JSMN parse example using parent links feature

jsmn_example.c

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

#define JSMN_PARENT_LINKS

#include "jsmn.h"

#define MAX_TOK 50

struct obj {
	struct {
		char *architecture;
		size_t sqm;
	} house;
	struct {
		char *brand;
		size_t horsepower;
	} car;
};

void fill_object(char *str, jsmntok_t *tokens, struct obj *parsed, int toknum) {
	int i, parent;
	char *tmp;

	/*
	 * Iterate over tokens to find the keys of root object.
	 * Iteration is started from 1st object as 0th is root object.
	 * We know that key is the key of root object if it's `parent` value
	 * is set to 0.
	 */
	for(i = 1; i < toknum; i++) {
		if(tokens[i].parent != 0 ||
		   tokens[i].type != JSMN_STRING)
			continue;

		/*
		 * compare key name and also check if there's a value attached to the key
		 * and also whether that value is an object and whether that object is empty
		 */
		if(strncmp("house", &str[tokens[i].start], tokens[i].end - tokens[i].start) == 0
		   && tokens[i].size == 1) {
			if(tokens[i + 1].size == 0 ||
			   tokens[i + 1].type != JSMN_OBJECT)
				continue;

			parent = i + 1;
			/*
			 * Go over tokens untill we either hit end of them or arrive back at
			 * root object
			 */
			for(i++; i < toknum && tokens[i].parent != 0; i++) {
				if(strncmp("architecture", &str[tokens[i].start], tokens[i].end - tokens[i].start) == 0) {
					/*
					 * we check parent to make sure that we are not about to
					 * work on value of key that is part of unhandled key/value
					 */
					if(tokens[i].parent != parent ||
					   tokens[i].size != 1 ||
					   tokens[i + 1].type != JSMN_STRING)
						continue;

					i++; //now points at the value
					/*
					 * Using calloc will zero out memory and with that also add
					 * null at the end of the string in an inefficient way, heh
					 */
					parsed->house.architecture = calloc(1, tokens[i].end - tokens[i].start + 1);
					memcpy(parsed->house.architecture, &str[tokens[i].start], tokens[i].end - tokens[i].start);
				}
				else if(strncmp("sqm", &str[tokens[i].start], tokens[i].end - tokens[i].start) == 0) {
					if(tokens[i].parent != parent ||
					   tokens[i].size != 1 ||
					   tokens[i + 1].type != JSMN_PRIMITIVE)
						continue;

					i++;

					tmp = calloc(1, tokens[i].end - tokens[i].start + 1);
					memcpy(tmp, &str[tokens[i].start], tokens[i].end - tokens[i].start);

					parsed->house.sqm = atoi(tmp);
					free(tmp);
				}
			}
			i--; //without this the upper loop with increase the index by one and miss the key
		}
		else if(strncmp("car", &str[tokens[i].start], tokens[i].end - tokens[i].start) == 0) {
			if(tokens[i + 1].size == 0 ||
			   tokens[i + 1].type != JSMN_OBJECT)
				continue;

			parent = i + 1;
			for(i++; i < toknum && tokens[i].parent != 0; i++) {
				if(strncmp("brand", &str[tokens[i].start], tokens[i].end - tokens[i].start) == 0) {
					if(tokens[i].parent != parent ||
					   tokens[i].size != 1 ||
					   tokens[i + 1].type != JSMN_STRING)
						continue;

					i++;

					parsed->car.brand = calloc(1, tokens[i].end - tokens[i].start + 1);
					memcpy(parsed->car.brand, &str[tokens[i].start], tokens[i].end - tokens[i].start);
				}
				else if(strncmp("horsepower", &str[tokens[i].start], tokens[i].end - tokens[i].start) == 0) {
					if(tokens[i].parent != parent ||
					   tokens[i].size != 1 ||
					   tokens[i + 1].type != JSMN_PRIMITIVE)
						continue;

					i++;

					tmp = calloc(1, tokens[i].end - tokens[i].start + 1);
					memcpy(tmp, &str[tokens[i].start], tokens[i].end - tokens[i].start);

					parsed->car.horsepower = atoi(tmp);
					free(tmp);
				}
			}
			i--;
		}
	}
}

int main(void) {
	jsmn_parser parser;
	jsmntok_t tok[MAX_TOK];
	int ret;
	char *input = "{\n"
				  "\"house\": {\"architecture\": \"wood_frame\", \"sqm\": 100},\n"
				  "\"car\": {\"brand\": \"\", \"horsepower\": 40} \n"
				  "}";
	struct obj parsed;


	/*
	 * Initialize parser and parse JSON string
	 */
	jsmn_init(&parser);
	ret = jsmn_parse(&parser, input, strlen(input), tok, sizeof tok / sizeof *tok);
	if(ret < 0) {
		fprintf(stderr, "Parsing failed (%i)\n", ret);
		return ret;
	}

	/*
	 * Fill struct based on parsed data
	 */
	fill_object(input, tok, &parsed, ret);

	printf("House:\n\tArchitecture: %s\n\tSqm: %i\nCar:\n\tBrand: %s\n\tHorsepower: %i\n",
		   parsed.house.architecture, parsed.house.sqm, parsed.car.brand, parsed.car.horsepower);
}