source: azure_iot_hub/trunk/musl-1.1.18/src/locale/pleval.c

#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   = Prim | Mul '*' Prim | Mul '/' Prim | Mul '%' Prim
Prim  = '(' Expr ')' | '!' Prim | decimal | 'n'

internals:

recursive descent expression evaluator with stack depth limit.
for binary operators an operator-precedence parser is used.
eval* functions store the result of the parsed subexpression
and return a pointer to the next non-space character.
*/

struct st {
        unsigned long r;
        unsigned long n;
        int op;
};

static const char *skipspace(const char *s)
{
        while (isspace(*s)) s++;
        return s;
}

static const char *evalexpr(struct st *st, const char *s, int d);

static const char *evalprim(struct st *st, const char *s, int d)
{
        char *e;
        if (--d < 0) return "";
        s = skipspace(s);
        if (isdigit(*s)) {
                st->r = strtoul(s, &e, 10);
                if (e == s || st->r == -1) return "";
                return skipspace(e);
        }
        if (*s == 'n') {
                st->r = st->n;
                return skipspace(s+1);
        }
        if (*s == '(') {
                s = evalexpr(st, s+1, d);
                if (*s != ')') return "";
                return skipspace(s+1);
        }
        if (*s == '!') {
                s = evalprim(st, s+1, d);
                st->r = !st->r;
                return s;
        }
        return "";
}

static int binop(struct st *st, int op, unsigned long left)
{
        unsigned long a = left, b = st->r;
        switch (op) {
        case 0: st->r = a||b; return 0;
        case 1: st->r = a&&b; return 0;
        case 2: st->r = a==b; return 0;
        case 3: st->r = a!=b; return 0;
        case 4: st->r = a>=b; return 0;
        case 5: st->r = a<=b; return 0;
        case 6: st->r = a>b; return 0;
        case 7: st->r = a<b; return 0;
        case 8: st->r = a+b; return 0;
        case 9: st->r = a-b; return 0;
        case 10: st->r = a*b; return 0;
        case 11: if (b) {st->r = a%b; return 0;} return 1;
        case 12: if (b) {st->r = a/b; return 0;} return 1;
        }
        return 1;
}

static const char *parseop(struct st *st, const char *s)
{
        static const char opch[11] = "|&=!><+-*%/";
        static const char opch2[6] = "|&====";
        int i;
        for (i=0; i<11; i++)
                if (*s == opch[i]) {
                        /* note: >,< are accepted with or without = */
                        if (i<6 && s[1] == opch2[i]) {
                                st->op = i;
                                return s+2;
                        }
                        if (i>=4) {
                                st->op = i+2;
                                return s+1;
                        }
                        break;
                }
        st->op = 13;
        return s;
}

static const char *evalbinop(struct st *st, const char *s, int minprec, int d)
{
        static const char prec[14] = {1,2,3,3,4,4,4,4,5,5,6,6,6,0};
        unsigned long left;
        int op;
        d--;
        s = evalprim(st, s, d);
        s = parseop(st, s);
        for (;;) {
                /*
                st->r (left hand side value) and st->op are now set,
                get the right hand side or back out if op has low prec,
                if op was missing then prec[op]==0
                */
                op = st->op;
                if (prec[op] <= minprec)
                        return s;
                left = st->r;
                s = evalbinop(st, s, prec[op], d);
                if (binop(st, op, left))
                        return "";
        }
}

static const char *evalexpr(struct st *st, const char *s, int d)
{
        unsigned long a, b;
        if (--d < 0)
                return "";
        s = evalbinop(st, s, 0, d);
        if (*s != '?')
                return s;
        a = st->r;
        s = evalexpr(st, s+1, d);
        if (*s != ':')
                return "";
        b = st->r;
        s = evalexpr(st, s+1, d);
        st->r = a ? b : st->r;
        return s;
}

unsigned long __pleval(const char *s, unsigned long n)
{
        struct st st;
        st.n = n;
        s = evalexpr(&st, s, 100);
        return *s == ';' ? st.r : -1;
}