// Copyright 2016 The Hugo Authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package tpl

import (
	"errors"
	"html/template"
	"strings"
	"text/template/parse"
)

// decl keeps track of the variable mappings, i.e. $mysite => .Site etc.
type decl map[string]string

var paramsPaths = [][]string{
	{"Params"},
	{"Site", "Params"},

	// Site and Pag referenced from shortcodes
	{"Page", "Site", "Params"},
	{"Page", "Params"},

	{"Site", "Language", "Params"},
}

type templateContext struct {
	decl  decl
	templ *template.Template
}

func newTemplateContext(templ *template.Template) *templateContext {
	return &templateContext{templ: templ, decl: make(map[string]string)}

}

func applyTemplateTransformers(templ *template.Template) error {
	if templ == nil || templ.Tree == nil {
		return errors.New("expected template, but none provided")
	}

	c := newTemplateContext(templ)

	c.paramsKeysToLower(templ.Tree.Root)

	return nil
}

// paramsKeysToLower is made purposely non-generic to make it not so tempting
// to do more of these hard-to-maintain AST transformations.
func (c *templateContext) paramsKeysToLower(n parse.Node) {

	switch x := n.(type) {
	case *parse.ListNode:
		if x != nil {
			c.paramsKeysToLowerForNodes(x.Nodes...)
		}
	case *parse.ActionNode:
		c.paramsKeysToLowerForNodes(x.Pipe)
	case *parse.IfNode:
		c.paramsKeysToLowerForNodes(x.Pipe, x.List, x.ElseList)
	case *parse.WithNode:
		c.paramsKeysToLowerForNodes(x.Pipe, x.List, x.ElseList)
	case *parse.RangeNode:
		c.paramsKeysToLowerForNodes(x.Pipe, x.List, x.ElseList)
	case *parse.TemplateNode:
		subTempl := c.templ.Lookup(x.Name)
		if subTempl != nil {
			c.paramsKeysToLowerForNodes(subTempl.Tree.Root)
		}
	case *parse.PipeNode:
		for i, elem := range x.Decl {
			if len(x.Cmds) > i {
				// maps $site => .Site etc.
				c.decl[elem.Ident[0]] = x.Cmds[i].String()
			}
		}

		for _, cmd := range x.Cmds {
			c.paramsKeysToLower(cmd)
		}

	case *parse.CommandNode:
		for _, elem := range x.Args {
			switch an := elem.(type) {
			case *parse.FieldNode:
				c.updateIdentsIfNeeded(an.Ident)
			case *parse.VariableNode:
				c.updateIdentsIfNeeded(an.Ident)
			case *parse.PipeNode:
				c.paramsKeysToLower(an)
			}

		}
	}
}

func (c *templateContext) paramsKeysToLowerForNodes(nodes ...parse.Node) {
	for _, node := range nodes {
		c.paramsKeysToLower(node)
	}
}

func (c *templateContext) updateIdentsIfNeeded(idents []string) {
	index := c.decl.indexOfReplacementStart(idents)

	if index == -1 {
		return
	}

	for i := index; i < len(idents); i++ {
		idents[i] = strings.ToLower(idents[i])
	}
}

// indexOfReplacementStart will return the index of where to start doing replacement,
// -1 if none needed.
func (d decl) indexOfReplacementStart(idents []string) int {

	l := len(idents)

	if l == 0 {
		return -1
	}

	first := idents[0]
	firstIsVar := first[0] == '$'

	if l == 1 && !firstIsVar {
		// This can not be a Params.x
		return -1
	}

	if !firstIsVar {
		found := false
		for _, paramsPath := range paramsPaths {
			if first == paramsPath[0] {
				found = true
				break
			}
		}
		if !found {
			return -1
		}
	}

	var (
		resolvedIdents []string
		replacements   []string
		replaced       []string
	)

	// An Ident can start out as one of
	// [Params] [$blue] [$colors.Blue]
	// We need to resolve the variables, so
	// $blue => [Params Colors Blue]
	// etc.
	replacements = []string{idents[0]}

	// Loop until there are no more $vars to resolve.
	for i := 0; i < len(replacements); i++ {

		if i > 20 {
			// bail out
			return -1
		}

		potentialVar := replacements[i]

		if potentialVar == "$" {
			continue
		}

		if potentialVar == "" || potentialVar[0] != '$' {
			// leave it as is
			replaced = append(replaced, strings.Split(potentialVar, ".")...)
			continue
		}

		replacement, ok := d[potentialVar]

		if !ok {
			// Temporary range vars. We do not care about those.
			return -1
		}

		replacement = strings.TrimPrefix(replacement, ".")

		if replacement == "" {
			continue
		}

		if replacement[0] == '$' {
			// Needs further expansion
			replacements = append(replacements, strings.Split(replacement, ".")...)
		} else {
			replaced = append(replaced, strings.Split(replacement, ".")...)
		}
	}

	resolvedIdents = append(replaced, idents[1:]...)

	for _, paramPath := range paramsPaths {
		if index := indexOfFirstRealIdentAfterWords(resolvedIdents, idents, paramPath...); index != -1 {
			return index
		}
	}

	return -1

}

func indexOfFirstRealIdentAfterWords(resolvedIdents, idents []string, words ...string) int {
	if !sliceStartsWith(resolvedIdents, words...) {
		return -1
	}

	for i, ident := range idents {
		if ident == "" || ident[0] == '$' {
			continue
		}
		found := true
		for _, word := range words {
			if ident == word {
				found = false
				break
			}
		}
		if found {
			return i
		}
	}

	return -1
}

func sliceStartsWith(slice []string, words ...string) bool {

	if len(slice) < len(words) {
		return false
	}

	for i, word := range words {
		if word != slice[i] {
			return false
		}
	}
	return true
}