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#include "string.h"
#include "stdio.h"
#define VARIABLE_LIST 1
#define VARIABLE_VALUE 2
#define VARIABLE 3
#define NUMERAL 4
#define ADDITION 5
#define GREATER 6
#define ASSIGNMENT 7
#define SEQUENCE 8
#define WHILE 9
#define SKIP 10
typedef struct term {
short select;
union {
struct term **children;
int int_val;
char *string_val;
} term_body;
} term;
void panic(char *s) {
/*fprintf(stderr, "%s\n", s);*/
printf( "%s\n", s);
exit(-1);
}
term *make_term(int tag) {
term *ptr;
if(!(ptr = (term *)malloc(sizeof(term)))) {
panic("allocation failed in make_term");
}
ptr->select = tag;
return ptr;
}
term *make_binary_term(int tag, term *i1, term *i2) {
term *ptr = make_term(tag);
if(!(ptr->term_body.children
= (term **)malloc(sizeof(term*)*2))) {
panic("could not allocate memory in make_binary_term");
}
ptr->term_body.children[0] = i1;
ptr->term_body.children[1] = i2;
return ptr;
}
term *variable(char *s) {
term *ptr = make_term(VARIABLE);
ptr->term_body.string_val = s;
return ptr;
}
term *numeral(int i) {
term *ptr = make_term(NUMERAL);
ptr->term_body.int_val = i;
return ptr;
}
term *addition(term *i1, term *i2) {
return make_binary_term(ADDITION, i1, i2);
}
term *variable_list(term *i1, term *i2) {
return make_binary_term(VARIABLE_LIST, i1, i2);
}
term *variable_value(term *i1, term *i2) {
return make_binary_term(VARIABLE_VALUE, i1, i2);
}
term *greater(term *i1, term *i2) {
return make_binary_term(GREATER, i1, i2);
}
term *assignment(term *i1, term *i2) {
return make_binary_term(ASSIGNMENT, i1, i2);
}
term *sequence(term *i1, term *i2) {
return make_binary_term(SEQUENCE, i1, i2);
}
term *term_while(term *i1, term *i2) {
return make_binary_term(WHILE, i1, i2);
}
term *skip() {
return make_term(SKIP);
}
char *variable_name(term *v) {
return (v->term_body.string_val);
}
int numeral_value(term *t) {
return(t->term_body.int_val);
}
term *child(term *t, int rank) {
return (t->term_body.children[rank-1]);
}
int lookup(term *env, char *var) {
while(NULL != env) {
term *pair = child(env, 1);
if(strcmp(variable_name(child(pair, 1)), var)==0) {
return numeral_value(child(pair, 2));
} else {
env = child(env, 2);
}
}
/* fprintf(stderr, "%s : ", var);*/
printf( "%s : ", var);
panic("no such variable in environment for lookup");
}
term *update(term *env, char *var_name, term *val) {
if(NULL != env) {
term *var = child(child(env, 1), 1);
if(strcmp(variable_name(var), var_name) == 0) {
return(variable_list(variable_value(var, val),
child(env, 2)));
} else {
return(variable_list(child(env, 1),
update(child(env, 2), var_name, val)));
}
} else {
/*fprintf(stderr, "%s : ", var_name);*/
printf( "%s : ", var_name);
panic("no such variable in environment for update");
}
}
int evaluate(term *env, term *exp) {
int n1, n2;
switch(exp->select) {
case VARIABLE: return lookup(env, variable_name(exp));
case NUMERAL: return numeral_value(exp);
case ADDITION:
return (evaluate(env, child(exp, 1)) +
evaluate(env, child(exp, 2)));
}
}
int evaluate_bool(term *env, term *exp) {
int n1 = evaluate(env, child(exp, 1));
int n2 = evaluate(env, child(exp, 2));
return (n1 > n2);
}
term *execute(term *env, term *inst) {
switch(inst->select) {
case ASSIGNMENT:
return update(env,
variable_name(child(inst, 1)),
numeral(evaluate(env,child(inst, 2))));
case SEQUENCE:
return execute(execute(env,child(inst, 1)),child(inst, 2));
case WHILE:
if(evaluate_bool(env,child(inst, 1))) {
return execute(execute(env,child(inst, 2)), inst);
} else {
return env;
}
}
}
void print_env(term *env) {
while(env != NULL) {
printf("%s : %d\n", variable_name(child(child(env,1),1)),
numeral_value(child(child(env,1),2)));
env = child(env, 2);
}
}
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