
# Tape structure in simdjson

We parse a JSON document to a tape. A tape is an array of 64-bit values. Each node encountered in the JSON document is written to the tape using one or more 64-bit tape elements; the layout of the tape is in "document order": elements are stored as they are encountered in the JSON document.

Throughout, little endian encoding is assumed. The tape is indexed starting at 0 (the first element is at index 0).

## Example

It is sometimes useful to start with an example. Consider the following JSON document:

```json
{
	"Image": {
		"Width": 800,
		"Height": 600,
		"Title": "View from 15th Floor",
		"Thumbnail": {
			"Url": "http://www.example.com/image/481989943",
			"Height": 125,
			"Width": 100
		},
		"Animated": false,
		"IDs": [116, 943, 234, 38793]
	}
}
```

The following is a dump of the content of the tape, with the first number of each line representing the index of a tape element.

### The Tape
| index | element (64 bit word)                                               |
| ----- | ------------------------------------------------------------------- |
| 0     | r	// pointing to 38 (right after last node)                         |
| 1     | {	// pointing to next tape location 38 (first node after the scope) |
| 2     | string "Image"                                                      |
| 3     | {	// pointing to next tape location 37 (first node after the scope) |
| 4     | string "Width"                                                      |
| 5     | integer 800                                                         |
| 7     | string "Height"                                                     |
| 8     | integer 600                                                         |
| 10    | string "Title"                                                      |
| 11    | string "View from 15th Floor"                                       |
| 12    | string "Thumbnail"                                                  |
| 13    | {	// pointing to next tape location 23 (first node after the scope) |
| 14    | string "Url"                                                        |
| 15    | string "http://www.example.com/image/481989943"                     |
| 16    | string "Height"                                                     |
| 17    | integer 125                                                         |
| 19    | string "Width"                                                      |
| 20    | integer 100                                                         |
| 22    | }	// pointing to previous tape location 13 (start of the scope)     |
| 23    | string "Animated"                                                   |
| 24    | false                                                               |
| 25    | string "IDs"                                                        |
| 26    | [	// pointing to next tape location 36 (first node after the scope) |
| 27    | integer 116                                                         |
| 29    | integer 943                                                         |
| 31    | integer 234                                                         |
| 33    | integer 38793                                                       |
| 35    | ]	// pointing to previous tape location 26 (start of the scope)     |
| 36    | }	// pointing to previous tape location 3 (start of the scope)      |
| 37    | }	// pointing to previous tape location 1 (start of the scope)      |
| 38    | r	// pointing to 0 (start root)                                     |



## General formal of the tape elements

Most tape elements are written as `('c' << 56) + x` where `'c'` is some ASCII character determining the type of the element (out of 't', 'f', 'n', 'l', 'u', 'd', '"', '{', '}', '[', ']' ,'r') and where `x` is a 56-bit value called the payload. The payload is normally interpreted as an unsigned 56-bit integer. Note that 56-bit integers can be quite large.


Performance consideration: We believe that accessing the tape in regular units of 64 bits is more important for performance than saving memory.

## Simple JSON values

Simple JSON nodes are represented with one tape element:

- null is  represented as the 64-bit value `('n' << 56)` where `'n'` is the 8-bit code point values (in ASCII) corresponding to the letter `'n'`.
- true is  represented as the 64-bit value `('t' << 56)`.
- false is  represented as the 64-bit value `('f' << 56)`.


## Integer and Double values

Integer values are represented as two 64-bit tape elements:
- The 64-bit value `('l' << 56)` followed by the 64-bit integer value literally. Integer values are assumed to be signed 64-bit values, using two's complement notation.
- The 64-bit value `('u' << 56)` followed by the 64-bit integer value literally. Integer values are assumed to be unsigned 64-bit values.


Float values are represented as two 64-bit tape elements:
- The 64-bit value `('d' << 56)` followed by the 64-bit double value literally in standard IEEE 754 notation.

Performance consideration: We store numbers of the main tape because we believe that locality of reference is helpful for performance.

## Root node

Each JSON document will have two special 64-bit tape elements representing a root node, one at the beginning and one at the end.

- The first 64-bit tape element contains the value `('r' << 56) + x` where `x` is the location on the tape of the last root element.
- The last 64-bit tape element contains the value `('r' << 56)`.

All of the parsed document is located between these two 64-bit tape elements.

Hint: We can read the first tape element to determine the length of the tape.


## Strings

We prefix the string data itself by a 32-bit header to be interpreted as a 32-bit integer. It indicates the length of the string. The actual string data starts at an offset of 4 bytes.

We store string values using UTF-8 encoding with null termination on a separate tape. A string value is represented on the main tape as the 64-bit tape element `('"' << 56) + x` where the payload `x` is the location on the string tape of the null-terminated string.

## Arrays

JSON arrays are represented using two 64-bit tape elements.

- The first 64-bit tape element contains the value `('[' << 56) + x` where the payload `x` is 1 + the index of the second 64-bit tape element on the tape.
- The second 64-bit tape element contains the value `(']' << 56) + x` where the payload `x` contains the index of the first 64-bit tape element on the tape.

All the content of the array is located between these two tape elements, including arrays and objects.

Performance consideration: We can skip the content of an array entirely by accessing the first 64-bit tape element, reading the payload and moving to the corresponding index on the tape.

## Objects

JSON objects are represented using two 64-bit tape elements.

- The first 64-bit tape element contains the value `('{' << 56) + x` where the payload `x` is 1 + the index of the second 64-bit tape element on the tape.
- The second 64-bit tape element contains the value `('}' << 56) + x` where the payload `x` contains the index of the first 64-bit tape element on the tape.

In-between these two tape elements, we alternate between key (which must be strings) and values. A value could be an object or an array.

All the content of the object is located between these two tape elements, including arrays and objects.

Performance consideration: We can skip the content of an object entirely by accessing the first 64-bit tape element, reading the payload and moving to the corresponding index on the tape.
