package toml

import (
	"fmt"
	"strings"
	"unicode/utf8"
)

type itemType int

const (
	itemError itemType = iota
	itemNIL            // used in the parser to indicate no type
	itemEOF
	itemText
	itemString
	itemBool
	itemInteger
	itemFloat
	itemDatetime
	itemArray // the start of an array
	itemArrayEnd
	itemTableStart
	itemTableEnd
	itemArrayTableStart
	itemArrayTableEnd
	itemKeyStart
	itemCommentStart
)

const (
	eof             = 0
	tableStart      = '['
	tableEnd        = ']'
	arrayTableStart = '['
	arrayTableEnd   = ']'
	tableSep        = '.'
	keySep          = '='
	arrayStart      = '['
	arrayEnd        = ']'
	arrayValTerm    = ','
	commentStart    = '#'
	stringStart     = '"'
	stringEnd       = '"'
)

type stateFn func(lx *lexer) stateFn

type lexer struct {
	input string
	start int
	pos   int
	width int
	line  int
	state stateFn
	items chan item

	// A stack of state functions used to maintain context.
	// The idea is to reuse parts of the state machine in various places.
	// For example, values can appear at the top level or within arbitrarily
	// nested arrays. The last state on the stack is used after a value has
	// been lexed. Similarly for comments.
	stack []stateFn
}

type item struct {
	typ  itemType
	val  string
	line int
}

func (lx *lexer) nextItem() item {
	for {
		select {
		case item := <-lx.items:
			return item
		default:
			lx.state = lx.state(lx)
		}
	}
}

func lex(input string) *lexer {
	lx := &lexer{
		input: input + "\n",
		state: lexTop,
		line:  1,
		items: make(chan item, 10),
		stack: make([]stateFn, 0, 10),
	}
	return lx
}

func (lx *lexer) push(state stateFn) {
	lx.stack = append(lx.stack, state)
}

func (lx *lexer) pop() stateFn {
	if len(lx.stack) == 0 {
		return lx.errorf("BUG in lexer: no states to pop.")
	}
	last := lx.stack[len(lx.stack)-1]
	lx.stack = lx.stack[0 : len(lx.stack)-1]
	return last
}

func (lx *lexer) current() string {
	return lx.input[lx.start:lx.pos]
}

func (lx *lexer) emit(typ itemType) {
	lx.items <- item{typ, lx.current(), lx.line}
	lx.start = lx.pos
}

func (lx *lexer) emitTrim(typ itemType) {
	lx.items <- item{typ, strings.TrimSpace(lx.current()), lx.line}
	lx.start = lx.pos
}

func (lx *lexer) next() (r rune) {
	if lx.pos >= len(lx.input) {
		lx.width = 0
		return eof
	}

	if lx.input[lx.pos] == '\n' {
		lx.line++
	}
	r, lx.width = utf8.DecodeRuneInString(lx.input[lx.pos:])
	lx.pos += lx.width
	return r
}

// ignore skips over the pending input before this point.
func (lx *lexer) ignore() {
	lx.start = lx.pos
}

// backup steps back one rune. Can be called only once per call of next.
func (lx *lexer) backup() {
	lx.pos -= lx.width
	if lx.pos < len(lx.input) && lx.input[lx.pos] == '\n' {
		lx.line--
	}
}

// accept consumes the next rune if it's equal to `valid`.
func (lx *lexer) accept(valid rune) bool {
	if lx.next() == valid {
		return true
	}
	lx.backup()
	return false
}

// peek returns but does not consume the next rune in the input.
func (lx *lexer) peek() rune {
	r := lx.next()
	lx.backup()
	return r
}

// errorf stops all lexing by emitting an error and returning `nil`.
// Note that any value that is a character is escaped if it's a special
// character (new lines, tabs, etc.).
func (lx *lexer) errorf(format string, values ...interface{}) stateFn {
	lx.items <- item{
		itemError,
		fmt.Sprintf(format, values...),
		lx.line,
	}
	return nil
}

// lexTop consumes elements at the top level of TOML data.
func lexTop(lx *lexer) stateFn {
	r := lx.next()
	if isWhitespace(r) || isNL(r) {
		return lexSkip(lx, lexTop)
	}

	switch r {
	case commentStart:
		lx.push(lexTop)
		return lexCommentStart
	case tableStart:
		return lexTableStart
	case eof:
		if lx.pos > lx.start {
			return lx.errorf("Unexpected EOF.")
		}
		lx.emit(itemEOF)
		return nil
	}

	// At this point, the only valid item can be a key, so we back up
	// and let the key lexer do the rest.
	lx.backup()
	lx.push(lexTopEnd)
	return lexKeyStart
}

// lexTopEnd is entered whenever a top-level item has been consumed. (A value
// or a table.) It must see only whitespace, and will turn back to lexTop
// upon a new line. If it sees EOF, it will quit the lexer successfully.
func lexTopEnd(lx *lexer) stateFn {
	r := lx.next()
	switch {
	case r == commentStart:
		// a comment will read to a new line for us.
		lx.push(lexTop)
		return lexCommentStart
	case isWhitespace(r):
		return lexTopEnd
	case isNL(r):
		lx.ignore()
		return lexTop
	case r == eof:
		lx.ignore()
		return lexTop
	}
	return lx.errorf("Expected a top-level item to end with a new line, "+
		"comment or EOF, but got %q instead.", r)
}

// lexTable lexes the beginning of a table. Namely, it makes sure that
// it starts with a character other than '.' and ']'.
// It assumes that '[' has already been consumed.
// It also handles the case that this is an item in an array of tables.
// e.g., '[[name]]'.
func lexTableStart(lx *lexer) stateFn {
	if lx.peek() == arrayTableStart {
		lx.next()
		lx.emit(itemArrayTableStart)
		lx.push(lexArrayTableEnd)
	} else {
		lx.emit(itemTableStart)
		lx.push(lexTableEnd)
	}
	return lexTableNameStart
}

func lexTableEnd(lx *lexer) stateFn {
	lx.emit(itemTableEnd)
	return lexTopEnd
}

func lexArrayTableEnd(lx *lexer) stateFn {
	if r := lx.next(); r != arrayTableEnd {
		return lx.errorf("Expected end of table array name delimiter %q, "+
			"but got %q instead.", arrayTableEnd, r)
	}
	lx.emit(itemArrayTableEnd)
	return lexTopEnd
}

func lexTableNameStart(lx *lexer) stateFn {
	switch lx.next() {
	case tableEnd, eof:
		return lx.errorf("Unexpected end of table. (Tables cannot " +
			"be empty.)")
	case tableSep:
		return lx.errorf("Unexpected table separator. (Tables cannot " +
			"be empty.)")
	}
	return lexTableName
}

// lexTableName lexes the name of a table. It assumes that at least one
// valid character for the table has already been read.
func lexTableName(lx *lexer) stateFn {
	switch lx.peek() {
	case eof:
		return lx.errorf("Unexpected end of table name %q.", lx.current())
	case tableStart:
		return lx.errorf("Table names cannot contain %q or %q.",
			tableStart, tableEnd)
	case tableEnd:
		lx.emit(itemText)
		lx.next()
		return lx.pop()
	case tableSep:
		lx.emit(itemText)
		lx.next()
		lx.ignore()
		return lexTableNameStart
	}
	lx.next()
	return lexTableName
}

// lexKeyStart consumes a key name up until the first non-whitespace character.
// lexKeyStart will ignore whitespace.
func lexKeyStart(lx *lexer) stateFn {
	r := lx.peek()
	switch {
	case r == keySep:
		return lx.errorf("Unexpected key separator %q.", keySep)
	case isWhitespace(r) || isNL(r):
		lx.next()
		return lexSkip(lx, lexKeyStart)
	}

	lx.ignore()
	lx.emit(itemKeyStart)
	lx.next()
	return lexKey
}

// lexKey consumes the text of a key. Assumes that the first character (which
// is not whitespace) has already been consumed.
func lexKey(lx *lexer) stateFn {
	r := lx.peek()

	// Keys cannot contain a '#' character.
	if r == commentStart {
		return lx.errorf("Key cannot contain a '#' character.")
	}

	// XXX: Possible divergence from spec?
	// "Keys start with the first non-whitespace character and end with the
	// last non-whitespace character before the equals sign."
	// Note here that whitespace is either a tab or a space.
	// But we'll call it quits if we see a new line too.
	if isNL(r) {
		lx.emitTrim(itemText)
		return lexKeyEnd
	}

	// Let's also call it quits if we see an equals sign.
	if r == keySep {
		lx.emitTrim(itemText)
		return lexKeyEnd
	}

	lx.next()
	return lexKey
}

// lexKeyEnd consumes the end of a key (up to the key separator).
// Assumes that any whitespace after a key has been consumed.
func lexKeyEnd(lx *lexer) stateFn {
	r := lx.next()
	if r == keySep {
		return lexSkip(lx, lexValue)
	}
	return lx.errorf("Expected key separator %q, but got %q instead.",
		keySep, r)
}

// lexValue starts the consumption of a value anywhere a value is expected.
// lexValue will ignore whitespace.
// After a value is lexed, the last state on the next is popped and returned.
func lexValue(lx *lexer) stateFn {
	// We allow whitespace to precede a value, but NOT new lines.
	// In array syntax, the array states are responsible for ignoring new lines.
	r := lx.next()
	if isWhitespace(r) {
		return lexSkip(lx, lexValue)
	}

	switch {
	case r == arrayStart:
		lx.ignore()
		lx.emit(itemArray)
		return lexArrayValue
	case r == stringStart:
		lx.ignore() // ignore the '"'
		return lexString
	case r == 't':
		return lexTrue
	case r == 'f':
		return lexFalse
	case r == '-':
		return lexNumberStart
	case isDigit(r):
		lx.backup() // avoid an extra state and use the same as above
		return lexNumberOrDateStart
	case r == '.': // special error case, be kind to users
		return lx.errorf("Floats must start with a digit, not '.'.")
	}
	return lx.errorf("Expected value but found %q instead.", r)
}

// lexArrayValue consumes one value in an array. It assumes that '[' or ','
// have already been consumed. All whitespace and new lines are ignored.
func lexArrayValue(lx *lexer) stateFn {
	r := lx.next()
	switch {
	case isWhitespace(r) || isNL(r):
		return lexSkip(lx, lexArrayValue)
	case r == commentStart:
		lx.push(lexArrayValue)
		return lexCommentStart
	case r == arrayValTerm:
		return lx.errorf("Unexpected array value terminator %q.",
			arrayValTerm)
	case r == arrayEnd:
		return lexArrayEnd
	}

	lx.backup()
	lx.push(lexArrayValueEnd)
	return lexValue
}

// lexArrayValueEnd consumes the cruft between values of an array. Namely,
// it ignores whitespace and expects either a ',' or a ']'.
func lexArrayValueEnd(lx *lexer) stateFn {
	r := lx.next()
	switch {
	case isWhitespace(r) || isNL(r):
		return lexSkip(lx, lexArrayValueEnd)
	case r == commentStart:
		lx.push(lexArrayValueEnd)
		return lexCommentStart
	case r == arrayValTerm:
		lx.ignore()
		return lexArrayValue // move on to the next value
	case r == arrayEnd:
		return lexArrayEnd
	}
	return lx.errorf("Expected an array value terminator %q or an array "+
		"terminator %q, but got %q instead.", arrayValTerm, arrayEnd, r)
}

// lexArrayEnd finishes the lexing of an array. It assumes that a ']' has
// just been consumed.
func lexArrayEnd(lx *lexer) stateFn {
	lx.ignore()
	lx.emit(itemArrayEnd)
	return lx.pop()
}

// lexString consumes the inner contents of a string. It assumes that the
// beginning '"' has already been consumed and ignored.
func lexString(lx *lexer) stateFn {
	r := lx.next()
	switch {
	case isNL(r):
		return lx.errorf("Strings cannot contain new lines.")
	case r == '\\':
		return lexStringEscape
	case r == stringEnd:
		lx.backup()
		lx.emit(itemString)
		lx.next()
		lx.ignore()
		return lx.pop()
	}
	return lexString
}

// lexStringEscape consumes an escaped character. It assumes that the preceding
// '\\' has already been consumed.
func lexStringEscape(lx *lexer) stateFn {
	r := lx.next()
	switch r {
	case 'b':
		fallthrough
	case 't':
		fallthrough
	case 'n':
		fallthrough
	case 'f':
		fallthrough
	case 'r':
		fallthrough
	case '"':
		fallthrough
	case '/':
		fallthrough
	case '\\':
		return lexString
	case 'u':
		return lexStringUnicode
	}
	return lx.errorf("Invalid escape character %q. Only the following "+
		"escape characters are allowed: "+
		"\\b, \\t, \\n, \\f, \\r, \\\", \\/, \\\\, and \\uXXXX.", r)
}

// lexStringBinary consumes two hexadecimal digits following '\x'. It assumes
// that the '\x' has already been consumed.
func lexStringUnicode(lx *lexer) stateFn {
	var r rune

	for i := 0; i < 4; i++ {
		r = lx.next()
		if !isHexadecimal(r) {
			return lx.errorf("Expected four hexadecimal digits after '\\x', "+
				"but got '%s' instead.", lx.current())
		}
	}
	return lexString
}

// lexNumberOrDateStart consumes either a (positive) integer, float or datetime.
// It assumes that NO negative sign has been consumed.
func lexNumberOrDateStart(lx *lexer) stateFn {
	r := lx.next()
	if !isDigit(r) {
		if r == '.' {
			return lx.errorf("Floats must start with a digit, not '.'.")
		} else {
			return lx.errorf("Expected a digit but got %q.", r)
		}
	}
	return lexNumberOrDate
}

// lexNumberOrDate consumes either a (positive) integer, float or datetime.
func lexNumberOrDate(lx *lexer) stateFn {
	r := lx.next()
	switch {
	case r == '-':
		if lx.pos-lx.start != 5 {
			return lx.errorf("All ISO8601 dates must be in full Zulu form.")
		}
		return lexDateAfterYear
	case isDigit(r):
		return lexNumberOrDate
	case r == '.':
		return lexFloatStart
	}

	lx.backup()
	lx.emit(itemInteger)
	return lx.pop()
}

// lexDateAfterYear consumes a full Zulu Datetime in ISO8601 format.
// It assumes that "YYYY-" has already been consumed.
func lexDateAfterYear(lx *lexer) stateFn {
	formats := []rune{
		// digits are '0'.
		// everything else is direct equality.
		'0', '0', '-', '0', '0',
		'T',
		'0', '0', ':', '0', '0', ':', '0', '0',
		'Z',
	}
	for _, f := range formats {
		r := lx.next()
		if f == '0' {
			if !isDigit(r) {
				return lx.errorf("Expected digit in ISO8601 datetime, "+
					"but found %q instead.", r)
			}
		} else if f != r {
			return lx.errorf("Expected %q in ISO8601 datetime, "+
				"but found %q instead.", f, r)
		}
	}
	lx.emit(itemDatetime)
	return lx.pop()
}

// lexNumberStart consumes either an integer or a float. It assumes that a
// negative sign has already been read, but that *no* digits have been consumed.
// lexNumberStart will move to the appropriate integer or float states.
func lexNumberStart(lx *lexer) stateFn {
	// we MUST see a digit. Even floats have to start with a digit.
	r := lx.next()
	if !isDigit(r) {
		if r == '.' {
			return lx.errorf("Floats must start with a digit, not '.'.")
		} else {
			return lx.errorf("Expected a digit but got %q.", r)
		}
	}
	return lexNumber
}

// lexNumber consumes an integer or a float after seeing the first digit.
func lexNumber(lx *lexer) stateFn {
	r := lx.next()
	switch {
	case isDigit(r):
		return lexNumber
	case r == '.':
		return lexFloatStart
	}

	lx.backup()
	lx.emit(itemInteger)
	return lx.pop()
}

// lexFloatStart starts the consumption of digits of a float after a '.'.
// Namely, at least one digit is required.
func lexFloatStart(lx *lexer) stateFn {
	r := lx.next()
	if !isDigit(r) {
		return lx.errorf("Floats must have a digit after the '.', but got "+
			"%q instead.", r)
	}
	return lexFloat
}

// lexFloat consumes the digits of a float after a '.'.
// Assumes that one digit has been consumed after a '.' already.
func lexFloat(lx *lexer) stateFn {
	r := lx.next()
	if isDigit(r) {
		return lexFloat
	}

	lx.backup()
	lx.emit(itemFloat)
	return lx.pop()
}

// lexConst consumes the s[1:] in s. It assumes that s[0] has already been
// consumed.
func lexConst(lx *lexer, s string) stateFn {
	for i := range s[1:] {
		if r := lx.next(); r != rune(s[i+1]) {
			return lx.errorf("Expected %q, but found %q instead.", s[:i+1],
				s[:i]+string(r))
		}
	}
	return nil
}

// lexTrue consumes the "rue" in "true". It assumes that 't' has already
// been consumed.
func lexTrue(lx *lexer) stateFn {
	if fn := lexConst(lx, "true"); fn != nil {
		return fn
	}
	lx.emit(itemBool)
	return lx.pop()
}

// lexFalse consumes the "alse" in "false". It assumes that 'f' has already
// been consumed.
func lexFalse(lx *lexer) stateFn {
	if fn := lexConst(lx, "false"); fn != nil {
		return fn
	}
	lx.emit(itemBool)
	return lx.pop()
}

// lexCommentStart begins the lexing of a comment. It will emit
// itemCommentStart and consume no characters, passing control to lexComment.
func lexCommentStart(lx *lexer) stateFn {
	lx.ignore()
	lx.emit(itemCommentStart)
	return lexComment
}

// lexComment lexes an entire comment. It assumes that '#' has been consumed.
// It will consume *up to* the first new line character, and pass control
// back to the last state on the stack.
func lexComment(lx *lexer) stateFn {
	r := lx.peek()
	if isNL(r) || r == eof {
		lx.emit(itemText)
		return lx.pop()
	}
	lx.next()
	return lexComment
}

// lexSkip ignores all slurped input and moves on to the next state.
func lexSkip(lx *lexer, nextState stateFn) stateFn {
	return func(lx *lexer) stateFn {
		lx.ignore()
		return nextState
	}
}

// isWhitespace returns true if `r` is a whitespace character according
// to the spec.
func isWhitespace(r rune) bool {
	return r == '\t' || r == ' '
}

func isNL(r rune) bool {
	return r == '\n' || r == '\r'
}

func isDigit(r rune) bool {
	return r >= '0' && r <= '9'
}

func isHexadecimal(r rune) bool {
	return (r >= '0' && r <= '9') ||
		(r >= 'a' && r <= 'f') ||
		(r >= 'A' && r <= 'F')
}

func (itype itemType) String() string {
	switch itype {
	case itemError:
		return "Error"
	case itemNIL:
		return "NIL"
	case itemEOF:
		return "EOF"
	case itemText:
		return "Text"
	case itemString:
		return "String"
	case itemBool:
		return "Bool"
	case itemInteger:
		return "Integer"
	case itemFloat:
		return "Float"
	case itemDatetime:
		return "DateTime"
	case itemTableStart:
		return "TableStart"
	case itemTableEnd:
		return "TableEnd"
	case itemKeyStart:
		return "KeyStart"
	case itemArray:
		return "Array"
	case itemArrayEnd:
		return "ArrayEnd"
	case itemCommentStart:
		return "CommentStart"
	}
	panic(fmt.Sprintf("BUG: Unknown type '%d'.", int(itype)))
}

func (item item) String() string {
	return fmt.Sprintf("(%s, %s)", item.typ.String(), item.val)
}