package transform

import (
	"bytes"
	"io"
	"net/url"
	"strings"
	"unicode/utf8"
)

type matchState int

const (
	matchStateNone matchState = iota
	matchStateWhitespace
	matchStatePartial
	matchStateFull
)

type contentlexer struct {
	content []byte

	pos   int // input position
	start int // item start position
	width int // width of last element

	matchers []absURLMatcher
	state    stateFunc

	ms      matchState
	matches [3]bool // track matches of the 3 prefixes
	i       int     // last index in matches checked

	w io.Writer
}

type stateFunc func(*contentlexer) stateFunc

type prefix struct {
	r []rune
	f func(l *contentlexer)
}

var prefixes = []*prefix{
	&prefix{r: []rune{'s', 'r', 'c', '='}, f: checkCandidateBase},
	&prefix{r: []rune{'h', 'r', 'e', 'f', '='}, f: checkCandidateBase},
	&prefix{r: []rune{'s', 'r', 'c', 's', 'e', 't', '='}, f: checkCandidateSrcset},
}

type absURLMatcher struct {
	match          []byte
	quote          []byte
	replacementURL []byte
}

func (l *contentlexer) match(r rune) {

	var found bool

	// note, the prefixes can start off on the same foot, i.e.
	// src and srcset.
	if l.ms == matchStateWhitespace {
		l.i = 0
		for j, p := range prefixes {
			if r == p.r[l.i] {
				l.matches[j] = true
				found = true
				if l.checkMatchState(r, j) {
					return
				}
			} else {
				l.matches[j] = false
			}
		}

		if !found {
			l.ms = matchStateNone
		}

		return
	}

	l.i++
	for j, m := range l.matches {
		// still a match?
		if m {
			if prefixes[j].r[l.i] == r {
				found = true
				if l.checkMatchState(r, j) {
					return
				}
			} else {
				l.matches[j] = false
			}
		}
	}

	if found {
		return
	}

	l.ms = matchStateNone
}

func (l *contentlexer) checkMatchState(r rune, idx int) bool {
	if r == '=' {
		l.ms = matchStateFull
		for k := range l.matches {
			if k != idx {
				l.matches[k] = false
			}
		}
		return true
	}

	l.ms = matchStatePartial

	return false
}

func (l *contentlexer) emit() {
	l.w.Write(l.content[l.start:l.pos])
	l.start = l.pos
}

func checkCandidateBase(l *contentlexer) {
	for _, m := range l.matchers {
		if !bytes.HasPrefix(l.content[l.pos:], m.match) {
			continue
		}
		// check for schemaless URLs
		posAfter := l.pos + len(m.match)
		if posAfter >= len(l.content) {
			return
		}
		r, _ := utf8.DecodeRune(l.content[posAfter:])
		if r == '/' {
			// schemaless: skip
			return
		}
		if l.pos > l.start {
			l.emit()
		}
		l.pos += len(m.match)
		l.w.Write(m.quote)
		l.w.Write(m.replacementURL)
		l.start = l.pos
	}
}

func checkCandidateSrcset(l *contentlexer) {
	// special case, not frequent (me think)
	for _, m := range l.matchers {
		if !bytes.HasPrefix(l.content[l.pos:], m.match) {
			continue
		}

		// check for schemaless URLs
		posAfter := l.pos + len(m.match)
		if posAfter >= len(l.content) {
			return
		}
		r, _ := utf8.DecodeRune(l.content[posAfter:])
		if r == '/' {
			// schemaless: skip
			continue
		}

		posLastQuote := bytes.Index(l.content[l.pos+1:], m.quote)

		// safe guard
		if posLastQuote < 0 || posLastQuote > 2000 {
			return
		}

		if l.pos > l.start {
			l.emit()
		}

		section := l.content[l.pos+len(m.quote) : l.pos+posLastQuote+1]

		fields := bytes.Fields(section)
		l.w.Write([]byte(m.quote))
		for i, f := range fields {
			if f[0] == '/' {
				l.w.Write(m.replacementURL)
				l.w.Write(f[1:])

			} else {
				l.w.Write(f)
			}

			if i < len(fields)-1 {
				l.w.Write([]byte(" "))
			}
		}

		l.w.Write(m.quote)
		l.pos += len(section) + (len(m.quote) * 2)
		l.start = l.pos
	}
}

func (l *contentlexer) replace() {
	contentLength := len(l.content)
	var r rune

	for {
		if l.pos >= contentLength {
			l.width = 0
			break
		}

		var width = 1
		r = rune(l.content[l.pos])
		if r >= utf8.RuneSelf {
			r, width = utf8.DecodeRune(l.content[l.pos:])
		}
		l.width = width
		l.pos += l.width
		if r == ' ' {
			l.ms = matchStateWhitespace
		} else if l.ms != matchStateNone {
			l.match(r)
			if l.ms == matchStateFull {
				var p *prefix
				for i, m := range l.matches {
					if m {
						p = prefixes[i]
					}
					l.matches[i] = false
				}
				if p == nil {
					panic("illegal state: curr is nil when state is full")
				}
				l.ms = matchStateNone
				p.f(l)
			}
		}
	}

	// Done!
	if l.pos > l.start {
		l.emit()
	}
}

func doReplace(ct contentTransformer, matchers []absURLMatcher) {
	lexer := &contentlexer{
		content:  ct.Content(),
		w:        ct,
		matchers: matchers}

	lexer.replace()
}

type absURLReplacer struct {
	htmlMatchers []absURLMatcher
	xmlMatchers  []absURLMatcher
}

func newAbsURLReplacer(baseURL string) *absURLReplacer {
	u, _ := url.Parse(baseURL)
	base := []byte(strings.TrimRight(u.String(), "/") + "/")

	// HTML
	dqHTMLMatch := []byte("\"/")
	sqHTMLMatch := []byte("'/")

	// XML
	dqXMLMatch := []byte("&#34;/")
	sqXMLMatch := []byte("&#39;/")

	dqHTML := []byte("\"")
	sqHTML := []byte("'")

	dqXML := []byte("&#34;")
	sqXML := []byte("&#39;")

	return &absURLReplacer{
		htmlMatchers: []absURLMatcher{
			{dqHTMLMatch, dqHTML, base},
			{sqHTMLMatch, sqHTML, base},
		},
		xmlMatchers: []absURLMatcher{
			{dqXMLMatch, dqXML, base},
			{sqXMLMatch, sqXML, base},
		}}

}

func (au *absURLReplacer) replaceInHTML(ct contentTransformer) {
	doReplace(ct, au.htmlMatchers)
}

func (au *absURLReplacer) replaceInXML(ct contentTransformer) {
	doReplace(ct, au.xmlMatchers)
}