// Package timezone 提供随计划冻结的时区快照，禁止读取宿主机或 ZONEINFO 环境配置。
package timezone

import (
	"archive/zip"
	"bytes"
	_ "embed"
	"errors"
	"io"
	"sync"
	"time"
)

// Version 标识 Go 1.27.1 附带的 IANA 快照及文件摘要；升级时须保留旧版本可读能力。
const Version = "go1.27.1-b2d18a7c8fa8"

//go:embed zoneinfo.zip
var data []byte
var zones sync.Map
var archive = sync.OnceValues(func() (*zip.Reader, error) { return zip.NewReader(bytes.NewReader(data), int64(len(data))) })

// Load 仅接受精确的内置名称，返回只读 Location；不接受 Local 或文件系统路径。
func Load(name, version string) (*time.Location, error) {
	if version != Version || name == "" || name == "Local" || len(name) > 64 {
		return nil, errors.New("unsupported frozen timezone version or name")
	}
	if name == "UTC" {
		return time.UTC, nil
	}
	if v, ok := zones.Load(name); ok {
		return v.(*time.Location), nil
	}
	a, err := archive()
	if err != nil {
		return nil, err
	}
	for _, entry := range a.File {
		if entry.Name != name {
			continue
		}
		r, err := entry.Open()
		if err != nil {
			return nil, err
		}
		b, err := io.ReadAll(io.LimitReader(r, 1<<20))
		closeErr := r.Close()
		if err != nil {
			return nil, err
		}
		if closeErr != nil {
			return nil, closeErr
		}
		loc, err := time.LoadLocationFromTZData(name, b)
		if err != nil {
			return nil, err
		}
		actual, _ := zones.LoadOrStore(name, loc)
		return actual.(*time.Location), nil
	}
	return nil, errors.New("unknown IANA timezone")
}