path: root/json/read.go

package json

import (
	"bufio"
	"main/walk"
	"strings"
	"strconv"
	"errors"
)

func isWhitespace(r rune) bool {
	for _, ws := range " \t\r\n" {
		if r == ws {
			return true
		}
	}
	return false
}

func isNumberRune(r rune) bool {
	return '0' <= r && r <= '9' || r == '.'
}

type JSONReaderStructure int
const (
	JSONReaderStructureArray JSONReaderStructure = iota
	JSONReaderStructureMap
)

type JSONReaderState int
const (
	// Immediately before a value
	JSONReaderStateValue JSONReaderState = iota
	// Immediate after a value
	JSONReaderStateValueEnd
)

func NewJSONReader(reader *bufio.Reader) *JSONReader {
	return &JSONReader {
		path: nil,
		structure: nil,
		state: JSONReaderStateValue,
		reader: reader,
	}
}

type JSONReader struct {
	path []walk.PathSegment
	structure []JSONReaderStructure
	state JSONReaderState
	reader *bufio.Reader
	prevStart bool
}

func (reader *JSONReader) Read() (walk.WalkItem, error) {
	switch reader.state {
	case JSONReaderStateValue:
		if len(reader.structure) == 0 {
			// Before the start of a root JSON value
			return reader.readValue()
		}
		switch reader.structure[len(reader.structure) - 1] {
		case JSONReaderStructureArray:
			// Before a value inside an array
			r, err := reader.nextNonWsRune()
			if err != nil {
				panic("Missing rest of array")
			}
			if r == ']' {
				// End of an array
				reader.structure = reader.structure[:len(reader.structure) - 1]
				reader.path = reader.path[:len(reader.path) - 1]
				reader.state = JSONReaderStateValueEnd
				item := walk.WalkItem {
					Value: reader.buildWalkItemValue(walk.ArrayValue([]walk.ArrayElement{})),
					Start: false,
					PrevStart: reader.prevStart,
					End: true,
					NextEnd: reader.isNextEnd(),
				}
				reader.prevStart = false
				return item, nil
			}
			// Element in array
			reader.reader.UnreadRune()
			prevIndex := reader.path[len(reader.path) - 1].(int)
			reader.path[len(reader.path) - 1] = prevIndex + 1
			return reader.readValue()
		case JSONReaderStructureMap:
			// Before a value inside a map
			r, err := reader.nextNonWsRune()
			if err != nil {
				panic("Reached EOF inside JSON map")
			}
			if r == '}' {
				// End of a map
				reader.structure = reader.structure[:len(reader.structure) - 1]
				reader.path = reader.path[:len(reader.path) - 1]
				reader.state = JSONReaderStateValueEnd
				item := walk.WalkItem {
					Value: reader.buildWalkItemValue(walk.MapValue([]walk.MapElement{})),
					Start: false,
					PrevStart: reader.prevStart,
					End: true,
					NextEnd: reader.isNextEnd(),
				}
				reader.prevStart = false
				return item, nil
			}
			// Element in map
			if r != '"' {
				panic("Expected key in map, found something else")
			}
			key := reader.readString()
			reader.path[len(reader.path) - 1] = key
			r, err = reader.nextNonWsRune()
			if err != nil {
				panic("Reached EOF after map key")
			}
			if r != ':' {
				panic("Expected : after map key, found something else")
			}
			return reader.readValue()
		default:
			panic("Invalid JSONReaderStructure")
		}
	case JSONReaderStateValueEnd:
		if len(reader.structure) == 0 {
			_, err := reader.nextNonWsRune()
			if err == nil {
				panic("input continues after JSON value")
			}
			return walk.WalkItem{}, errors.New("eof")
		}
		switch reader.structure[len(reader.structure) - 1] {
		case JSONReaderStructureArray:
			r, err := reader.nextNonWsRune()
			if err != nil {
				panic("Missing end of array")
			}
			if r == ']' {
				reader.path = reader.path[:len(reader.path) - 1]
				reader.structure = reader.structure[:len(reader.structure) - 1]
				reader.state = JSONReaderStateValueEnd
				item := walk.WalkItem {
					Value: reader.buildWalkItemValue(walk.ArrayValue([]walk.ArrayElement{})),
					Start: false,
					PrevStart: reader.prevStart,
					End: true,
					NextEnd: reader.isNextEnd(),
				}
				reader.prevStart = false
				return item, nil
			}
			if r != ',' {
				panic("Missing , after array value")
			}
			reader.state = JSONReaderStateValue
			return reader.Read()
		case JSONReaderStructureMap:
			r, err := reader.nextNonWsRune()
			if err != nil {
				panic("Missing end of map")
			}
			if r == '}' {
				reader.path = reader.path[:len(reader.path) - 1]
				reader.structure = reader.structure[:len(reader.structure) - 1]
				reader.state = JSONReaderStateValueEnd
				item := walk.WalkItem {
					Value: reader.buildWalkItemValue(walk.MapValue([]walk.MapElement{})),
					Start: false,
					PrevStart: reader.prevStart,
					End: true,
					NextEnd: reader.isNextEnd(),
				}
				reader.prevStart = false
				return item, nil
			}
			if r != ',' {
				panic("Missing , after map value")
			}
			reader.state = JSONReaderStateValue
			return reader.Read()
		default:
			panic("Invalid JSONReaderStructure")
		}
	default:
		panic("Invalid JSONReaderState")
	}
}

func (reader *JSONReader) readValue() (walk.WalkItem, error) {
	r, err := reader.nextNonWsRune()
	if err != nil {
		panic("Missing value in JSON")
	}
	switch r {
		case 'n':
			reader.requireString("ull")
			reader.state = JSONReaderStateValueEnd
			value := walk.WalkItem {
				Value: reader.buildWalkItemValue(walk.NullValue{}),
				Start: false,
				PrevStart: reader.prevStart,
				End: false,
				NextEnd: reader.isNextEnd(),
			}
			reader.prevStart = false
			return value, nil
		case 'f':
			reader.requireString("alse")
			reader.state = JSONReaderStateValueEnd
			value := walk.WalkItem {
				Value: reader.buildWalkItemValue(walk.BoolValue(false)),
				Start: false,
				PrevStart: reader.prevStart,
				End: false,
				NextEnd: reader.isNextEnd(),
			}
			reader.prevStart = false
			return value, nil
		case 't':
			reader.requireString("rue")
			reader.state = JSONReaderStateValueEnd
			value := walk.WalkItem {
				Value: reader.buildWalkItemValue(walk.BoolValue(true)),
				Start: false,
				PrevStart: reader.prevStart,
				End: false,
				NextEnd: reader.isNextEnd(),
			}
			reader.prevStart = false
			return value, nil
		case '"':
			v := reader.readString()
			reader.state = JSONReaderStateValueEnd
			value := walk.WalkItem {
				Value: reader.buildWalkItemValue(walk.StringValue(v)),
				Start: false,
				PrevStart: reader.prevStart,
				End: false,
				NextEnd: reader.isNextEnd(),
			}
			reader.prevStart = false
			return value, nil
		case '{':
			reader.state = JSONReaderStateValue
			reader.structure = append(reader.structure, JSONReaderStructureMap)
			value := walk.WalkItem {
				Value: reader.buildWalkItemValue(walk.MapValue([]walk.MapElement{})),
				Start: true,
				PrevStart: reader.prevStart,
				End: false,
				NextEnd: reader.isNextEnd(),
			}
			reader.prevStart = true
			reader.path = append(reader.path, walk.StringValue(""))
			return value, nil
		case '[':
			reader.state = JSONReaderStateValue
			reader.structure = append(reader.structure, JSONReaderStructureArray)
			value := walk.WalkItem {
				Value: reader.buildWalkItemValue(walk.ArrayValue([]walk.ArrayElement{})),
				Start: true,
				PrevStart: reader.prevStart,
				End: false,
				NextEnd: reader.isNextEnd(),
			}
			reader.prevStart = true
			reader.path = append(reader.path, -1)
			return value, nil
	}
	if isNumberRune(r) {
		var builder strings.Builder
		builder.WriteRune(r)
		for {
			r, _, err = reader.reader.ReadRune()
			if err != nil {
				break
			}
			if !isNumberRune(r) {
				reader.reader.UnreadRune()
				break
			}
			builder.WriteRune(r)
		}
		number, parseError := strconv.ParseFloat(builder.String(), 64)
		if parseError != nil {
			panic("Invalid number")
		}
		reader.state = JSONReaderStateValueEnd
		value := walk.WalkItem {
			Value: reader.buildWalkItemValue(walk.NumberValue(number)),
			Start: false,
			PrevStart: reader.prevStart,
			End: false,
			NextEnd: reader.isNextEnd(),
		}
		reader.prevStart = false
		return value, nil
	}
	panic("Invalid JSON value starting with: " + string(r))
}

func (reader *JSONReader) buildWalkItemValue(value walk.Value) walk.Value {
	for i := len(reader.path) - 1; i >= 0; i -= 1 {
		switch segment := reader.path[i].(type) {
		case int:
			value = walk.ArrayValue {
				walk.ArrayElement {
					Index: segment,
					Value: value,
				},
			}
		case string:
			value = walk.MapValue {
				walk.MapElement {
					Key: segment,
					Value: value,
				},
			}
		default:
			panic("Invalid segment type")
		}
	}
	return value
}

func (reader *JSONReader) readString() string {
	var builder strings.Builder
	for {
		r, _, err := reader.reader.ReadRune()
		if err != nil {
			panic("Missing rest of string")
		}
		if r == '"' {
			break
		}
		if r == '\\' {
			r, _, err = reader.reader.ReadRune()
			if err != nil {
				panic("Missing rune after \\")
			}
			builder.WriteRune(r)
			continue
		}
		builder.WriteRune(r)
	}
	return builder.String()
}

func (reader *JSONReader) isNextEnd() bool {
	r, err := reader.peekNonWsRune()
	if err != nil {
		return false
	}
	return r == ']' || r == '}'
}

func (reader *JSONReader) peekNonWsRune() (rune, error) {
	r, err := reader.nextNonWsRune()
	if err != nil {
		return 0, err
	}
	reader.reader.UnreadRune()
	return r, nil
}

func (reader *JSONReader) nextNonWsRune() (rune, error) {
	for {
		r, _, err := reader.reader.ReadRune()
		if err != nil {
			return 0, err
		}
		if !isWhitespace(r) {
			return r, nil
		}
	}
}

func (reader *JSONReader) requireString(criteria string) {
	for _, r := range criteria {
		reader.require(r)
	}
}

func (reader *JSONReader) require(criterion rune) {
	r, _, err := reader.reader.ReadRune()
	if err != nil {
		panic("Error while reading required rune: " + err.Error())
	}
	if r != criterion {
		panic("Required rune not read")
	}
}

func (reader *JSONReader) AssertDone() {
	// TODO
}