// Package project assembles copies of conventions inside a consuming
// repository.
//
// A copy is flat: one file per topic, the layers of the topic inside it as
// sections in the order base, language, stack. The paths of the suite are not
// reproduced — whoever checks code against a convention reads one file and does
// not gather a topic out of three places.
//
// Everything below the local marker belongs to the repository and survives
// reassembly; everything above it is rewritten. There is no three-way merge and
// no report of divergence: after a reassembly the difference is shown by git,
// and the decision is taken by a person before the commit.
package project

import (
	"fmt"
	"os"
	"path/filepath"
	"strings"

	"git.vakhrushev.me/av/convy/internal/doc"
	"git.vakhrushev.me/av/convy/internal/lang"
	"git.vakhrushev.me/av/convy/internal/manifest"
	"git.vakhrushev.me/av/convy/internal/suite"
)

// LocalMarker is the one piece of markup inside a copy that means something to
// the tool. It is single and nameless, so there is no name to be orphaned by a
// rename.
const LocalMarker = "<!-- conv:local -->"

// Copy is the outcome of assembling one topic for one component.
type Copy struct {
	Topic string
	// Path is where the file lies, from the root of the project.
	Path string
	// Layers lists the paths in the suite the file was built from.
	Layers []string
	// Created says the file did not exist before.
	Created bool
	// Kept says a local part below the marker was carried over.
	Kept bool
}

// Axis is what a component asks the suite for.
func Axis(c manifest.Component) suite.Component {
	return suite.Component{Lang: c.Lang, Stack: c.Stack}
}

// Assemble builds one topic for one component and writes the file. root is the
// root of the project.
func Assemble(s *suite.Suite, root string, c manifest.Component, topic string) (Copy, error) {
	if !s.Manifest.TopicLive(topic) {
		if s.Manifest.TopicRetired(topic) {
			return Copy{}, fmt.Errorf("the suite has retired the topic %q: %s", topic, s.Manifest.Topics.Retired[topic])
		}
		return Copy{}, fmt.Errorf("the suite declares no live topic %q", topic)
	}
	layers, _ := s.Assemble(topic, Axis(c))
	if len(layers) == 0 {
		return Copy{}, fmt.Errorf("the topic %q has no layer this component takes", topic)
	}

	rel := filepath.ToSlash(filepath.Join(c.Dir, topic+".md"))
	name := filepath.Join(root, filepath.FromSlash(rel))
	made := Copy{Topic: topic, Path: rel}
	for _, d := range layers {
		made.Layers = append(made.Layers, d.Path)
	}

	existing, err := os.ReadFile(name)
	switch {
	case os.IsNotExist(err):
		made.Created = true
	case err != nil:
		return Copy{}, err
	default:
		if err := ours(rel, string(existing), topic); err != nil {
			return Copy{}, err
		}
	}

	local := localPart(string(existing))
	made.Kept = strings.TrimSpace(strings.TrimPrefix(local, LocalMarker)) != ""

	body := Render(topic, layers, s.Vocab) + "\n\n" + local
	if err := os.MkdirAll(filepath.Dir(name), 0o755); err != nil {
		return Copy{}, err
	}
	if err := os.WriteFile(name, []byte(body), 0o644); err != nil {
		return Copy{}, err
	}
	return made, nil
}

// ReadingName is what the reader's guide of the language is called next to the
// copies, whatever it is called where it came from.
const ReadingName = "READING.md"

// Reading puts the reader's guide of the language next to the copies. The guide
// belongs to the level above and is overwritten whole; README.md standing next
// to it belongs to the repository and is never touched.
//
// The guide travels because the copy names the language by a version and by no
// path: without the guide an agent reading a copy takes ДОПУСКАЕТСЯ for the
// everyday "you can" and loses exactly what the word was introduced for.
func Reading(langRoot, path, root, dir string) (string, error) {
	if path == "" {
		return "", fmt.Errorf("the suite manifest names no reading guide, and it is what travels next to the copies")
	}
	body, err := os.ReadFile(filepath.Join(langRoot, filepath.FromSlash(path)))
	if err != nil {
		return "", fmt.Errorf("reading the guide of the language: %w", err)
	}
	rel := filepath.ToSlash(filepath.Join(dir, ReadingName))
	name := filepath.Join(root, filepath.FromSlash(rel))
	if err := os.MkdirAll(filepath.Dir(name), 0o755); err != nil {
		return "", err
	}
	if err := os.WriteFile(name, body, 0o644); err != nil {
		return "", err
	}
	return rel, nil
}

// ours refuses to overwrite a file that is not a copy of this topic. A file
// whose origin key was taken away has stopped being a copy and become a
// document of the repository, and the tool has no business rewriting it.
func ours(rel, existing, topic string) error {
	d, err := doc.Parse(rel, existing)
	if err != nil {
		return fmt.Errorf("%s is in the way and cannot be read: %w", rel, err)
	}
	switch {
	case d.Front.Origin == "" && d.Front.Topic == "" && d.Front.Prefix == "":
		return fmt.Errorf("%s carries no origin key: it is a document of the repository rather than a copy, and assembly would overwrite it", rel)
	case d.Front.Origin == "":
		return fmt.Errorf("%s carries no origin key, while it does carry the front matter of a suite file: it looks like a layer put here by hand", rel)
	case d.Front.Origin != topic:
		return fmt.Errorf("%s is a copy of the topic %q, and the topic %q would be assembled into the same file", rel, d.Front.Origin, topic)
	}
	return nil
}

// localPart returns everything from the marker down, together with the marker.
// A file that has none gets one: the marker is placed by the assembler, and a
// copy without it has nowhere to put a derogation.
func localPart(existing string) string {
	lines := strings.Split(existing, "\n")
	for i, line := range lines {
		if strings.TrimSpace(line) != LocalMarker {
			continue
		}
		return strings.TrimRight(strings.Join(lines[i:], "\n"), "\n") + "\n"
	}
	return LocalMarker + "\n"
}

// Render lays out the part of a copy that comes from the suite: the front
// matter of the copy and the layers of the topic.
func Render(topic string, layers []*doc.Document, v lang.Vocabulary) string {
	parts := make([]string, 0, len(layers)+1)
	parts = append(parts, "---\norigin: "+topic+"\n---")
	for i, d := range layers {
		parts = append(parts, layerText(d, v, i == 0))
	}
	return strings.Join(parts, "\n\n")
}

// layerText renders one layer.
//
// The first layer is the document: its title, its introduction, its sections.
// Every layer after it becomes a section of that document, because a layer only
// implements and narrows the base rather than standing beside it — so its own
// headings step down one level and its title becomes the heading of the
// section. The line about the language version is dropped from all but the
// first: it says the same thing three times over otherwise, and it is what
// makes the copy self-contained rather than decoration to be repeated.
func layerText(d *doc.Document, v lang.Vocabulary, first bool) string {
	skip := map[int]bool{}
	if !first {
		if from, to, ok := versionLines(d, v); ok {
			for n := from; n <= to; n++ {
				skip[n] = true
			}
		}
	}

	var out []string
	for n := d.Body; n <= d.Len(); n++ {
		if skip[n] {
			continue
		}
		line := d.Line(n)
		if !first && !d.Fenced(n) && strings.HasPrefix(line, "#") {
			line = "#" + line
		}
		out = append(out, line)
	}
	return strings.Trim(strings.Join(out, "\n"), "\n")
}

// versionLines finds the paragraph carrying the language version line: the one
// listing every key word of the vocabulary.
func versionLines(d *doc.Document, v lang.Vocabulary) (from, to int, ok bool) {
	start, end := d.Preamble()
	words := v.Words()
	for _, p := range d.Paragraphs(start, end) {
		text := p.Text()
		found := true
		for _, w := range words {
			if !strings.Contains(text, w) {
				found = false
				break
			}
		}
		if found {
			return p.Start, p.End, true
		}
	}
	return 0, 0, false
}