json.c

#include <stdio.h>
#include <string.h>

enum jstates {JFAILED, JNXT, JSEP, JUP, JDWN, JQUP, JQDWN, JESC, JUNESC, JBARE, JUNBARE, JUTF8_2,
    JUTF8_3, JUTF8_4, JUTF8_N, JSTATE_CNT};
enum jtype {JEOS,JERR,JTSEP,JOBJB,JOBJE,JARRB,JARRE,JNUM,JSTR,JTRUE,JFALSE,JNULL};
struct jtok {int type, len; const char *str;};
struct jctx {const unsigned char *tbl; const char *cur, *end;};

static const unsigned char jtbl[256] = {
    [48 ... 56]=JBARE, ['\t']=JNXT, ['\r']=JNXT, ['\n']=JNXT, [' ']=JNXT,
    ['"']=JQUP, [':']=JSEP, ['=']=JSEP, [',']=JNXT, ['[']=JUP, [']']=JDWN,
    ['{']=JUP, ['}']=JDWN, ['-']=JBARE, ['t']=JBARE, ['f']=JBARE, ['n']=JBARE};
static const unsigned char jbare[256] = {
    [32 ... 43]=JNXT, [44]=JUNBARE, [45 ... 92]=JNXT, [93]=JUNBARE, [94 ... 124]=JNXT,
    [125]=JUNBARE, [126]=JNXT, ['\t']=JUNBARE, ['\r']=JUNBARE, ['\n']=JUNBARE};
static const unsigned char jstr[256] = {
    [32 ... 33]=JNXT, [34]=JQDWN, [35 ... 92]=JNXT, [93]=JESC, [94 ... 126]=JNXT,
    [192 ... 223]=JUTF8_2, [224 ... 239]=JUTF8_3, [240 ... 247]=JUTF8_4};
static const unsigned char jutf8[256] = {[128 ... 191]=JUTF8_N};
static const unsigned char jesc[256] = { ['"']=JUNESC, ['\\']=JUNESC, ['/']=JUNESC,
    ['b']=JUNESC, ['f']=JUNESC, ['n']=JUNESC, ['r']=JUNESC, ['t']=JUNESC, ['u']=JUNESC};

static struct jtok
jparse(struct jctx *ctx)
{
    const char *str = 0;
    int remain = 0, c = 0;
    ctx->tbl = (ctx->tbl) ? ctx->tbl: jtbl;
    #define t(...)(struct jtok){__VA_ARGS__}
    while (ctx->cur < ctx->end) {
        switch (ctx->tbl[c = (unsigned char)(*ctx->cur++)]) {
        case JFAILED: return t(.type = JERR);
        case JESC: ctx->tbl = jesc; break;
        case JUNESC: ctx->tbl = jstr; break;
        case JQUP: str = ctx->cur, ctx->tbl = jstr; break;
        case JUTF8_2: ctx->tbl = jutf8, remain = 1; break;
        case JUTF8_3: ctx->tbl = jutf8, remain = 2; break;
        case JUTF8_4: ctx->tbl = jutf8, remain = 3; break;
        case JUTF8_N: if (!--remain) ctx->tbl = jstr; break;
        case JBARE: ctx->tbl=jbare, str = ctx->cur-1; break;
        case JSEP: return t(.type=JTSEP, .str=ctx->cur-1, .len=1);
        case JQDWN: ctx->tbl=jtbl; return t(.type = JSTR, .str=str, .len=(int)((ctx->cur-1)-str));
        case JUP: return t(.type=(c=='{')?JOBJB:JARRB, .str=ctx->cur-1, .len=1);
        case JDWN: return t(.type=(c=='}')?JOBJE:JARRE, .str=ctx->cur-1, .len=1);
        case JUNBARE: ctx->tbl=jtbl; return t(.str=str, .len=(int)(--ctx->cur - str),
            .type=(str[0] == 't') ? JTRUE:(str[0] == 'f') ? JFALSE:(str[0] == 'n') ? JNULL:JNUM);}
    } return t(.type=JEOS);
    #undef t
}
int main(void)
{
    struct jtok t;
    const char json[] = "{\"name\":\"test\", \"age\":42, \"utf8\":\"äöü\", \"alive\":true}";
    struct jctx ctx = {.cur = json, .end = json + sizeof(json)};
    while ((t = jparse(&ctx)).type) {
        switch (t.type) {case JERR: goto err;
        case JOBJB: printf("Token(Object_Begin)\n"); break;
        case JOBJE: printf("Token(Object_End)\n"); break;
        case JARRB: printf("Token(Array_Begin)\n"); break;
        case JARRE: printf("Token(Array_End)\n"); break;
        case JNUM: printf("Token(Number): %.*s\n", t.len, t.str); break;
        case JSTR: printf("Token(String): %.*s\n", t.len, t.str); break;
        case JTRUE: printf("Token(True): %.*s\n", t.len, t.str); break;
        case JFALSE: printf("Token(False): %.*s\n", t.len, t.str); break;
        case JNULL: printf("Token(NULL): %.*s\n", t.len, t.str); break;
        case JTSEP: printf("Token(SEP): '%.*s'\n", t.len, t.str); break;}
    } err: return 0;
}