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

void *emalloc(size_t n) {	/* allocate n bytes or die trying */
    void *r = malloc(n);
    assert(r != 0);
    return r;
}

typedef struct Expr_Node *expr;
#define LIT 'c'
struct Literal { int val; };
#define NEG 'n'
struct Negated { expr arg; };
#define ADD '+'
#define SUB '-'
#define MUL '*'
#define DIV '/'
#define MOD '%'
struct Binary  { expr lhs, rhs; };

struct Expr_Node {
    char op;
    union {
	/* op == LIT */ struct Literal l;
	/* op == NEG */ struct Negated n;
        /* others    */ struct Binary  b;
    } u;
};

#define lit_val(p) ((p)->u.l.val)
#define neg_arg(p) ((p)->u.n.arg)
#define bin_lhs(p) ((p)->u.b.lhs)
#define bin_rhs(p) ((p)->u.b.rhs)

void print(expr e) {
    switch (e->op) {
	case LIT:
	    printf("%d", lit_val(e));
	    break;
	case NEG:
	    printf("-");
	    print(neg_arg(e));
	    break;
	default:
	    printf("(");
	    print(bin_lhs(e));
	    printf("%c", e->op);
	    print(bin_rhs(e));
	    printf(")");
	    break;
    }
}

int value(expr e) {
    switch (e->op) {
	case LIT:
	    return lit_val(e);
	case NEG:
	    return -value(neg_arg(e));
	case ADD:
	    return value(bin_lhs(e)) + value(bin_rhs(e));
	case SUB:
	    return value(bin_lhs(e)) - value(bin_rhs(e));
	case MUL:
	    return value(bin_lhs(e)) * value(bin_rhs(e));
	case DIV:
	    return value(bin_lhs(e)) / value(bin_rhs(e));
	case MOD:
	    return value(bin_lhs(e)) * value(bin_rhs(e));
        default:
	    abort();	/* shut the compiler up */
    }
}

expr mk_literal(int v) {
    expr r = emalloc(sizeof *r);
    r->op = LIT, lit_val(r) = v;
    return r;
}

expr mk_negated(expr e) {
    expr r = emalloc(sizeof *r);
    r->op = NEG, neg_arg(r) = e;
    return r;
}

expr mk_sum(expr e, expr f) {
    expr r = emalloc(sizeof *r);
    r->op = ADD, bin_lhs(r) = e, bin_rhs(r) = f;
    return r;
}

expr mk_difference(expr e, expr f) {
    expr r = emalloc(sizeof *r);
    r->op = SUB, bin_lhs(r) = e, bin_rhs(r) = f;
    return r;
}

expr mk_product(expr e, expr f) {
    expr r = emalloc(sizeof *r);
    r->op = MUL, bin_lhs(r) = e, bin_rhs(r) = f;
    return r;
}

expr mk_quotient(expr e, expr f) {
    expr r = emalloc(sizeof *r);
    r->op = DIV, bin_lhs(r) = e, bin_rhs(r) = f;
    return r;
}

expr mk_remainder(expr e, expr f) {
    expr r = emalloc(sizeof *r);
    r->op = MOD, bin_lhs(r) = e, bin_rhs(r) = f;
    return r;
}


int main(void) {
    expr e = mk_product(mk_difference(mk_literal(1), mk_literal(2)),
                        mk_sum(mk_literal(3), mk_literal(4)));
    print(e);
    printf(" == %d\n", value(e));
    return 0;
}