Move JSON serialising, deserialising and walking code into a separate package

1 files changed, 316 insertions, 0 deletions

diff --git a/walk/walk.go b/walk/walk.go
new file mode 100644
index 0000000..19180b4
--- /dev/null
+++ b/walk/walk.go
@@ -0,0 +1,316 @@
+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")
+}