package skate

import (
	"crypto/sha1"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path"

	"git.archive.org/martin/cgraph/skate/atomic"
)

var ErrCacheMiss = errors.New("cache miss")

type Cache struct {
	Dir         string
	initialized bool
}

// Init creates all directories.
func (c *Cache) init() error {
	for i := 0; i < 256; i++ {
		s := fmt.Sprintf("%02x", i)
		d := path.Join(c.Dir, s)
		if _, err := os.Stat(d); os.IsNotExist(err) {
			if err := os.MkdirAll(d, 0755); err != nil {
				return err
			}
		}
	}
	c.initialized = true
	return nil
}

func (c *Cache) Has(k string) bool {
	var (
		shard, name = shardedHash(k)
		spath       = path.Join(c.Dir, shard, name)
	)
	if _, err := os.Stat(spath); os.IsNotExist(err) {
		return false
	}
	return true
}

func (c *Cache) Get(k string) ([]byte, error) {
	var (
		shard, name = shardedHash(k)
		spath       = path.Join(c.Dir, shard, name)
	)
	if !c.initialized {
		if err := c.init(); err != nil {
			return nil, err
		}
	}
	if _, err := os.Stat(spath); os.IsNotExist(err) {
		return nil, ErrCacheMiss
	}
	f, err := os.Open(spath)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	return ioutil.ReadAll(f)
}

func (c *Cache) Set(k string, v []byte) error {
	var (
		shard, name = shardedHash(k)
		spath       = path.Join(c.Dir, shard, name)
	)
	if !c.initialized {
		if err := c.init(); err != nil {
			return err
		}
	}
	if err := atomic.WriteFile(spath, v, 0755); err != nil {
		return err
	}
	return nil
}

// shardedHash returns a sha1 shard (2) and name (38) for a given string value
// (e.g. a URL).
func shardedHash(v string) (shard, name string) {
	h := sha1.New()
	_, _ = io.WriteString(h, v)
	name = fmt.Sprintf("%x", h.Sum(nil))
	return name[:2], name[2:]
}