overflow!");
vm->stack[vm->stackSize++] = value;
}
Object* pop(VM* vm) {
assert(vm->stackSize > 0, "Stack underflow!");
return vm->stack[--vm->stackSize];
}
void mark(Object* object) {
/* If already marked, we're done. Check this first to avoid recursing
on cycles in the object graph. */
if (object->marked) return;
object->marked = 1;
if (object->type == OBJ_PAIR) {
mark(object->head);
mark(object->tail);
}
}
void markAll(VM* vm)
{
for (int i = 0; i < vm->stackSize; i++) {
mark(vm->stack[i]);
}
}
void sweep(VM* vm)
{
Object** object = &vm->firstObject;
while (*object) {
if (!(*object)->marked) {
/* This object wasn't reached, so remove it from the list and free it. */
Object* unreached = *object;
*object = unreached->next;
free(unreached);
vm->numObjects--;
} else {
/* This object was reached, so unmark it (for the next GC) and move on to
the next. */
(*object)->marked = 0;
object = &(*object)->next;
}
}
}
void gc(VM* vm) {
int numObjects = vm->numObjects;
markAll(vm);
sweep(vm);
vm->maxObjects = vm->numObjects * 2;
printf("Collected %d objects, %d remaining.\n", numObjects - vm->numObjects,
vm->numObjects);
}
Object* newObject(VM* vm, ObjectType type) {
if (vm->numObjects == vm->maxObjects) gc(vm);
Object* object = malloc(sizeof(Object));
object->type = type;
object->next = vm->firstObject;
vm->firstObject = object;
object->marked = 0;
vm->numObjects++;
return object;
}
void pushInt(VM* vm, int intValue) {
Object* object = newObject(vm, OBJ_INT);
object->value = intValue;
push(vm, object);
}
Object* pushPair(VM* vm) {
Object* object = newObject(vm, OBJ_PAIR);
object->tail = pop(vm);
object->head = pop(vm);
push(vm, object);
return object;
}
void objectPrint(Object* object) {
switch (object->type) {
case OBJ_INT:
printf("%d", object->value);
break;
case OBJ_PAIR:
printf("(");
objectPrint(object->head);
printf(", ");
objectPrint(object->tail);
printf(")");
break;
}
}
void test1() {
printf("Test 1: Objects on stack are preserved.\n");
VM* vm = newVM();
pushInt(vm, 1);
pushInt(vm, 2);
gc(vm);
assert(vm->numObjects == 2, "Should have preserved objects.");
}
void test2() {
printf("Test 2: Unreached objects are collected.\n");
VM* vm = newVM();
pushInt(vm, 1);
pushInt(vm, 2);
pop(vm);
pop(vm);
gc(vm);
assert(vm->numObjects == 0, "Should have collected objects.");
}
void test3() {
printf("Test 3: Reach nested objects.\n");
VM* vm = newVM();
pushInt(vm, 1);
pushInt(vm, 2);
pushPair(vm);
pushInt(vm, 3);
pushInt(vm, 4);
pushPair(vm);
pushPair(vm);
gc(vm);
assert(vm->numObjects == 7, "Should have reached objects.");
}
void test4() {
printf("Test 4: Handle cycles.\n");
VM* vm = newVM();
pushInt(vm, 1);
pushInt(vm, 2);
Object* a = pushPair(vm);
pushInt(vm, 3);
pushInt(vm, 4);
Object* b = pushPair(vm);
a->tail = b;
b->tail = a;
gc(vm);
assert(vm->numObjects == 4, "Should have collected objects.");
}
void perfTest() {
printf("Performance Test.\n");
VM* vm = newVM();
for (int i = 0; i < 1000; i++) {
for (int j = 0; j < 20; j++) {
pushInt(vm, i);
}
for (int k = 0; k < 20; k++) {
pop(vm);
}
}
}
int main(int argc, const char * argv[]) {
test1();
test2();
test3();
test4();
perfTest();
return 0;
}