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

import (
	"encoding/json"
	"fmt"
	"io/fs"
	"os"
	"path"
	"strings"

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

// 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 {
	FS      fs.FS
	Label   string
	OnDisk  bool
	BaseDir string
}

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

// ReadRows reads a rows file beside the category, marking it named.
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)
}

func (s Source) base() string {
	if s.BaseDir == "" {
		return "."
	}
	return s.BaseDir
}

// 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
// never data.
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 dir names, and reports whether it handed over any category.
func (s Source) walkDir(dir []string, compile func(Category) error) (bool, error) {
	full := path.Join(append([]string{s.base()}, dir...)...)
	entries, err := fs.ReadDir(s.FS, 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(), ".") { // hidden: a checkout or an editor's file, never data
			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
	}
	for name, named := range rows.tsv {
		if !named && !strings.HasPrefix(name, ".") {
			return false, fmt.Errorf("%s: no category names it in its rows; a table's rows file sits beside a category file naming it", s.labelled(path.Join(full, name)))
		}
	}
	return handed, nil
}

// walkFolder walks a subfolder, and refuses its name once 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.CheckName(dir[len(dir)-1]); err != nil {
		return false, fmt.Errorf("%s: folder %w", s.labelled(path.Join(append([]string{s.base()}, dir...)...)), err)
	}
	return true, nil
}

// Load hands compile the one category file name in the folder dir.
func (s Source) Load(dir []string, name string, compile func(Category) error) error {
	full := path.Join(append([]string{s.base()}, dir...)...)
	entries, err := fs.ReadDir(s.FS, 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.CheckName(name); err != nil {
		return false, fmt.Errorf("%s: category %w", s.labelled(full), err)
	}
	b, err := fs.ReadFile(s.FS, full)
	if err != nil {
		return false, fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	var raw any
	if err := json.Unmarshal(b, &raw); err != nil {
		return false, fmt.Errorf("%s: %w", s.labelled(full), err)
	}
	if err := compile(Category{Dir: 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,
// each marked once a category names it.
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()] = false
		}
	}
	return files
}

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