qb-json-next
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2.1.3 • Public • Published

qb-json-next

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A very fast, very light, zero-dependency, validating JSON tokenizer (~300 MB/sec running node 6 on 2.2 GHz Intel i7).

qb-json-next provides core parsing and state management for converting raw JSON buffers into tokens in a tiny library.

Check out qb-json-tokenizer for a wrapper over this tokenization that makes it easier to parse arbitrary-size streaming chunks of JSON.

Complies with the 100% test coverage and minimum dependency requirements of qb-standard .

install

npm install qb-json-next

next (ps, opt)

next() is the function returned by require('qb-json-next'). It updates a ps or "parse-state" object. The ps object can be any object with a 'next_src' buffer.

ps - the "parse state" object which is updated with every call to next() (see following section)

opt {
    err - a function that handles errors.  if not specified, errors will be thrown instead.
}

for example, we can parse and print all tokens in a buffer like this:

const next = require('qb-json-next')

const ps = { next_src: new Buffer( '{ "a": [1,2,3] }' ) } 
while ( next(ps) ) {
  console.log( next.tokstr(ps) )                    // see documentation for tokstr() details 
}

output:

> {@0                       // start of object at 0
> k3@2:[@7                  // key (3 bytes) at 2, start of array at 7                        
> d1@8                      // decimal (1 byte) at 8
> d1@10                     // decimal (1 byte) at 10
> d1@12                     // decimal (1 byte) at 12
> ]@13                      // end array at 13
> }@15                      // end object at 15

next() returns the last token parsed, the ps.tok property, which is zero if a complete token was not parsable. ps.tok and other properties are also explained in the documentation below.

next() has other helpful properties such as TOK, POS, ECODE, posname() and tokstr() which are explained in the following section.

The parse-state object (ps)

Each call to next(ps) updates the parse-state object or 'ps'. A parse-state object can be any object you like and can be as simple as:

ps = { next_src: a_source_buffer }

Upon calling next(ps), the parse state will be enriched with a complete collection of parse state information:

{
    // user-populated:
    next_src    // [byt] - users must set this to the src buffer/chunk of JSON to process before calling next(). 
                           Calling next() will move this buffer to the 'src' field for processing, at which point user
                           may set next_src to the next chunk to be processed.

    // qb-next managed fields:
    src         // [byt] - set to the current buffer processed
    soff        // int   - the prior src offset.  e.g. ps.soff + ps.vlim = total byte offset from start
    vcount      // int   - value count - number of complete values or key-values read so far
    koff        // int   - key offset
    klim        // int   - key limit
    tok         // int   - token - integer indicating what was parsed or encountered, see chart
    voff        // int   - value offset
    vlim        // int   - value limit
    stack       // [byt] - ascii open braces/brackets representing array or object containers and depth
    pos         // int   - relative parse position code (before value, after key...) 
    ecode       // int   - end-code used to indicate special termination state such as truncated or illegal values      
}

Parsing across buffers with whole values

The ps.next_src property is checked before returning from next() whenever ps.src is empty or finished. If set, ps.next_src is moved to ps.src, limits are reset, and parsing continues with the new buffer. This allows parsing across buffers:

const next = require('qb-json-next')
const ps = {}

while (ps.next_src = get_next_buffer_somehow()) {
    while (next(ps)) {
      console.log(next.tokstr(ps))  
    }
}

Parsing stopping at any point retains exact information about the stop state, but next() can only continue parsing after a whole value or key-value pair.

(You can use qb-json-align to handle arbitrary buffer splits)

next() cannot continue after part of a key, part of a value or after a key with no subsequent value (array and object start brackets count as a whole value).

src1                        src2                            split allowed?
'['                         '11, 12, 13]'                   YES 
'[11, 12'                   ', 13]'                         YES 
'[11, 12, 13'               ']'                             YES 
'{ '                        '}'                             YES   
'{ "a": true '              '}'                             YES  
'{ "a": true'               ', "b": false }'                YES  
'{ "a": true, '             '"b": false }'                  YES  
'{ "a": true, "b": ['       '82] }'                         YES - key "b" is in same buffer as array start
                            
'[ 11, 1'                   '2, 13 ]'                       NO - split number 12
'{ "a'                      '": true, "b": [82] }'          NO - split key "a" 
'{ "a": true, "b": '        '[82] }'                        NO - key "b" is in different buffer as array start [  

Parsing buffers with any arbitrary split

The module qb-json-align supports continued parsing from any split buffer including truncated keys and values.
It's simple to use - just call align(ps) after setting ps.next_src and before calling next(ps) and split values are handled for you:

while (ps.next_src = get_next_buffer_somehow()) {
    align(ps)                                               // add this call after next_src is set to handle any split.
    while (next(ps)) {
      console.log(next.tokstr(ps))
    }
}

ps.vcount

The number of completed values. Array and object start brackets are not counted, while array and object end brackets are counted. Strings are only counted after vlim passes the closing quote and numbers are only counted after a non-number delimiter byte is read. Keys are not counted, only the completed value following the key is counted counting every key-value pair as "1" value.

offsets: ps.koff, ps.klim, ps.voff, and ps.vlim

The offsets describe key and value start and end points. ps.koff and ps.voff are inclusive offsets where the key and value start (starting at zero), while ps.klim and ps.vlim are exclusive limits which are one byte after the last key or value byte.

When ps.koff = ps.klim, then there is no key. Object values always have a key, while array values never do.

When ps.voff = ps.vlim, then there is no value.

Remember that src data is UTF-8 encouded, not UTF-16 encoded like javascript strings so non-asci will not directly translate to strings. qb-utf8-to-str-tiny is a light-weight library (though not fast) for converting UTF-8 to javascript strings. Also note that string offsets include the quotes (ascii 34)

For example:

                                                                              (ascii as strings)
                                                                              
                                          koff    klim    voff    vlim        key   value   stack
|-----------------------------------------   0       0       0       0                         
|                                                                                              
| |---------------------------------------   0       0       1       2                 {       {         
| |                                                                                            
| |       |-------------------------------   9      10      10      10        "                {
| |       |                                                                                    
| |       | |-----------------------------   9      12      12      12        "a"              {
| |       | |                                                                                  
| |       | | |---------------------------   9      12      14      14        "a"              {
| |       | | |                                                                                
| |       | | | |-------------------------   9      12      15      16        "a"      "       {
| |       | | | |                                                                              
| |       | | | |  |----------------------   9      12      15      19        "a"   "hi"       {
| |       | | | |  |                                                                           
| |       | | | |  |                       (key/value flushed out - to callback)               
| |       | | | |  |                        12      12      19      19                         {
| |       | | | |  |                                                                           
| |       | | | |  |     |----------------  21      24      25      25        "b"              {
| |       | | | |  |     |                                                                     
| |       | | | |  |     | |--------------  21      24      26      27        "b"      [      {[
| |       | | | |  |     | |                                                                   
| |       | | | |  |     | |    |---------  24      24      32      32                        {[
| |       | | | |  |     | |    |                                                              
| |       | | | |  |     | |    | |-------  24      24      32      33                 ]       {
| |       | | | |  |     | |    | |
| |       | | | |  |     | |    | | |-----  24      24      35      36                 ]       
| |       | | | |  |     | |    | | |
          1         2         3      
01234567890123456789012345678901234567
 {       "a":  "hi", "b": [ 1, 2 ] }                           

ps.tok

The ps.tok property holds an ascii byte representing the type of value last read, or zero if parsing ended (at src limit or on error). Possible values for tok are defined in the next.TOK object property:

next.TOK

The TOK object maps names to the ps.tok integers returned by next(). For all but string and decimal, the token returned is simply the same as the first byte encountered. 't' for true, '{' for object start, etc. Here they are just as they are defined in the code:

const TOK = {
  ARR: 91,        // '['    - array start   
  ARR_END: 93,    // ']'    - array end
  DEC: 100,       // 'd'    - a decimal value starting with: -, 0, 1, ..., 9
  FAL: 102,       // 'f'    - false
  NUL: 110,       // 'n'    - null
  STR: 115,       // 's'    - a string value starting with "
  TRU: 116,       // 't'    - true
  OBJ: 123,       // '{'    - object start
  OBJ_END:  125,  // '}'    - object end
}

next.tokstr(ps, detail)

Given a parse state, next.tokstr() returns an abbreviated string representation of the type of token, the length of key and value and the byte offset into ps.src.

Setting detail option to true will include the abbreviated position name (see POS), and a string representation of the stack. For example:

const next = require('qb-json-next')

console.log('First object:')
const ps = {next_src: new Buffer('{ "a": [1,2,3] }')}
while (next(ps)) {
  console.log(next.tokstr(ps))
}

console.log('Second object with detail = true:')
ps.next_src = new Buffer(', { "b": [4,5,6] }')
while (next(ps)) {
  console.log(next.tokstr(ps, true))
}

outputs:

First object:
{@0
k3@2:[@7
d1@8
d1@10
d1@12
]@13
}@15
Second object with detail = true:
{@2:O_BF:{
k3@4:[@9:A_BF:{[
d1@10:A_AV:{[
d1@12:A_AV:{[
d1@14:A_AV:{[
]@15:O_AV:{
}@17:A_AV

ps.stack and ps.pos

ps.stack and ps.pos keep track of the nested context and relative position of parsing.

ps.stack uses an array of integers (object and array open-braces ascii) to track depth and context

91      // ascii for '[' - array start
123     // ascii for '{' - object start                                    

stack:  [] | [123]                      | [123,91]       | [123]  | []
           |                            |                |        |
           {  name :  "Samuel" , tags : [ "Sam", "Sammy" ]        } ,  "another value"

ps.pos describes parser position relative to a JSON key or value. Note that both the start and end brackets of arrays and objects are considered values when describing position.

  value                                          value
   |   key    value    key  value            value |
   |    |       |       |     | value   value  |   |
   |    |       |       |     |   |       |    |   |
   {  name : "Samuel", tags : [ "Sam", "Sammy" ]   }

There are 2 possible positions before, and after that define parse position relative to a key or value plus a first indicator to indicate if it is the first item in a new context:

before-first-value                                                          stack = ''
  |  
  |  before-first-key                                                       stack = '{'
  |  |
  |  |    after-key
  |  |    |
  |  |    | before-value
  |  |    | |
  |  |    | | 
  |  |    | |  
  |  |    | |         after-value
  |  |    | |         |
  |  |    | |         | before-key        
  |  |    | |         | |
  |  |    | |         | | 
  |  |    | |         | |  
  |  |    | |         | |    after-key
  |  |    | |         | |    |
  |  |    | |         | |    | before-value
  |  |    | |         | |    | |
  |  |    | |         | |    | | before-first-value                         stack = '{['
  |  |    | |         | |    | | |
  |  |    | |         | |    | | |  
  |  |    | |         | |    | | |    
  |  |    | |         | |    | | |     after-value
  |  |    | |         | |    | | |     |
  |  |    | |         | |    | | |     |before-value
  |  |    | |         | |    | | |     ||
  |  |    | |         | |    | | |     ||
  |  |    | |         | |    | | |     ||   
  |  |    | |         | |    | | |     ||       after-value
  |  |    | |         | |    | | |     ||       | 
  |  |    | |         | |    | | |     ||       | after-value               stack = '{'
  |  |    | |         | |    | | |     ||       | |           after-value   stack = ''
  |  |    | |         | |    | | |     ||       | |           |
  |  |    | |         | |    | | |     ||       | |           | before-value
  |  |    | |         | |    | | |     ||       | |           | |
  |  |    | |         | |    | | |     ||       | |           | | 
  |  |    | |         | |    | | |     ||       | |           | |  
  |  |    | |         | |    | | |     ||       | |           | |                after-value
  |  |    | |         | |    | | |     ||       | |           | |                |
   {  name :  "Samuel" , tags : [ "Sam", "Sammy" ]        }    ,  "another value"

Most state management in next() is the matter of a bitwise-or and one or two array lookups per token, which is why next() can validate so quickly.

next.pos positions are defined in the next.POS object:

next.POS

The POS object maps names to the ps.pos values used by next().
ps.pos integers allow super-fast performance, but are not user-friendly, so these POS names give some level readability (though admittedly still very brief). Here they are as defined in the code:

// values for ps.pos(ition).  LSB (0x7F) are reserved for token ascii value.
const POS = {
  A_BF: 0x080,   // in array, before first value
  A_BV: 0x100,   // in array, before value
  A_AV: 0x180,   // in array, after value
  O_BF: 0x200,   // in object, before first key
  O_BK: 0x280,   // in object, before key
  O_AK: 0x300,   // in object, after key
  O_BV: 0x380,   // in object, before value
  O_AV: 0x400,   // in object, after value
}

(Design note - the object/array context is redundant with stack information, but is included in ps.pos to allow one-step lookup and validation which is a factor in making next() validation so fast.)

next.posname (pos)

Given a ps.pos integer as input next.posname() returns a brief string code (A_BF, A_BV, O_BF...)- the same names used for next.POS

ps.ecode

The ps.ecode or "end-code" describes special parsing end states. The four possible end states are defined by the next.ECODE object:

next.ECODE

The ECODE object maps names of special parsing "end" states to integers used by ps.ecode. When one of these special states occurs, ps.tok is set to zero and ps.ecode is set to one of the following:

const ECODE = {
  BAD_VALUE: 66,    // 'B'  encountered invalid byte or series of bytes
  TRUNC_DEC: 68,    // 'D'  end of buffer was value was a decimal ending with a digit (0-9). it is *possibly* unfinished
  KEY_NO_VAL: 75,   // 'K'  object key complete, but value did not start
  TRUNCATED: 84,    // 'T'  key or value was unfinished at end of buffer
  UNEXPECTED: 85,   // 'U'  encountered a recognized token in wrong place/context
}

How it works - Understanding the parse graph (state and stack)

The parse graph is a simple mapping of parse states to other allowed states. For next(), we employ an integer array to do this in the most efficent manner possible. We encode position state (ps.pos) along with ascii values in the same low integer to create the graph.

let state1 = states[state0 + ascii-value]

If the state isn't allowed, then state1 is zero. If allowed, it is the new ps.pos value - the next position.

let state2 = states[state1 + ascii-value]

This simple mechanism works for all state transitions, except when we leave context of an object or array.
When a '}' or ']' is encountered, the new state will have no context set (you can see this for yourself in the Adding Custom Rules to Parsing section, below).

When closing an object or array, the 'stack' is used to supplement missing context (91 is ascii for array-close):

if (stack.length !== 0) { state1 |= (stack[stack.length - 1] === 91 ? in_arr : in_obj) }

Adding Custom Rules to Parsing

Though qb-json-next uses bit manipulation, I have tried to make the rules as readable as possible so even if you aren't comfortable with bit twiddling, you may understand and modify the parse rules. Can you see how to make parsing tolerant of trailing commas by looking at the states below? (the answer is at the bottom of this section).

First, the setup. We create an integer-to-integer mapping of all the allowed states (see generate-maps.js for this script that generates the ascii state array). The full parse graph is defined with 10 lines of configuration:

//   position(s) + token(s) -> new position     (state + token -> new state)
map([POS.A_BF, POS.A_BV], 'ntfds', POS.A_AV)
map([POS.A_AV], ',', POS.A_BV)

map([POS.A_BF, POS.A_BV, POS.O_BV], '[', POS.A_BF)
map([POS.A_BF, POS.A_BV, POS.O_BV], '{', POS.O_BF)

map([POS.A_BF, POS.A_AV], ']', POS.A_AV)      // use any non-zero value here - stack is used to check new position
map([POS.O_BF, POS.O_AV], '}', POS.A_AV)      // use any non-zero value here - stack is used to check new position

map([POS.O_AV], ',', POS.O_BK)
map([POS.O_BF, POS.O_BK], 's', POS.O_AK)      // s = string
map([POS.O_AK], ':', POS.O_BV)
map([POS.O_BV], 'ntfds', POS.O_AV)

That's pretty dense, and the codes look cryptic, but it is easy to see the 'big picture' once you understand the abbreviations.

map([POS.A_BF, POS.A_BV], 'ntfds', POS.A_AV)

... maps positions

    "in-array, before-first-value" and "in-array, before-value"
        for tokens (null, true, false, decimal, string)
            to position ("in-array, after-value")

And this statement:

map([POS.O_AV], ',', POS.O_BK)

... maps position

    "in-object, after-value" 
        for token ',' (comma) 
            to position "in-object, before-key"

If that made sense, I encourage looking at the map code - it is just as understandable as that...

To make the graph tolerate trailing commas in arrays [1,2,3,], add an array-end rule where a value is expected (before-value):

  map([POS.A_BV], ']', POS.A_AV )

In fact, if you look above, this looks extremely similar to the existing 'A_BF' + ']' rule that allows arrays to close without any content at all:

  map([POS.A_BF, POS.A_AV], ']', POS.A_AV)        
  
  ... and we could have just added our case to that rule instead, if we liked
  
  map([POS.A_BF, POS.A_AV, POS.A_BV], ']', POS.A_AV)        

To make the graph also tolerate trailing commas in an empty array [,], add an array-comma rule where a first value is expected (before-first-value):

  map([POS.A_BF], ',',  POS.A_BV )

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