json_parser.c

#include <stdio.h>
#include <stddef.h>
#include <stdbool.h>
#include <stdlib.h>
#include <ctype.h>

struct json_array_t;
typedef struct json_array_t json_array_t;

struct json_object_t;
typedef struct json_object_t json_object_t;

struct json_value_t;
typedef struct json_value_t json_value_t;

struct json_array_t {
    size_t alloc;
    size_t len;
    json_value_t *data;
};

struct json_object_t {
    size_t alloc;
    size_t len;
    json_value_t *keys;
    json_value_t *values;
};

struct json_value_t {
    union {
        bool boolean;
        double number;
        const char *string;
        json_array_t *array;
        json_object_t *object;
    };
    enum {
        JSON_INVALID,
        JSON_NULL,
        JSON_BOOLEAN,
        JSON_NUMBER,
        JSON_STRING,
        JSON_ARRAY,
        JSON_OBJECT,
    } type;
};

void file_trim(FILE *file) {
    while (true) {
        char c = fgetc(file);
        if (!isspace(c)) {
            ungetc(c, file);
            break;
        }
    }
}

json_value_t read_json(FILE *file) {
    file_trim(file);
    char c = fgetc(file);
    switch (c) {
        case 'f': {
            if (fgetc(file) == 'a' && fgetc(file) == 'l' && fgetc(file) == 's' && fgetc(file) == 'e') {
                return (json_value_t) {
                    .type = JSON_BOOLEAN,
                    .boolean = false,
                };  
            }
        }
        case 't': {
            if (fgetc(file) == 'r' && fgetc(file) == 'u' && fgetc(file) == 'e') {
                return (json_value_t) {
                    .type = JSON_BOOLEAN,
                    .boolean = true,
                };  
            }
        }
        case '"': {
            size_t alloc = 0;
            size_t len = 0;
            char *string = NULL;
            while (true) {
                char c = fgetc(file);
                if (c == '"') {
                    break;
                }
                if (len + 1 >= alloc) {
                    alloc = (len + 1) * 2;
                    string = realloc(string, sizeof(char) * alloc);
                }
                string[len++] = c;
            }
            return (json_value_t) {
                .type = JSON_STRING,
                .string = string,
            };  
        }
        case '[': {
            ungetc(c, file);
            json_array_t *array = malloc(sizeof(json_array_t));
            *array = (json_array_t) {0};
            while (true) {
                file_trim(file);
                char last = fgetc(file);
                if (last == ']') {
                    break;
                }
                json_value_t entry = read_json(file);
                if (array->len + 1 >= array->alloc) {
                    array->alloc = (array->len + 1) * 2;
                    array->data = realloc(array->data, sizeof(json_value_t) * array->alloc);
                }
                array->data[array->len++] = entry;
            }
            return (json_value_t) {
                .type = JSON_ARRAY,
                .array = array,
            };  
        }
        default: {
            ungetc(c, file);
            if ('0' <= c && c <= '9') {
                double number;
                fscanf(file, "%lf", &number);
                return (json_value_t) {
                    .type = JSON_NUMBER,
                    .number = number,
                };  
            }
            break;
        }
    }
    return (json_value_t) {
        .type = JSON_INVALID,
    };
}

void write_json(FILE *out, json_value_t value) {
    switch (value.type) {
        case JSON_INVALID: {
            fprintf(out, "ERROR\n");
            break;
        }
        case JSON_NULL: {
            fprintf(out, "null");
            break;
        }
        case JSON_BOOLEAN: {
            if (value.boolean) {
                fprintf(out, "true");
            } else {
                fprintf(out, "false");
            }
            break;
        }
        case JSON_NUMBER: {
            fprintf(out, "%.17g", value.number);
            break;
        }
        case JSON_STRING: {
            fprintf(out, "\"%s\"", value.string);
            break;
        }
        case JSON_ARRAY: {
            fprintf(out, "[");
            for (size_t i = 0; i < value.array->len; i++) {
                if (i != 0) {
                    fprintf(out, ",");
                }
                write_json(out, value.array->data[i]);
            }
            fprintf(out, "]");
            break;
        }
        case JSON_OBJECT: {
            fprintf(out, "OBJECT\n");
            break;
        }
    }
}

int main() {
    FILE *in_file = fopen("in.json", "r");
    json_value_t value = read_json(in_file);
    fclose(in_file);
    FILE *out_file = fopen("out.json", "w");
    write_json(out_file, value);
    fclose(out_file);
}