package walk

import (
	"bufio"
	"math"
	"strings"
	"strconv"
	"fmt"
)

type ReadAction int
const (
	ActionReadValue ReadAction = iota
	ActionAppendPath
	ActionPopPath
	ActionIncrementPath
	ActionAppendPathNull
)

type JSONInStructure int
const (
	JSONInRoot JSONInStructure = iota
	JSONInMap
	JSONInArray
	JSONInString
	JSONInValueEnd
	JSONInKey
	JSONInKeyEnd
)

type JSONIn struct {
	path []Atom
	reader *bufio.Reader
	structure []JSONInStructure
	readBuffer []Atom
	readIndex int
	readBufferCapacity int
	actionBuffer []ReadAction
	actionIndex int
}

func NewJSONIn(reader *bufio.Reader) JSONIn {
	return JSONIn {
		path: make([]Atom, 0, 256),
		reader: reader,
		structure: []JSONInStructure{JSONInRoot},
		readBuffer: make([]Atom, 0, 256),
		readIndex: 0,
		readBufferCapacity: 256,
		actionBuffer: make([]ReadAction, 0, 256),
		actionIndex: 0,
	}
}

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 == '.'
}

func (in *JSONIn) popPath() {
	if len(in.path) == 0 {
		panic("Tried to pop from empty path")
	}
	finalAtom := in.path[len(in.path) - 1]
	if finalAtom.Typ != AtomStringTerminal {
		in.path = in.path[:len(in.path) - 1]
		return
	}
	i := len(in.path) - 2
	for {
		if i < 0 {
			panic("Missing string begin in path")
		}
		if in.path[i].Typ == AtomStringTerminal {
			break
		}
		i--
	}
	in.path = in.path[:i]
}

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

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

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

// Returns the first full value of a list of atoms and also a boolean to indicate if there isn't a value at the beginning
func firstValue(atoms []Atom) ([]Atom, bool) {
	if len(atoms) == 0 {
		return nil, true
	}
	if atoms[0].Typ != AtomStringTerminal {
		return atoms[0:1], false
	}
	i := 1
	for {
		if i == len(atoms) {
			return nil, true
		}
		if atoms[i].Typ == AtomStringTerminal {
			return atoms[0:i+1], false
		}
		i++
	}
}

func (in *JSONIn) Read() (WalkItem, error) {
	actionLoop: for {
		if in.actionIndex == len(in.actionBuffer) {
			in.actionIndex = 0
			in.readIndex = 0
			in.actionBuffer = in.actionBuffer[:0]
			in.readBuffer = in.readBuffer[:0]
			structure, err := in.fillReadBuffer(in.structure)
			in.structure = structure
			if len(in.actionBuffer) == 0 {
				return WalkItem{}, err
			}
		}
		action := in.actionBuffer[in.actionIndex]
		switch action {
			case ActionReadValue:
				value, incomplete := firstValue(in.readBuffer[in.readIndex:])
				if incomplete {
					if in.readIndex == 0 {
						newReadBuffer := make([]Atom, len(in.readBuffer), in.readBufferCapacity * 2)
						in.readBufferCapacity *= 2
						copy(newReadBuffer, in.readBuffer)
						in.readBuffer = newReadBuffer
						structure, _ := in.fillReadBuffer(in.structure)
						in.structure = structure
						continue actionLoop
					}
					copy(in.readBuffer, in.readBuffer[in.readIndex:])
					in.readBuffer = in.readBuffer[:len(in.readBuffer) - in.readIndex]
					in.readIndex = 0
					copy(in.actionBuffer, in.actionBuffer[in.actionIndex:])
					in.actionBuffer = in.actionBuffer[:len(in.actionBuffer) - in.actionIndex]
					in.actionIndex = 0
					structure, _ := in.fillReadBuffer(in.structure)
					in.structure = structure
					continue actionLoop
				}
				in.readIndex += len(value)
				in.actionIndex++
				return WalkItem {
					Value: value,
					Path: in.path,
				}, nil
			case ActionAppendPath:
				value, incomplete := firstValue(in.readBuffer[in.readIndex:])
				if incomplete {
					if in.readIndex == 0 {
						newReadBuffer := make([]Atom, len(in.readBuffer), in.readBufferCapacity * 2)
						in.readBufferCapacity *= 2
						copy(newReadBuffer, in.readBuffer)
						in.readBuffer = newReadBuffer
						structure, _ := in.fillReadBuffer(in.structure)
						in.structure = structure
						continue actionLoop
					}
					copy(in.readBuffer, in.readBuffer[in.readIndex:])
					in.readBuffer = in.readBuffer[:len(in.readBuffer) - in.readIndex]
					in.readIndex = 0
					copy(in.actionBuffer, in.actionBuffer[in.actionIndex:])
					in.actionBuffer = in.actionBuffer[:len(in.actionBuffer) - in.actionIndex]
					in.actionIndex = 0
					structure, _ := in.fillReadBuffer(in.structure)
					in.structure = structure
					continue actionLoop
				}
				in.readIndex += len(value)
				in.path = append(in.path, value...)
			case ActionAppendPathNull:
				in.path = append(in.path, NewAtomNull())
			case ActionPopPath:
				in.popPath()
			case ActionIncrementPath:
				prevIndex := in.path[len(in.path) - 1]
				if prevIndex.Typ == AtomNull {
					prevIndex.Typ = AtomNumber
					prevIndex.data = math.Float64bits(0)
				} else if prevIndex.Typ == AtomNumber {
					prevIndex.data = math.Float64bits(math.Float64frombits(prevIndex.data) + 1)
				} else {
					panic("Invalid index in array input. Type: " + fmt.Sprintf("%v", prevIndex.Typ))
				}
				in.path[len(in.path) - 1] = prevIndex
			default:
				panic("Invalid ReadAction")
		}
		in.actionIndex++
	}
}

func (in *JSONIn) AssertDone() {
	if len(in.structure) != 1 || in.structure[0] != JSONInRoot {
		panic("Input ended on incomplete JSON root")
	}
}

func (in *JSONIn) pushReadBuffer(atom Atom) bool {
	in.readBuffer = append(in.readBuffer, atom)
	return len(in.readBuffer) == in.readBufferCapacity
}

func (in *JSONIn) pushActionBuffer(action ReadAction) {
	in.actionBuffer = append(in.actionBuffer, action)
}

func (in *JSONIn) fillReadBuffer(structure []JSONInStructure) ([]JSONInStructure, error) {
	valueStart: {
		state := structure[len(structure) - 1]
		switch state {
			case JSONInString:
				structure = structure[:len(structure) - 1]
				goto string
			case JSONInValueEnd:
				structure = structure[:len(structure) - 1]
				goto valueEnd
			case JSONInKey:
				structure = structure[:len(structure) - 1]
				goto key
			case JSONInKeyEnd:
				structure = structure[:len(structure) - 1]
				goto keyEnd
			case JSONInMap:
				goto mapValue
			case JSONInArray:
				goto arrayValue
			case JSONInRoot:
				goto value
		}
	}
	value: {
		r, err := in.nextNonWsRune()
		if err != nil {
			return structure, err
		}
		switch r {
			case 'n':
				in.requireString("ull")
				in.pushActionBuffer(ActionReadValue)
				if in.pushReadBuffer(NewAtomNull()) {
					return append(structure, JSONInValueEnd), nil
				}
				goto valueEnd
			case 'f':
				in.requireString("alse")
				in.pushActionBuffer(ActionReadValue)
				if in.pushReadBuffer(NewAtomBool(false)) {
					return append(structure, JSONInValueEnd), nil
				}
				goto valueEnd
			case 't':
				in.requireString("rue")
				in.pushActionBuffer(ActionReadValue)
				if in.pushReadBuffer(NewAtomBool(true)) {
					return append(structure, JSONInValueEnd), nil
				}
				goto valueEnd
			case '"':
				in.pushActionBuffer(ActionReadValue)
				if in.pushReadBuffer(NewAtomStringTerminal()) {
					return append(structure, JSONInString), nil
				}
				goto string
			case '{':
				structure = append(structure, JSONInMap)
				in.pushActionBuffer(ActionReadValue)
				in.pushActionBuffer(ActionAppendPathNull)
				if in.pushReadBuffer(NewAtomTerminal(MapBegin)) {
					return structure, nil
				}
				goto mapValue
			case '[':
				structure = append(structure, JSONInArray)
				in.pushActionBuffer(ActionReadValue)
				in.pushActionBuffer(ActionAppendPathNull)
				if in.pushReadBuffer(NewAtomTerminal(ArrayBegin)) {
					return structure, nil
				}
				goto arrayValue
		}
		if isNumberRune(r) {
			var builder strings.Builder
			builder.WriteRune(r)
			for {
				r, _, err = in.reader.ReadRune()
				if err != nil {
					break
				}
				if !isNumberRune(r) {
					in.reader.UnreadRune()
					break
				}
				builder.WriteRune(r)
			}
			number, parseError := strconv.ParseFloat(builder.String(), 64)
			if parseError != nil {
				panic("Invalid number")
			}
			in.pushActionBuffer(ActionReadValue)
			if in.pushReadBuffer(NewAtomNumber(number)) {
				return append(structure, JSONInValueEnd), nil
			}
			goto valueEnd
		}
		panic("Invalid JSON value starting with: " + string(r))
	}
	string: {
		r, _, err := in.reader.ReadRune()
		if err != nil {
			panic("Missing closing terminal in string input: " + err.Error())
		}
		if r == '"' {
			if in.pushReadBuffer(NewAtomStringTerminal()) {
				return append(structure, JSONInValueEnd), nil
			}
			goto valueEnd
		}
		if r == '\\' {
			r, _, err = in.reader.ReadRune()
			if err != nil {
				panic("Missing rune after \\")
			}
			if in.pushReadBuffer(NewAtomStringRune(r)) {
				return append(structure, JSONInString), nil
			}
			goto string
		}
		if in.pushReadBuffer(NewAtomStringRune(r)) {
			return append(structure, JSONInString), nil
		}
		goto string
	}
	key: {
		r, _, err := in.reader.ReadRune()
		if err != nil {
			panic("Missing closing terminal in string input: " + err.Error())
		}
		if r == '"' {
			if in.pushReadBuffer(NewAtomStringTerminal()) {
				return append(structure, JSONInKeyEnd), nil
			}
			goto keyEnd
		}
		if r == '\\' {
			r, _, err = in.reader.ReadRune()
			if err != nil {
				panic("Missing rune after \\")
			}
			if in.pushReadBuffer(NewAtomStringRune(r)) {
				return append(structure, JSONInKey), nil
			}
			goto key
		}
		if in.pushReadBuffer(NewAtomStringRune(r)) {
			return append(structure, JSONInKey), nil
		}
		goto key
	}
	keyEnd: {
		r, err := in.nextNonWsRune()
		if err != nil {
			panic("Expected : got: " + err.Error())
		}
		if r != ':' {
			panic("Expected : after key")
		}
		goto value
	}
	valueEnd: {
		r, err := in.nextNonWsRune()
		if err != nil {
			return structure, err
		}
		underState := structure[len(structure) - 1]
		if underState == JSONInRoot {
			panic("More input after root JSON object ends")
		} else if underState == JSONInMap && r == '}' {
			structure = structure[:len(structure) - 1]
			in.pushActionBuffer(ActionPopPath)
			in.pushActionBuffer(ActionReadValue)
			if in.pushReadBuffer(NewAtomTerminal(MapEnd)) {
				return append(structure, JSONInValueEnd), nil
			}
			goto valueEnd
		} else if underState == JSONInArray && r == ']' {
			structure = structure[:len(structure) - 1]
			in.pushActionBuffer(ActionPopPath)
			in.pushActionBuffer(ActionReadValue)
			if in.pushReadBuffer(NewAtomTerminal(ArrayEnd)) {
				return append(structure, JSONInValueEnd), nil
			}
			goto valueEnd
		}
		if r != ',' {
			panic("Expected , after JSON value, found: \"" + string(r) + "\"")
		}
		goto valueStart
	}
	mapValue: {
		in.pushActionBuffer(ActionPopPath)
		r, err := in.nextNonWsRune()
		if err != nil {
			panic("Missing value inside object")
		}
		if r == '}' {
			structure = structure[:len(structure) - 1]
			in.pushActionBuffer(ActionReadValue)
			if in.pushReadBuffer(NewAtomTerminal(MapEnd)) {
				return append(structure, JSONInValueEnd), nil
			}
			goto valueEnd
		}
		if r != '"' {
			panic("Expected key found something else")
		}
		in.pushActionBuffer(ActionAppendPath)
		if in.pushReadBuffer(NewAtomStringTerminal()) {
			return append(structure, JSONInKey), nil
		}
		goto key
	}
	arrayValue: {
		r, err := in.nextNonWsRune()
		if err != nil {
			panic("Missing value inside array")
		}
		if r == ']' {
			structure = structure[:len(structure) - 1]
			in.pushActionBuffer(ActionPopPath)
			in.pushActionBuffer(ActionReadValue)
			if in.pushReadBuffer(NewAtomTerminal(ArrayEnd)) {
				return append(structure, JSONInValueEnd), nil
			}
			goto valueEnd
		}
		in.reader.UnreadRune()
		in.pushActionBuffer(ActionIncrementPath)
		goto value
	}
}