package main

import (
	"io"
	"unicode"
	"unicode/utf8"

	"github.com/pkg/errors"
	"github.com/prodhe/poe/gapbuffer"
)

const (
	DirForward uint8 = iota
	DirBackward
	DirAbsolute
)

// Text is a buffer for editing. It uses an underlying gap buffer for storage and manages all things text related, like insert, delete, selection, searching and undo/redo.
//
// Although the underlying buffer is a pure byte slice, Text only works with runes and UTF-8.
type Text struct {
	buf      *gapbuffer.Buffer
	q0, q1   int     // dot/cursor
	off      int     // offset for reading runes in buffer
	lastRune rune    // save the last read rune
	runeBuf  []byte  // temp buf to read rune at a time from gap buffer
	history  History // undo/redo stack
}

func (t *Text) Write(p []byte) (int, error) {
	// handle replace
	if len(t.ReadDot()) > 0 {
		t.Delete()
	}

	// do the actual insertion
	c := Change{t.q0, HInsert, p}
	n, err := t.commit(c)
	if err != nil {
		return n, err
	}
	t.history.Do(c)
	t.SeekDot(n, 1) // move dot
	return n, nil
}

func (t *Text) Delete() (int, error) {
	if len(t.ReadDot()) == 0 {
		t.q0--
		c, _ := t.buf.ByteAt(t.q0)
		for !utf8.RuneStart(c) {
			t.q0--
			c, _ = t.buf.ByteAt(t.q0)
		}
		if t.q0 < 0 {
			return 0, nil
		}
	}
	c := Change{t.q0, HDelete, []byte(t.ReadDot())}
	n, err := t.commit(c)
	if err != nil {
		return n, err
	}
	t.history.Do(c)
	return n, nil
}

// Len returns the number of bytes in buffer.
func (t *Text) Len() int {
	return t.buf.Len()
}

// ReadRune reads a rune from buffer and advances the internal offset. This could be called in sequence to get all runes from buffer. This populates LastRune().
func (t *Text) ReadRune() (r rune, size int, err error) {
	r, size, err = t.ReadRuneAt(t.off)
	if err != nil {
		return 0, 0, err
	}
	t.off += size
	t.lastRune = r
	return
}

func (t *Text) UnreadRune() (r rune, size int, err error) {
	t.off--
	r, size, err = t.ReadRuneAt(t.off)
	t.off++
	if err != nil {
		return 0, 0, err
	}
	t.off -= size
	return
}

// ReadRuneAt returns the rune and its size at offset. If the given offset (in byte count) is not a valid rune, it will try to back up until it finds a valid starting point for a rune and return that one.
func (t *Text) ReadRuneAt(offset int) (r rune, size int, err error) {
	var c byte
	c, err = t.buf.ByteAt(offset)
	if err != nil {
		return 0, 0, err
	}
	for !utf8.RuneStart(c) {
		offset--
		c, err = t.buf.ByteAt(offset)
		if err != nil {
			return 0, 0, err
		}
	}

	if c < utf8.RuneSelf {
		return rune(c), 1, nil
	}

	if cap(t.runeBuf) < 4 {
		t.runeBuf = make([]byte, 4) // max length of a rune
	}
	_, err = t.buf.ReadAt(t.runeBuf, offset)
	if err != nil {
		return 0, 0, err
	}
	r, n := utf8.DecodeRune(t.runeBuf)

	return r, n, nil
}

// LastRune returns the last rune read by ReadRune().
func (t *Text) LastRune() rune {
	return t.lastRune
}

// ReadDot returns content of current dot.
func (t *Text) ReadDot() string {
	if t.q0 == t.q1 {
		return ""
	}
	buf := make([]byte, t.q1-t.q0)
	_, err := t.buf.ReadAt(buf, t.q0)
	if err != nil {
		printMsg("dot: %s", err)
		return ""
	}
	return string(buf)
}

// Dot returns current offsets for dot.
func (t *Text) Dot() (int, int) {
	return t.q0, t.q1
}

// Seek implements io.Seeker and sets the internal offset for next ReadRune(). If the offset is not a valid rune start, it will backup until it finds one.
func (t *Text) Seek(offset, whence int) (int, error) {
	t.off = offset

	switch whence {
	case io.SeekStart:
		t.off = offset
	case io.SeekCurrent:
		t.off += offset
	case io.SeekEnd:
		t.off = t.Len() + offset
	default:
		return 0, errors.New("invalid whence")
	}

	c, _ := t.buf.ByteAt(t.off)
	for !utf8.RuneStart(c) {
		t.off--
		c, _ = t.buf.ByteAt(t.off)
	}

	return t.off, nil
}

// SeekDot sets the dot to a single offset in the text buffer.
func (t *Text) SeekDot(offset, whence int) (int, error) {
	switch whence {
	case io.SeekStart:
		q0, _, err := t.SetDot(offset, offset)
		return q0, err
	case io.SeekCurrent:
		q0, _, err := t.SetDot(t.q0+offset, t.q0+offset)
		return q0, err
	case io.SeekEnd:
		q0, _, err := t.SetDot(t.Len()+offset, t.Len()+offset)
		return q0, err
	default:
		return 0, errors.New("invalid whence")
	}
}

// SetDot sets both ends of the dot into an absolute position. It will check the given offsets and adjust them accordingly, so they are not out of bounds or on an invalid rune start. It returns the final offsets. Error is always nil.
func (t *Text) SetDot(q0, q1 int) (int, int, error) {
	t.q0, t.q1 = q0, q1

	// check out of bounds
	if t.q0 < 0 {
		t.q0 = 0
	}
	if t.q1 < 0 {
		t.q1 = 0
	}
	if t.q0 >= t.buf.Len() {
		t.q0 = t.buf.Len()
	}
	if t.q1 >= t.buf.Len() {
		t.q1 = t.buf.Len()
	}

	// q0 must never be greater than q1
	if t.q0 > t.q1 {
		t.q0 = t.q1
	}
	if t.q1 < t.q0 {
		t.q1 = t.q0
	}

	// set only to valid rune start
	var c byte
	c, _ = t.buf.ByteAt(t.q0)
	for !utf8.RuneStart(c) {
		t.q0--
		c, _ = t.buf.ByteAt(t.q0)
	}
	c, _ = t.buf.ByteAt(t.q1)
	for !utf8.RuneStart(c) {
		t.q1--
		c, _ = t.buf.ByteAt(t.q1)
	}

	return t.q0, t.q1, nil
}

// ExpandDot expands the current selection in positive or negative offset. A positive offset expands forwards and a negative expands backwards. Q is 0 or 1, either the left or the right end of the dot.
func (t *Text) ExpandDot(q, offset int) {
	if q < 0 || q > 1 {
		return
	}

	if q == 0 {
		t.SetDot(t.q0+offset, t.q1)
	} else {
		t.SetDot(t.q0, t.q1+offset)
	}
}

// Select expands current dot into something useful. Most likely a word, but if given pos is adjacent to a quote or bracket character, it tries to select into the matching pair.
func (t *Text) Select(pos int) {
	start, end := pos, pos
	c, _ := t.buf.ByteAt(start)

	// word
	for unicode.IsLetter(rune(c)) || unicode.IsDigit(rune(c)) {
		start--
		c, _ = t.buf.ByteAt(start)
	}
	if start < t.q0 {
		start++
	}
	c, _ = t.buf.ByteAt(end)
	for unicode.IsLetter(rune(c)) || unicode.IsDigit(rune(c)) {
		end++
		c, _ = t.buf.ByteAt(end)
	}

	// return a single char selection if no word was found
	if start == end {
		end++
	}

	// Set dot
	t.q0 = start
	t.q1 = end
}

// NextDelim returns number of bytes from given offset up until next delimiter.
func (t *Text) NextDelim(delim rune, offset int) (n int) {
	t.Seek(offset, io.SeekStart)

	r, size, err := t.ReadRune()
	if err != nil {
		return 0
	}

	for r != delim {
		n += size
		r, size, err = t.ReadRune()
		if err != nil {
			if err == io.EOF {
				return n
			}
			return 0
		}
	}

	return n
}

// PrevDelim returns number of bytes from given offset up until next delimiter.
func (t *Text) PrevDelim(delim rune, offset int) (n int) {
	t.Seek(offset, io.SeekStart)
	r, size, err := t.UnreadRune()
	if err != nil {
		return 0
	}
	n += size

	for r != delim {
		r, size, err = t.UnreadRune()
		n += size
		if err != nil {
			if err == gapbuffer.ErrOutOfRange {
				return n
			}
			return 0
		}
	}

	return n
}

func (t *Text) Undo() error {
	c, err := t.history.Undo()
	if err != nil {
		return errors.Wrap(err, "undo")
	}
	t.commit(c)

	// highlight text
	t.SetDot(c.offset, c.offset+len(c.content))

	return nil
}

func (t *Text) Redo() error {
	c, err := t.history.Redo()
	if err != nil {
		return errors.Wrap(err, "redo")
	}
	t.commit(c)

	if c.action == HDelete {
		t.SetDot(c.offset, c.offset)
	} else {
		t.SetDot(c.offset+len(c.content), c.offset+len(c.content))
	}
	return nil
}

func (t *Text) commit(c Change) (int, error) {
	switch c.action {
	case HInsert:
		t.buf.Seek(c.offset) // sync gap buffer
		n, err := t.buf.Write([]byte(c.content))
		if err != nil {
			return 0, err
		}
		return n, err
	case HDelete:
		n := len(c.content)
		t.buf.Seek(c.offset + n) // sync gap buffer
		for i := n; i > 0; i-- {
			t.buf.Delete() // gap buffer deletes one byte at a time
		}
		return n, nil
	default:
		return 0, errors.New("invalid action in change")
	}
}

/* History */
type HistoryAction uint8

const (
	HInsert HistoryAction = iota
	HDelete
)

type Change struct {
	offset  int
	action  HistoryAction
	content []byte
}

type ChangeSet []*Change

type History struct {
	done   []Change
	recall []Change
}

func (h *History) Do(c Change) {
	h.done = append(h.done, c)
	h.recall = nil // clear old recall stack on new do
}

func (h *History) Undo() (Change, error) {
	if len(h.done) == 0 {
		return Change{}, errors.New("no history")
	}
	lastdone := h.done[len(h.done)-1]
	h.recall = append(h.recall, lastdone)
	h.done = h.done[:len(h.done)-1] // remove last one

	// Reverse the done action so the returned change can be applied directly.
	switch lastdone.action {
	case HInsert:
		lastdone.action = HDelete
	case HDelete:
		lastdone.action = HInsert
	}

	return lastdone, nil
}

func (h *History) Redo() (Change, error) {
	if len(h.recall) == 0 {
		return Change{}, errors.New("no recall history")
	}
	lastrecall := h.recall[len(h.recall)-1]
	h.done = append(h.done, lastrecall)
	h.recall = h.recall[:len(h.recall)-1] //remove last one
	return lastrecall, nil
}