// Package datafiles walks a data tree and reads its category files and their rows.
package datafiles

import (
	"bytes"
	"embed"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"os"
	"path"
	"strings"
	"sync"

	"github.com/larvit/fejkdata/internal/grammar"
	"github.com/larvit/fejkdata/internal/jsonvalue"
)

// Source is one tree to load: an fs.FS and the directory in it to start from. label
// prefixes file names in errors; onDisk marks label as a directory that must exist.
type Source struct {
	fsys   fs.FS
	label  string
	onDisk bool
	base   string
}

// Dir is the tree in the directory path, its errors labelled with it.
func Dir(path string) Source {
	return Source{fsys: os.DirFS(path), label: path, onDisk: true}
}

// FS is the tree in fsys below base, "" for its root.
func FS(fsys fs.FS, base string) Source { return Source{fsys: fsys, base: base} }

// Category is one category file: the folders above it from the source's base, its name,
// and its parsed JSON.
type Category struct {
	Folders []string
	Name    string
	JSON    jsonvalue.Value
	rows    *rowsFiles
}

// ReadRows reads a rows file beside the category.
func (c Category) ReadRows(name string) (string, error) { return c.rows.read(name) }

func (s Source) labelled(p string) string {
	if s.label == "" {
		return p
	}
	return path.Join(s.label, p)
}

// dirPath is the folder that dir names, as fsys spells it.
func (s Source) dirPath(dir []string) string {
	if p := path.Join(append([]string{s.base}, dir...)...); p != "" {
		return p
	}
	return "."
}

// ManifestFile is the file at a source's root that describes the source.
const ManifestFile = ".fejkdata.json"

// Manifest is what a source's manifest says of it.
type Manifest struct {
	// Index is every category of the source by dot path; nil where the manifest has none.
	Index map[string]IndexEntry `json:"index"`
}

// IndexEntry is what an index says of a category: its parent table, "" for none, the paths
// List advertises below it, and the categories its templates reference.
type IndexEntry struct {
	Parent string   `json:"parent,omitempty"`
	Paths  []string `json:"paths"`
	Reads  []string `json:"reads,omitempty"`
}

// embeddedManifests holds each manifest read from an embed.FS, whose files never change.
var embeddedManifests sync.Map

type embeddedManifest struct {
	fsys embed.FS
	base string
}

// Manifest reads the manifest at the source's root, the zero Manifest where there is none.
func (s Source) Manifest() (Manifest, error) {
	e, embedded := s.fsys.(embed.FS)
	key := embeddedManifest{fsys: e, base: s.base}
	if m, read := embeddedManifests.Load(key); embedded && read {
		return m.(Manifest), nil
	}
	m, err := s.readManifest()
	if embedded && err == nil {
		embeddedManifests.Store(key, m)
	}
	return m, err
}

func (s Source) readManifest() (Manifest, error) {
	if s.onDisk && s.label == "" {
		return Manifest{}, nil // Walk refuses the empty data path
	}
	full := path.Join(s.base, ManifestFile)
	b, err := fs.ReadFile(s.fsys, full)
	if errors.Is(err, fs.ErrNotExist) {
		return Manifest{}, nil
	}
	if err != nil {
		return Manifest{}, fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	m, err := decodeManifest(b)
	if err != nil {
		return Manifest{}, fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	return m, nil
}

func decodeManifest(b []byte) (Manifest, error) {
	dec := json.NewDecoder(bytes.NewReader(b))
	dec.DisallowUnknownFields()
	var m Manifest
	if err := dec.Decode(&m); err != nil {
		return Manifest{}, err
	}
	if _, err := dec.Token(); err != io.EOF {
		return Manifest{}, fmt.Errorf("more than one JSON value")
	}
	for p := range m.Index {
		for _, seg := range strings.Split(p, ".") {
			if err := grammar.CheckIdentifier(seg); err != nil {
				return Manifest{}, fmt.Errorf("index names %q: %w", p, err)
			}
		}
	}
	return m, nil
}

// Walk hands compile every category of the tree, folders and files in name order. A
// folder holding no category anywhere below it is skipped; a hidden file or folder is no
// category, though a category may name a hidden rows file beside it.
func (s Source) Walk(compile func(Category) error) error {
	if s.onDisk {
		if s.label == "" {
			return fmt.Errorf("a data path is empty")
		}
		info, err := os.Stat(s.label)
		if err != nil {
			return fmt.Errorf("data path %s: %w", s.label, err)
		}
		if !info.IsDir() {
			return fmt.Errorf("%s is not a directory", s.label)
		}
	}
	_, err := s.walkDir(nil, compile)
	return err
}

// walkDir walks the folder that dir names, and reports whether it handed over any category.
func (s Source) walkDir(dir []string, compile func(Category) error) (bool, error) {
	full := s.dirPath(dir)
	entries, err := fs.ReadDir(s.fsys, full)
	if err != nil {
		return false, fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	rows := newRowsFiles(s, full, entries)
	handed := false
	for _, e := range entries {
		if strings.HasPrefix(e.Name(), ".") {
			continue
		}
		var holds bool
		if e.IsDir() {
			holds, err = s.walkFolder(append(dir[:len(dir):len(dir)], e.Name()), compile)
		} else {
			holds, err = s.compileFile(dir, e.Name(), rows, compile)
		}
		if err != nil {
			return false, err
		}
		handed = handed || holds
	}
	return handed, nil
}

// walkFolder walks a subfolder, and refuses its name if it holds a category.
func (s Source) walkFolder(dir []string, compile func(Category) error) (bool, error) {
	holds, err := s.walkDir(dir, compile)
	if err != nil || !holds {
		return false, err
	}
	if err := grammar.CheckIdentifier(dir[len(dir)-1]); err != nil {
		return false, fmt.Errorf("%s: folder %w", s.labelled(s.dirPath(dir)), err)
	}
	return true, nil
}

// Load hands compile the category file name.json in the folder dir.
func (s Source) Load(dir []string, name string, compile func(Category) error) error {
	full := s.dirPath(dir)
	entries, err := fs.ReadDir(s.fsys, full)
	if err != nil {
		return fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	_, err = s.compileFile(dir, name+".json", newRowsFiles(s, full, entries), compile)
	return err
}

// compileFile hands compile a *.json file as a category named after it, and reports
// whether it was one; any other file is skipped.
func (s Source) compileFile(dir []string, file string, rows *rowsFiles, compile func(Category) error) (bool, error) {
	if !strings.HasSuffix(file, ".json") {
		return false, nil
	}
	full := path.Join(rows.dir, file)
	name := strings.TrimSuffix(file, ".json")
	if err := grammar.CheckIdentifier(name); err != nil {
		return false, fmt.Errorf("%s: category %w", s.labelled(full), err)
	}
	b, err := fs.ReadFile(s.fsys, full)
	if err != nil {
		return false, fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	raw, err := jsonvalue.Decode(b)
	if err != nil {
		return false, fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	if err := compile(Category{Folders: dir, Name: name, JSON: raw, rows: rows}); err != nil {
		return false, fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	return true, nil
}

// rowsFiles is what a category may name beside itself: the rows files of its folder.
type rowsFiles struct {
	src Source
	dir string
	tsv map[string]bool
}

func newRowsFiles(src Source, dir string, entries []fs.DirEntry) *rowsFiles {
	files := &rowsFiles{src: src, dir: dir, tsv: map[string]bool{}}
	for _, e := range entries {
		if strings.HasSuffix(e.Name(), ".tsv") && !e.IsDir() {
			files.tsv[e.Name()] = true
		}
	}
	return files
}

func (r *rowsFiles) read(name string) (string, error) {
	if !r.tsv[name] {
		return "", fmt.Errorf("rows names %s, which is not beside it in %s", name, r.src.labelled(r.dir))
	}
	b, err := fs.ReadFile(r.src.fsys, path.Join(r.dir, name))
	if err != nil {
		return "", fmt.Errorf("%s: %w", r.src.labelled(path.Join(r.dir, name)), err)
	}
	return string(b), nil
}