任务

给定一条特定的Sum语言语句， 将它编译成Stack语言的语句.
在Sum语言中，语句由数字或数字与+组成，例如

• 1
• 1 + 2
• 1 + 2 + 3

由于加法满足左结合，因此第三条式子可理解为 ((1 + 2) + 3).
而在Stack语言中存在两个操作行为，分别是Push, Add, 具体为
Push x； x表示整型数字。
Add ： 从栈顶弹出两个数字，相加后再次把结果压入栈顶，可把*Add用以下四个步骤来描述：

1. Pop a
2. Pop b
3. c = a + b
4. Push c

程序

#include <stdio.h>
#include <stdlib.h>

#define TODO()                                  \
  do{                                                           \
    printf ("\nAdd your code here: file \"%s\", line %d\n",     \
            __FILE__, __LINE__);                                \
  }while(0)

///////////////////////////////////////////////
// Data structures for the Sum language.
enum Exp_Kind_t {EXP_INT, EXP_SUM};
struct Exp_t
{
  enum Exp_Kind_t kind;
};

struct Exp_Int
{
  enum Exp_Kind_t kind;
  int i;
};

struct Exp_Sum
{
  enum Exp_Kind_t kind;
  struct Exp_t *left;
  struct Exp_t *right;
};

// "constructors"
struct Exp_t *Exp_Int_new (int i)
{
  struct Exp_Int *p = malloc (sizeof(*p));
  p->kind = EXP_INT;
  p->i = i;
  return (struct Exp_t *)p;
}

struct Exp_t *Exp_Sum_new (struct Exp_t *left, struct Exp_t *right)
{
  struct Exp_Sum *p = malloc (sizeof(*p));
  p->kind = EXP_SUM;
  p->left = left;
  p->right = right;
  return (struct Exp_t *)p;
}

// "printer"
void Exp_print (struct Exp_t *exp)
{
  switch (exp->kind){
  case EXP_INT:{
    struct Exp_Int *p = (struct Exp_Int *)exp;
    printf ("%d", p->i);
    break;
  }
  case EXP_SUM:{
    struct Exp_Sum *p = (struct Exp_Sum *)exp;
    Exp_print (p->left);
    printf ("+");
    Exp_print (p->right);
    break;
  }
  default:
    break;
  }
}

//////////////////////////////////////////////
// Data structures for the Stack language.
enum Stack_Kind_t {STACK_ADD, STACK_PUSH};
struct Stack_t
{
  enum Stack_Kind_t kind;
};

struct Stack_Add
{
  enum Stack_Kind_t kind;
};

struct Stack_Push
{
  enum Stack_Kind_t kind;
  int i;
};

// "constructors"
struct Stack_t *Stack_Add_new ()
{
  struct Stack_Add *p = malloc (sizeof(*p));
  p->kind = STACK_ADD;
  return (struct Stack_t *)p;
}

struct Stack_t *Stack_Push_new (int i)
{
  struct Stack_Push *p = malloc (sizeof(*p));
  p->kind = STACK_PUSH;
  p->i = i;
  return (struct Stack_t *)p;
}

/// instruction list
struct List_t
{
  struct Stack_t *instr;
  struct List_t *next;
};

struct List_t *List_new (struct Stack_t *instr, struct List_t *next)
{
  struct List_t *p = malloc (sizeof (*p));
  p->instr = instr;
  p->next = next;
  return p;
}

// "printer"
void List_reverse_print (struct List_t *list)
{
  TODO();
  struct List_t *p = list;
  while (p) 
  {
      switch (p->instr->kind) 
      {
          case STACK_ADD:{
            printf("Add\n");
            break;
          }
          case STACK_PUSH:{
            struct Stack_Push *comm = (struct Stack_Push *)p->instr;
            printf("Push %d\n", comm -> i);
            break;
          }
            
          default:
            break;
      }
      p = p -> next;
  }
}

//////////////////////////////////////////////////
// a compiler from Sum to Stack
struct List_t *all = 0;

void emit (struct Stack_t *instr)
{
  all = List_new (instr, all);
}

void compile (struct Exp_t *exp)
{
  switch (exp->kind){
  case EXP_INT:{
    struct Exp_Int *p = (struct Exp_Int *)exp;
    emit (Stack_Push_new (p->i));
    break;
  }
  case EXP_SUM:{
    TODO();
    struct Exp_Sum *q = (struct Exp_Sum *)exp;
    compile(q->left);
    compile(q->right);
    emit(Stack_Add_new());
    break;
  }
  default:
    break;
  }
}

//////////////////////////////////////////////////
// program entry
int main()
{
  printf("Compile starting\n");
  // build an expression tree:
  //            +
  //           / \
  //          +   4
  //         / \
  //        2   3
  struct Exp_t *exp = Exp_Sum_new (Exp_Sum_new(Exp_Int_new (2)
                                               , Exp_Int_new (3))
                                   , Exp_Int_new (4));
  // print out this tree:
  printf ("the expression is:\n");
  Exp_print (exp);
  // compile this tree to Stack machine instructions
  compile (exp);

  // print out the generated Stack instructons:
  List_reverse_print (all);
  
  printf("\nCompile finished\n");
  return 0;
}