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#include <stdlib.h>
#include <ctype.h>
/*
grammar:
Start = Expr ';'
Expr = Or | Or '?' Expr ':' Expr
Or = And | Or '||' And
And = Eq | And '&&' Eq
Eq = Rel | Eq '==' Rel | Eq '!=' Rel
Rel = Add | Rel '<=' Add | Rel '>=' Add | Rel '<' Add | Rel '>' Add
Add = Mul | Add '+' Mul | Add '-' Mul
Mul = Term | Mul '*' Term | Mul '/' Term | Mul '%' Term
Term = '(' Expr ')' | '!' Term | decimal | 'n'
internals:
recursive descent expression evaluator with stack depth limit.
eval* functions return the value of the subexpression and set
the current string pointer to the next non-space char.
*/
struct st {
const char *s;
unsigned long n;
int err;
};
static const char *skipspace(const char *s)
{
while (isspace(*s)) s++;
return s;
}
static unsigned long evalconst(struct st *st)
{
char *e;
unsigned long n;
n = strtoul(st->s, &e, 10);
if (!isdigit(*st->s) || e == st->s || n == -1)
st->err = 1;
st->s = skipspace(e);
return n;
}
static unsigned long evalexpr(struct st *st, int d);
static unsigned long evalterm(struct st *st, int d)
{
unsigned long a;
if (d <= 0) {
st->err = 1;
return 0;
}
st->s = skipspace(st->s);
if (*st->s == '!') {
st->s++;
return !evalterm(st, d-1);
}
if (*st->s == '(') {
st->s++;
a = evalexpr(st, d-1);
if (*st->s != ')') {
st->err = 1;
return 0;
}
st->s = skipspace(st->s + 1);
return a;
}
if (*st->s == 'n') {
st->s = skipspace(st->s + 1);
return st->n;
}
return evalconst(st);
}
static unsigned long evalmul(struct st *st, int d)
{
unsigned long b, a = evalterm(st, d-1);
int op;
for (;;) {
op = *st->s;
if (op != '*' && op != '/' && op != '%')
return a;
st->s++;
b = evalterm(st, d-1);
if (op == '*') {
a *= b;
} else if (!b) {
st->err = 1;
return 0;
} else if (op == '%') {
a %= b;
} else {
a /= b;
}
}
}
static unsigned long evaladd(struct st *st, int d)
{
unsigned long a = 0;
int add = 1;
for (;;) {
a += (add?1:-1) * evalmul(st, d-1);
if (*st->s != '+' && *st->s != '-')
return a;
add = *st->s == '+';
st->s++;
}
}
static unsigned long evalrel(struct st *st, int d)
{
unsigned long b, a = evaladd(st, d-1);
int less, eq;
for (;;) {
if (*st->s != '<' && *st->s != '>')
return a;
less = st->s[0] == '<';
eq = st->s[1] == '=';
st->s += 1 + eq;
b = evaladd(st, d-1);
a = (less ? a < b : a > b) || (eq && a == b);
}
}
static unsigned long evaleq(struct st *st, int d)
{
unsigned long a = evalrel(st, d-1);
int neg;
for (;;) {
if ((st->s[0] != '=' && st->s[0] != '!') || st->s[1] != '=')
return a;
neg = st->s[0] == '!';
st->s += 2;
a = evalrel(st, d-1) == a;
a ^= neg;
}
}
static unsigned long evaland(struct st *st, int d)
{
unsigned long a = evaleq(st, d-1);
for (;;) {
if (st->s[0] != '&' || st->s[1] != '&')
return a;
st->s += 2;
a = evaleq(st, d-1) && a;
}
}
static unsigned long evalor(struct st *st, int d)
{
unsigned long a = evaland(st, d-1);
for (;;) {
if (st->s[0] != '|' || st->s[1] != '|')
return a;
st->s += 2;
a = evaland(st, d-1) || a;
}
}
static unsigned long evalexpr(struct st *st, int d)
{
unsigned long a1, a2, a3;
if (d <= 0) {
st->err = 1;
return 0;
}
a1 = evalor(st, d-1);
if (*st->s != '?')
return a1;
st->s++;
a2 = evalexpr(st, d-1);
if (*st->s != ':') {
st->err = 1;
return 0;
}
st->s++;
a3 = evalexpr(st, d-1);
return a1 ? a2 : a3;
}
unsigned long __pleval(const char *s, unsigned long n)
{
unsigned long a;
struct st st;
st.s = s;
st.n = n;
st.err = 0;
a = evalexpr(&st, 100);
if (st.err || *st.s != ';')
return -1;
return a;
}
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