README.md

parser-go-json

Parser for JSON in Go, which tries to not allocate any memory.

We can parse json without unmarshaling it into a Go struct using the Parser interface:

import "github.com/katydid/parser-go-json/json"

func main() {
	jsonString := `{"otherfield": 123, "myfield": "myvalue"}`
	jsonParser := json.NewParser()
	jsonParser.Init([]byte(jsonString))
	myvalue, err := GetMyField(jsonParser)
	if err != nil {
		panic(err)
	}
	println(myvalue)
}

We can then use the parser to decode only myfield and skip over other fields and return "myvalue":

import (
	"github.com/katydid/parser-go/cast"
	"github.com/katydid/parser-go/parse"
)


func GetMyField(p parse.Parser) (string, error) {
	hint, err := p.Next()
	if hint != parse.EnterHint {
		return "", errors.New("expected object")
	}
	if err != nil {
		return "", err
	}
	for {
		hint, err = p.Next()
		if err != nil {
			return "", err
		}
		if hint != parse.FieldHint {
			return "", errors.New("expected field")
		}
		kind, fieldName, err := p.Token()
		if err != nil {
			return "", err
		}
		if kind != parse.StringKind {
			return "", errors.New("expected string")
		}
		if cast.ToString(fieldName) == "myfield" {
			hint, err = p.Next()
			if err != nil {
				return "", err
			}
			if hint != parse.ValueHint {
				return "", errors.New("expected field")
			}
			kind, val, err := p.Token()
			if err != nil {
				return "", err
			}
			if kind != parse.StringKind {
				return "", errors.New("expected string")
			}
			return cast.ToString(val), nil
		} else {
			p.Skip()
		}
	}
}

Special Considerations

• The parser uses a buffer pool, which will allocate memory until it is warmed up.
• Buffers are reused and pooled. This means that the String and Bytes methods, returns a string and []byte respectively that should be copied if it is needed again before calling Next, Token or Skip.
• Arrays are indexed, which means that ["a","b","c"] will be parsed into something that looks like an integer indexed map: {O: "a", 1: "b", 2: "c"}.

Thank you

Thanks to the following people for consulting on the project:

• Jacques Marais
• Johan Brandhorst-Satzkorn

I still made all the bad design decisions, so don't blame them. They only made the project better than it would have been without their advice.