path: root/walk/walk.go

package walk

import (
	"io"
	"encoding/json"
	"fmt"
)

type PathSegment interface {}
type Path []PathSegment

type TerminalValue int
const (
	ArrayBegin TerminalValue = iota
	ArrayEnd
	MapBegin
	MapEnd
)
type ValueNull struct {}
type ValueBool bool
type ValueNumber float64
type ValueString string

type WalkValue interface {}

type WalkItem struct {
	Value WalkValue
	Path Path
}

type WalkItemStream struct {
	channel chan WalkItem
	rewinds []WalkItem
}

func (stream *WalkItemStream) next() (WalkItem, bool) {
	if len(stream.rewinds) == 0 {
		item, hasItem := <- stream.channel
		return item, hasItem
	}
	item := stream.rewinds[len(stream.rewinds)-1]
	stream.rewinds = stream.rewinds[0:len(stream.rewinds)-1]
	return item, true
}

func (stream *WalkItemStream) rewind(item WalkItem) {
	stream.rewinds = append(stream.rewinds, item)
}

func (stream *WalkItemStream) peek() (WalkItem, bool) {
	item, hasItem := stream.next()
	if !hasItem {
		return item, false
	}
	stream.rewind(item)
	return item, true
}

func tokenToValue(token json.Token) WalkValue {
	switch token.(type) {
		case nil:
			return ValueNull {}
		case bool:
			return ValueBool(token.(bool))
		case float64:
			return ValueNumber(token.(float64))
		case string:
			return ValueString(token.(string))
		default:
			panic("Can't convert JSON token to value")
	}
}

func readValue(dec *json.Decoder, path Path, out chan WalkItem) bool {
	if !dec.More() {
		return true
	}
	t, err := dec.Token()
	if err == io.EOF {
		return true
	} else if err != nil {
		panic("Invalid JSON")
	}
	switch t.(type) {
		case nil, string, float64, bool:
			v := tokenToValue(t)
			out <- WalkItem {v, path}
			return false
		case json.Delim:
			switch rune(t.(json.Delim)) {
				case '[':
					out <- WalkItem {ArrayBegin, path}
					index := 0
					for dec.More() {
						empty := readValue(dec, append(path, index), out)
						if empty {
							break
						}
						index += 1
					}
					t, err := dec.Token()
					if err != nil {
						panic("Invalid JSON")
					}
					delim, isDelim := t.(json.Delim)
					if !isDelim || delim != ']' {
						panic("Expected ] in JSON")
					}
					out <- WalkItem{ArrayEnd, path}
					return false
				case '{':
					out <- WalkItem {MapBegin, path}
					for dec.More() {
						t, _ := dec.Token()
						key, keyIsString := t.(string)
						if !keyIsString {
							panic("Invalid JSON")
						}
						empty := readValue(dec, append(path, key), out)
						if empty {
							panic("Invalid JSON")
						}
					}
					t, err := dec.Token()
					if err != nil {
						panic("Invalid JSON")
					}
					delim, isDelim := t.(json.Delim)
					if !isDelim || delim != '}' {
						panic("Expected } in JSON")
					}
					out <- WalkItem {MapEnd, path}
					return false
				default:
					panic("Error parsing JSON")
			}
		default:
			panic("Invalid JSON token")
	}
}

func startWalk(dec *json.Decoder, out chan WalkItem) {
	isEmpty := readValue(dec, nil, out)
	if isEmpty {
		panic("Missing JSON input")
	}
	close(out)
}

func Json(r io.Reader) chan WalkItem {
	dec := json.NewDecoder(r)
	out := make(chan WalkItem)
	go startWalk(dec, out)
	return out
}

func printIndent(indent int) {
	for i := 0; i < indent; i += 1 {
		fmt.Print("\t")
	}
}

func jsonOutArray(in *WalkItemStream, indent int) {
	fmt.Println("[")
	token, hasToken := in.next()
	if !hasToken {
		panic("Missing ] in output JSON")
	}
	terminal, isTerminal := token.Value.(TerminalValue)
	if isTerminal && terminal == ArrayEnd {
		fmt.Print("\n")
		printIndent(indent)
		fmt.Print("]")
		return
	}
	in.rewind(token)
	for {
		valueToken := jsonOutValue(in, indent + 1, true)
		if valueToken != nil {
			panic("Missing value in output JSON array")
		}
		token, hasToken := in.next()
		if !hasToken {
			panic("Missing ] in output JSON")
		}
		terminal, isTerminal := token.Value.(TerminalValue)
		if isTerminal && terminal == ArrayEnd {
			fmt.Print("\n")
			printIndent(indent)
			fmt.Print("]")
			return
		}
		in.rewind(token)
		fmt.Println(",")
	}
}

func jsonOutMap(in *WalkItemStream, indent int) {
	fmt.Println("{")
	token, hasToken := in.next()
	if !hasToken {
		panic("Missing } in output JSON")
	}
	terminal, isTerminal := token.Value.(TerminalValue)
	if isTerminal && terminal == MapEnd {
		fmt.Print("\n")
		printIndent(indent)
		fmt.Print("}")
		return
	}
	in.rewind(token)
	for {
		keyToken, hasKeyToken := in.peek()
		if !hasKeyToken {
			panic("Missing map element")
		}
		printIndent(indent + 1)
		if len(keyToken.Path) == 0 {
			panic("Map element missing key")
		}
		key := keyToken.Path[len(keyToken.Path)-1]
		switch key.(type) {
			case int:
				fmt.Print(key.(int))
			case string:
				fmt.Printf("%q", key.(string))
			default:
				panic("Invalid path segment")
		}
		fmt.Print(": ")
		valueToken := jsonOutValue(in, indent + 1, false)
		if valueToken != nil {
			panic("Missing value int output JSON map")
		}
		token, hasToken := in.next()
		if !hasToken {
			panic("Missing } in output JSON")
		}
		terminal, isTerminal := token.Value.(TerminalValue)
		if isTerminal && terminal == MapEnd {
			fmt.Print("\n")
			printIndent(indent)
			fmt.Print("}")
			return
		}
		in.rewind(token)
		fmt.Println(",")
	}
}

func jsonOutValue(in *WalkItemStream, indent int, doIndent bool) WalkValue {
	token, hasToken := in.next()
	if !hasToken {
		panic("Missing JSON token in output")
	}
	switch token.Value.(type) {
		case ValueNull:
			if doIndent {
				printIndent(indent)
			}
			fmt.Printf("null")
			return nil
		case ValueBool:
			if doIndent {
				printIndent(indent)
			}
			if token.Value.(ValueBool) {
				fmt.Print("true")
			} else {
				fmt.Print("false")
			}
			return nil
		case ValueNumber:
			if doIndent {
				printIndent(indent)
			}
			fmt.Printf("%v", token.Value)
			return nil
		case ValueString:
			if doIndent {
				printIndent(indent)
			}
			fmt.Printf("%q", token.Value)
			return nil
		case TerminalValue:
			switch token.Value.(TerminalValue) {
				case ArrayBegin:
					if doIndent {
						printIndent(indent)
					}
					jsonOutArray(in, indent)
					return nil
				case MapBegin:
					if doIndent {
						printIndent(indent)
					}
					jsonOutMap(in, indent)
					return nil
				default:
					return token
			}
		default:
			panic("Invalid WalkValue")
	}
}

func JsonOut(in chan WalkItem) {
	stream := WalkItemStream {
		channel: in,
		rewinds: nil,
	}
	if jsonOutValue(&stream, 0, true) != nil {
		panic("Invalid output JSON")
	}
	fmt.Print("\n")
}