package main

import (
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"slices"
	"strings"
	"sync"
	"unicode/utf8"
)

// A nest is a room where a team of crows works together. Its core crows are
// picked first; when a job needs more, the Sentry borrows from the rest of the
// flock. The Sentry and Elder serve every nest and are never core members.
type Nest struct {
	ID    string   `json:"nest_id"`
	Name  string   `json:"name"`
	Crows []string `json:"crows"`
}

const (
	defaultNestID = "todo"
	maxNests      = 20 // includes the default nest
	maxNestName   = 40
)

// defaultNest has an empty core: its crows come from the whole flock.
var defaultNest = Nest{ID: defaultNestID, Name: "To do", Crows: []string{}}

// NestError is a rejected nest. Clash means the nest_id is already taken (409);
// anything else is bad input or too many nests (400).
type NestError struct {
	Clash bool
	Msg   string
}

func (e *NestError) Error() string { return e.Msg }

func invalid(format string, a ...any) error {
	return &NestError{Msg: fmt.Sprintf(format, a...)}
}

// NestStore holds the nests and saves the ones people create to disk.
type NestStore struct {
	path string
	mu   sync.Mutex
	made []Nest // created nests, in creation order; the default nest is not stored
}

func LoadNests(path string) (*NestStore, error) {
	s := &NestStore{path: path}
	data, err := os.ReadFile(path)
	if err != nil && !os.IsNotExist(err) {
		return nil, err
	}
	if err == nil {
		if err := json.Unmarshal(data, &s.made); err != nil {
			return nil, fmt.Errorf("nests file is unreadable: %w", err)
		}
	}
	return s, nil
}

// All returns the default nest first, then the others in creation order.
func (s *NestStore) All() []Nest {
	s.mu.Lock()
	defer s.mu.Unlock()
	out := []Nest{copyNest(defaultNest)}
	for _, n := range s.made {
		out = append(out, copyNest(n))
	}
	return out
}

// Get finds a nest by ID.
func (s *NestStore) Get(id string) (Nest, bool) {
	if id == defaultNestID {
		return copyNest(defaultNest), true
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, n := range s.made {
		if n.ID == id {
			return copyNest(n), true
		}
	}
	return Nest{}, false
}

// Create checks the guardrails, adds the nest, and saves. A rejection is a
// *NestError; any other error means the save failed.
func (s *NestStore) Create(name string, crows []string) (Nest, error) {
	name = strings.TrimSpace(name)
	if n := utf8.RuneCountInString(name); n < 1 || n > maxNestName {
		return Nest{}, invalid("A nest name is 1 to %d characters.", maxNestName)
	}
	id := slugify(name)
	if id == "" {
		return Nest{}, invalid("A nest name needs at least one letter or digit (a-z, 0-9).")
	}
	if len(crows) < 1 || len(crows) > len(roster) {
		return Nest{}, invalid("A nest has 1 to %d crows.", len(roster))
	}
	for i, c := range crows {
		if !slices.Contains(roster, c) {
			return Nest{}, invalid("%q is not in the flock. Choose from %s.", c, strings.Join(roster, ", "))
		}
		if slices.Contains(crows[:i], c) {
			return Nest{}, invalid("%s is listed twice.", c)
		}
	}

	s.mu.Lock()
	defer s.mu.Unlock()
	if id == defaultNestID || slices.ContainsFunc(s.made, func(n Nest) bool { return n.ID == id }) {
		return Nest{}, &NestError{Clash: true, Msg: fmt.Sprintf("A nest called %q already exists.", id)}
	}
	if 1+len(s.made) >= maxNests {
		return Nest{}, invalid("The flock keeps at most %d nests.", maxNests)
	}
	n := Nest{ID: id, Name: name, Crows: slices.Clone(crows)}
	s.made = append(s.made, n)
	if err := s.save(); err != nil {
		s.made = s.made[:len(s.made)-1]
		return Nest{}, err
	}
	return copyNest(n), nil
}

// save writes atomically, like the ledger. The caller holds s.mu.
func (s *NestStore) save() error {
	data, err := json.MarshalIndent(s.made, "", "  ")
	if err != nil {
		return err
	}
	if err := os.MkdirAll(filepath.Dir(s.path), 0o755); err != nil {
		return err
	}
	tmp := s.path + ".tmp"
	if err := os.WriteFile(tmp, data, 0o644); err != nil {
		return err
	}
	return os.Rename(tmp, s.path)
}

func copyNest(n Nest) Nest {
	n.Crows = append([]string{}, n.Crows...)
	return n
}

// slugify lowercases and keeps a-z and 0-9; every other run of characters
// becomes one dash, and dashes at the ends are dropped.
func slugify(name string) string {
	var sb strings.Builder
	dash := false
	for _, r := range strings.ToLower(name) {
		if (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') {
			if dash && sb.Len() > 0 {
				sb.WriteByte('-')
			}
			sb.WriteRune(r)
			dash = false
		} else {
			dash = true
		}
	}
	return sb.String()
}