docs/Writerside/topics/json.h.md

# JSON

The UCX JSON API allows [parsing](#parser) and [formatting](#writer) of JSON data.

The parser API is similar to the [properties](properties.h.md) parser,
but - due to the nature of JSON - is not allocation-free.

## Parser

The following listing shows the JSON parser API.

> To simplify documentation, we introduce the pseudo-type `AnyStr` with the meaning that
> any UCX string and any C string are supported.
> The implementation is actually hidden behind a macro which uses `cx_strcast()` to guarantee compatibility.
{style="note"}

```C
#include <cx/json.h>

void cxJsonInit(CxJson *json, const CxAllocator *allocator);

void cxJsonReset(CxJson *json);

int cxJsonFilln(CxJson *json, const char *buf, size_t len);

int cxJsonFill(CxJson *json, AnyStr str);

CxJsonStatus cxJsonNext(CxJson *json, CxJsonValue **value);

void cxJsonDestroy(CxJson *json);

CxJsonStatus cxJsonFromString(const CxAllocator *allocator,
        AnyStr str, CxJsonValue **value);
```

The first step is to initialize a `CxJson` structure with a call to `cxJsonInit()`,
specifying the allocator that shall be used for allocating values of type `CxJsonValue`.

Specifying `NULL` as `allocator` is allowed, in which case the `cxDefaultAllocator` will be used.

The actual parsing is an interleaving invocation of the `cxJsonFill()` (or `cxJsonFilln()`) and `cxJsonNext()` functions.
The `cxJsonFill()` function is a convenience function that accepts UCX strings and normal zero-terminated C strings.

Calling `cxJsonNext()` will return with `CX_JSON_NO_ERROR` (= zero) for each JSON value that is successfully parsed,
and stores the pointer to the allocated value in the variable pointed to by `value`.

> The parser is capable of parsing multiple consecutive JSON values.
> If those values are not objects or arrays, they must, however, be separated by any whitespace character. 

When all the data from the input buffer was successfully consumed, `cxJsonNext()` returns `CX_JSON_NO_DATA`.

If `cxJsonNext()` returns `CX_JSON_INCOMPLETE_DATA` it means that the input buffer is exhausted,
but the parsed input does not constitute a complete JSON value. 
In that case, you can call `cxJsonFill()` again to add more data and continue with `cxJsonNext()`.

A complete list of all status codes can be seen [below](#list-of-status-codes).

If you want to reuse a `CxJson` structure, you can call `cxJsonReset()`.
In particular, you _must_ reset the parser when the last operation was a failure.
Otherwise, you need to call `cxJsonDestroy()` when you are done with the parser.

The function `cxJsonFromString()` combines the above procedure. 

### List of Status Codes

Below is a full list of status codes for `cxJsonNext()`.

| Status Code                           | Meaning                                                                                           |
|---------------------------------------|---------------------------------------------------------------------------------------------------|
| CX_JSON_NO_ERROR                      | A value was successfully parsed.                                                                  |                                                                                                                                                                                       |
| CX_JSON_NO_DATA                       | The input buffer does not contain more data.                                                      |
| CX_JSON_INCOMPLETE_DATA               | The input ends unexpectedly. Use `cxJsonFill()` to add more data before retrying.                 |
| CX_JSON_NULL_DATA                     | The input buffer was never initialized. Probably you forgot to call `cxJsonFill()` at least once. |
| CX_JSON_BUFFER_ALLOC_FAILED           | More internal buffer was needed, but could not be allocated.                                      |
| CX_JSON_VALUE_ALLOC_FAILED            | Allocating memory for a json value failed.                                                        |
| CX_JSON_FORMAT_ERROR_NUMBER           | A number value is incorrectly formatted.                                                          |
| CX_JSON_FORMAT_ERROR_UNEXPECTED_TOKEN | The tokenizer found something unexpected, i.e. the JSON string contains a syntax error.           |

## Access Values

```C
#include <cx/json.h>

bool cxJsonIsObject(const CxJsonValue *value);

bool cxJsonIsArray(const CxJsonValue *value);

bool cxJsonIsString(const CxJsonValue *value);

bool cxJsonIsNumber(const CxJsonValue *value);

bool cxJsonIsInteger(const CxJsonValue *value);

bool cxJsonIsLiteral(const CxJsonValue *value);

bool cxJsonIsBool(const CxJsonValue *value);

bool cxJsonIsTrue(const CxJsonValue *value);

bool cxJsonIsFalse(const CxJsonValue *value);

bool cxJsonIsNull(const CxJsonValue *value);

char *cxJsonAsString(const CxJsonValue *value);

cxstring cxJsonAsCxString(const CxJsonValue *value);

cxmutstr cxJsonAsCxMutStr(const CxJsonValue *value);

double cxJsonAsDouble(const CxJsonValue *value);

int64_t cxJsonAsInteger(const CxJsonValue *value);

bool cxJsonAsBool(const CxJsonValue *value);

size_t cxJsonArrSize(const CxJsonValue *value);

CxJsonValue *cxJsonArrGet(const CxJsonValue *value, size_t index);

CxJsonValue *cxJsonArrRemove(const CxJsonValue *value, size_t index);

size_t cxJsonObjSize(const CxJsonValue *value);

CxJsonValue *cxJsonObjGet(const CxJsonValue *value, AnyStr name);

CxJsonValue *cxJsonObjRemove(const CxJsonValue *value, AnyStr name);

CxIterator cxJsonArrIter(const CxJsonValue *value);

CxIterator cxJsonObjIter(const CxJsonValue *value);
```

The `cxJsonIsXYZ()` family functions check the type of the specified JSON value.

The JSON specification only defines numbers, therefore `cxJsonIsNumber()` returns true for both floating-point and integer numbers.
On the other hand, `cxJsonIsInteger()` only returns true for integral numbers.

The function `cxJsonIsBool()` returns true if `cxJsonIsLiteral()` returns true, but `cxJsonIsNull()` does not.

> Since a literal can be `true`, `false`, or `null`, note carefully that `!cxJsonIsTrue(v)`
> is in general _not_ equivalent to `cxJsonIsFalse(v)`.
> 
> Additionally, UCX does implement the JavaScript concept of a "falsy" value, meaning that
> `cxJsonIsFalse()` _only_ returns true, if the value is a literal `false`.
>{style="note"}

The `cxJsonAsXYZ()` family of functions return the value with its corresponding C type.

The functions `cxJsonAsInteger()` and `cxJsonAsDouble()` can be used for any number value.
For example, if `cxJsonAsInteger()` is used on a non-integral number, a double-to-int conversion is performed.

The function `cxJsonArraySize()` returns the number of items in an array value,
which can be accessed via index with `cxJsonArrGet()` or via an iterator created with `cxJsonArrIter()`.
An element can be removed from an array with `cxJsonArrRemove()`.

The function `cxJsonObjGet()` returns the member in a JSON object associated with the specified `name`.
To remove a member, use `cxJsonObjRemove()`.

> Both `cxJsonArrGet()` and `cxJsonObjGet()` are safe regarding access to non-existing values.
> 
> When `cxJsonArrGet()` is used with an out-of-bounds index, or `cxJsonObjGet()` is used with a non-existent name,
> they return a JSON value, that returns `false` for any `cxJsonIsXYZ()` function.
> 
> This is **not** the case for `cxJsonArrRemove()` and `cxJsonObjRemove()`, which return `NULL` in that case.

> If you don't have full control over the JSON data, you should always check the datatype of a value first before accessing it.
>{style="note"}

## Deallocate Memory

```C
#include <cx/json.h>

void cxJsonValueFree(CxJsonValue *value);
```

Once a JSON value is not needed anymore, the memory can be deallocated with `cxJsonValueFree()`.
Nested values are also recursively deallocated. 

> Make sure that you are not accidentally deallocating values that are still part of an object or array.
> When deallocating the enclosing object/array, this will lead to a double-free.
>{style="warning"}

## Create Objects

```C
#include <cx/json.h>

CxJsonValue* cxJsonCreateObj(const CxAllocator* allocator);

CxJsonValue* cxJsonCreateArr(const CxAllocator* allocator);

CxJsonValue* cxJsonCreateNumber(
        const CxAllocator* allocator, double num);

CxJsonValue* cxJsonCreateInteger(
        const CxAllocator* allocator, int64_t num);

CxJsonValue* cxJsonCreateString(const CxAllocator* allocator,
        AnyStr str);

CxJsonValue* cxJsonCreateLiteral(
        const CxAllocator* allocator, CxJsonLiteral lit);

int cxJsonArrAddNumbers(CxJsonValue* arr,
        const double* num, size_t count);

int cxJsonArrAddIntegers(CxJsonValue* arr,
        const int64_t* num, size_t count);

int cxJsonArrAddStrings(CxJsonValue* arr,
        const char* const* str, size_t count);

int cxJsonArrAddCxStrings(CxJsonValue* arr,
        const cxstring* str, size_t count);

int cxJsonArrAddLiterals(CxJsonValue* arr,
        const CxJsonLiteral* lit, size_t count);

int cxJsonArrAddValues(CxJsonValue* arr,
        CxJsonValue* const* val, size_t count);

int cxJsonObjPut(CxJsonValue* obj, AnyStr name, CxJsonValue* child);

CxJsonValue* cxJsonObjPutObj(CxJsonValue* obj, AnyStr name);

CxJsonValue* cxJsonObjPutArr(CxJsonValue* obj, AnyStr name);

CxJsonValue* cxJsonObjPutNumber(CxJsonValue* obj,
        AnyStr name, double num);

CxJsonValue* cxJsonObjPutInteger(CxJsonValue* obj,
        AnyStr name, int64_t num);

CxJsonValue* cxJsonObjPutString(CxJsonValue* obj,
        AnyStr name, AnyStr str);

CxJsonValue* cxJsonObjPutLiteral(CxJsonValue* obj,
        AnyStr name, CxJsonLiteral lit);
```

The `cxJsonCreateXY()`-family of functions can be used to create JSON values which are allocated by the specified `allocator`.
If you specify `NULL` as allocator, the `cxDefaultAllocator` is used.

If you want to add created values to a JSON array or JSON object,
you can use `cxJsonArrAddValues()` or `cxJsonObjPut()`, respectively.
However, it is usually more convenient to use one of the other functions, as they automatically create the JSON value for.

```C
#include <cx/json.h>

CxJsonValue* arr = cxJsonCreateArr(NULL);

// this is equivalent...
CxJsonValue* x = cxJsonCreateInteger(NULL, 47);
CxJsonValue* y = cxJsonCreateInteger(NULL, 11);
cxJsonArrAddValues(arr, (CxJsonValue*[]){x, y}, 2);

// ... to this
cxJsonArrAddIntegers(arr, (int64_t[]){47, 11}, 2);
```

The following example shows how to construct a complete JSON object.

```C
CxJsonValue *obj = cxJsonCreateObj(NULL);

cxJsonObjPutLiteral(obj, "bool", CX_JSON_FALSE);
cxJsonObjPutInteger(obj, "int", 47);

CxJsonValue *strings = cxJsonObjPutArr(obj, "strings");
cxJsonArrAddStrings(strings, (const char*[]) {"hello", "world"}, 2);

CxJsonValue *nested = cxJsonObjPutObj(obj, "nested");
CxJsonValue *objects = cxJsonObjPutArr(nested, "objects");
CxJsonValue *obj_in_arr[2] = {
    cxJsonCreateObj(NULL),
    cxJsonCreateObj(NULL),
};
cxJsonObjPutInteger(obj_in_arr[0], "name1", 1);
cxJsonObjPutInteger(obj_in_arr[0], "name2", 3);

cxJsonObjPutInteger(obj_in_arr[1], "name2", 7);
cxJsonObjPutInteger(obj_in_arr[1], "name1", 3);

cxJsonArrAddValues(objects, obj_in_arr, 2);

cxJsonArrAddNumbers(cxJsonObjPutArr(nested, "floats"),
    (double[]){3.1415, 47.11, 8.15}, 3);
    
cxJsonArrAddLiterals(cxJsonObjPutArr(nested, "literals"),
    (CxJsonLiteral[]){CX_JSON_TRUE, CX_JSON_NULL, CX_JSON_FALSE}, 3);
 
CxJsonValue *ints = cxJsonObjPutArr(nested, "ints");
cxJsonArrAddIntegers(ints, (int64_t[]){4, 8, 15}, 3);

CxJsonValue *nested_array = cxJsonCreateArr(NULL);
cxJsonArrAddValues(ints, &nested_array, 1);
cxJsonArrAddIntegers(nested_array, (int64_t[]){16, 23}, 2);
cxJsonArrAddIntegers(ints, (int64_t[]){42}, 1);

CxJsonWriter w = cxJsonWriterPretty(true);
cxJsonWrite(stdout, obj, (cx_write_func) fwrite, &w);

cxJsonValueFree(obj);
```

The above code writes the following output to `stdout`:

```JSON
{
    "bool": false,
    "int": 47,
    "strings": ["hello", "world"],
    "nested": {
        "objects": [{
            "name1": 1,
            "name2": 3
        }, {
            "name2": 7,
            "name1": 3
        }],
        "floats": [3.1415, 47.11, 8.15],
        "literals": [true, null, false],
        "ints": [4, 8, 15, [16, 23], 42]
    }
}
```

## Writer

```C
#include <cx/json.h>

typedef struct cx_json_writer_s {
    bool pretty;
    uint8_t frac_max_digits;
    bool indent_space;
    uint8_t indent;
    bool escape_slash;
} CxJsonWriter;

CxJsonWriter cxJsonWriterCompact(void);

CxJsonWriter cxJsonWriterPretty(bool use_spaces);

int cxJsonWrite(void* target, const CxJsonValue* value,
        cx_write_func wfunc, const CxJsonWriter* settings);
```

A JSON value can be formatted with the `cxJsonWrite()` function.

The `target` can be a stream, a UCX [buffer](buffer.h.md), or anything else that can be written to with a write function.
The behavior of the function is controlled via a `CxJsonWriter` struct.
With the functions `cxJsonWriterCompact()` and `cxJsonWriterPretty()` you can create default settings,
which you may modify to suit your needs.

| Setting           | Compact Default | Pretty Default                   | Description                                                                                                                                                      |
|-------------------|-----------------|----------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `pretty`          | `false`         | `true`                           | If true, the JSON will be formatted with line breaks and tabs or spaces. If false, output is as compact as possible without extra characters.                    |
| `frac_max_digits` | 6               | 6                                | The maximum number of fractional digits in a number value.                                                                                                       |
| `indent_space`    | ignored         | depends on `use_spaces` argument | If true, use spaces for indentation, otherwise use tabs.                                                                                                         |
| `indent`          | ignored         | 4                                | If `indent_space` is `true`, this is the number of spaces per tab. Ignored otherwise.                                                                            |
| `escape_slash`    | `false`         | `false`                          | If `true`, the slash character (a.k.a forward solidus: `/`) is also escaped. This is usually only needed when you want to use JSON as part of an HTML attribute. |


<seealso>
<category ref="apidoc">
<a href="https://ucx.sourceforge.io/api/json_8h.html">json.h</a>
</category>
</seealso>