// mirror files from upstream NVD source
package nvdmirror

import (
  "bytes"
  "crypto/sha256"
  "errors"
  "fmt"
  "github.com/pablotron/cvez/atomictemp"
  "github.com/pablotron/cvez/feed"
  "github.com/rs/zerolog/log"
  "io"
  "io/fs"
  "net/http"
  "net/url"
  "os"
  "path/filepath"
)

// Fetch result.
type fetchResult struct {
  src           string // source URL
  err           error // fetch result
  modified      bool // Was the result modified?
  path          string // Destination file.
  headers       http.Header // response headers
}

// Check result.
type checkResult struct {
  metaUrl   string  // meta full url
  metaPath  string  // meta file path
  fullPath  string  // full file path
  err       error   // error
  match     bool    // true if size and hash match
}

type syncMessage struct {
  fetch fetchResult // fetch result
  check checkResult // check result
}

// sync context
type syncContext struct {
  config  SyncConfig // sync config
  client  *http.Client // shared HTTP client
  cache   Cache // cache
  dstDir  string // destination directory
  ch      chan syncMessage // sync message channel
}

// Create sync context.
func newSyncContext(config SyncConfig, cache Cache, dstDir string) syncContext {
  // create shared transport and client
  tr := &http.Transport {
	  MaxIdleConns:     config.MaxIdleConns,
	  IdleConnTimeout:  config.IdleConnTimeout,
  }

  return syncContext {
    config: config,
    client: &http.Client{Transport: tr},
    cache:  cache,
    dstDir: dstDir,
    ch:     make(chan syncMessage),
  }
}

// Build request
func (me syncContext) getRequest(srcUrl string) (*http.Request, error) {
  // create HTTP request
  req, err := http.NewRequest("GET", srcUrl, nil)
  if err != nil {
    return nil, err
  }

  // Add user-agent, if-none-match, and if-modified-since headers.
  req.Header.Add("user-agent", me.config.GetUserAgent())
  if headers, ok := me.cache.Get(srcUrl); ok {
    for k, v := range(headers) {
      req.Header.Add(k, v)
    }
  }

  // return success
  return req, nil
}

// Fetch URL and write result to destination directory.
//
// Note: This method is called from a goroutine and writes the results
// back via the member channel.
func (me syncContext) fetch(srcUrl string) {
  // parse source url
  src, err := url.Parse(srcUrl)
  if err != nil {
    me.ch <- syncMessage {
      fetch: fetchResult { src: srcUrl, err: err },
    }
    return
  }

  // build destination path
  path := filepath.Join(me.dstDir, filepath.Base(src.Path))
  log.Debug().Str("url", srcUrl).Str("path", path).Send()

  // create request
  req, err := me.getRequest(srcUrl)
  if err != nil {
    me.ch <- syncMessage {
      fetch: fetchResult { src: srcUrl, err: err },
    }
    return
  }

  // send request
  resp, err := me.client.Do(req)
  if err != nil {
    me.ch <- syncMessage {
      fetch: fetchResult { src: srcUrl, err: err },
    }
    return
  }
  defer resp.Body.Close()

  switch resp.StatusCode {
  case 200: // success
    // write to output file
    err := atomictemp.Create(path, func(f io.Writer) error {
      _, err := io.Copy(f, resp.Body)
      return err
    })

    if err != nil {
      // write failed
      me.ch <- syncMessage {
        fetch: fetchResult { src: srcUrl, err: err },
      }
    } else {
      me.ch <- syncMessage {
        fetch: fetchResult {
          src: srcUrl,
          modified: true,
          path: path,
          headers: resp.Header,
        },
      }
    }
  case 304: // not modified
    me.ch <- syncMessage {
      fetch: fetchResult { src: srcUrl },
    }
  default: // error
    code := resp.StatusCode
    err := fmt.Errorf("%d: %s", code, http.StatusText(code))
    me.ch <- syncMessage {
      fetch: fetchResult { src: srcUrl, err: err },
    }
  }
}

// read hash from given meta file.
func (me syncContext) getMeta(path string) (*feed.Meta, error) {
  // open meta file
  f, err := os.Open(path)
  if err != nil {
    return nil, err
  }
  defer f.Close()

  // parse meta
  return feed.NewMeta(f)
}

// get hash of file in destination directory.
func (me syncContext) getFileHash(path string) ([32]byte, error) {
  var r [32]byte

  // open file
  f, err := os.Open(path)
  if err != nil {
    return r, err
  }
  defer f.Close()

  // hash file
  hash := sha256.New()
  if _, err := io.Copy(hash, f); err != nil {
    return r, err
  }

  // copy sum to result, return success
  hash.Sum(r[:])
  return r, nil
}

// Check the size and hash in the metadata file against the full file.
//
// Note: This method is called from a goroutine and returns it's value
// via the internal channel.
func (me syncContext) check(metaUrl, fullUrl string) {
  // build result
  r := syncMessage {
    check: checkResult {
      metaUrl:  metaUrl,
      // build paths
      metaPath: filepath.Join(me.dstDir, filepath.Base(metaUrl)),
      fullPath: filepath.Join(me.dstDir, filepath.Base(fullUrl)),
    },
  }

  // get size of full file
  size, err := getFileSize(r.check.fullPath)
  if errors.Is(err, fs.ErrNotExist) {
    r.check.match = false
    me.ch <- r
    return
  } else if err != nil {
    r.check.err = err
    me.ch <- r
    return
  }

  // get meta hash
  m, err := me.getMeta(r.check.metaPath)
  if err != nil {
    r.check.err = err
    me.ch <- r
    return
  }

  // check for file size match
  if size != m.GzSize {
    r.check.match = false
    me.ch <- r
    return
  }

  // get full hash
  fh, err := me.getFileHash(r.check.fullPath)
  if err != nil {
    r.check.err = err
    me.ch <- r
    return
  }

  // return result
  r.check.match = (bytes.Compare(m.Sha256[:], fh[:]) == 0)
  me.ch <- r
}

// Sync to destination directory and return an array of updated files.
func Sync(config SyncConfig, cache Cache, dstDir string) []string {
  // log.Debug().Str("dstDir", dstDir).Msg("Sync")

  // build sync context
  ctx := newSyncContext(config, cache, dstDir)

  // get meta URL to full URL map
  metaUrls := config.getMetaUrls()

  // fetch meta URLs
  for metaUrl, _ := range(metaUrls) {
    log.Debug().Str("url", metaUrl).Msg("init")
    go ctx.fetch(metaUrl)
  }

  // build list of metas to check
  checkUrls := make([]string, 0, len(metaUrls))

  // read meta results
  for range(metaUrls) {
    r := <-ctx.ch
    sl := log.With().Str("url", r.fetch.src).Logger()

    if r.fetch.err != nil {
      // URL error
      sl.Error().Err(r.fetch.err).Send()
    } else if !r.fetch.modified {
      // URL not modified
      sl.Debug().Msg("not modified")
    } else {
      // URL updated
      sl.Debug().Msg("update")

      // build request headers
      headers := map[string]string {
        "if-none-match": r.fetch.headers.Get("etag"),
        "if-modified-since": r.fetch.headers.Get("last-modified"),
      }

      // save headers to cache
      if err := cache.Set(r.fetch.src, headers); err != nil {
        sl.Error().Err(r.fetch.err).Msg("cache.Set")
      } else {
        // append to list of check URLs
        checkUrls = append(checkUrls, r.fetch.src)
      }
    }
  }

  // check size and hash in updated metas
  logArray("checkUrls", checkUrls)
  for _, metaUrl := range(checkUrls) {
    go ctx.check(metaUrl, metaUrls[metaUrl])
  }

  // build list of non-meta URLs to sync.
  syncUrls := make([]string, 0, len(metaUrls))
  syncUrls = append(syncUrls, config.GetCpeDictUrl())

  for range(checkUrls) {
    r := <-ctx.ch

    // create sublogger
    sl := log.With().
      Str("metaUrl", r.check.metaUrl).
      Str("metaPath", r.check.metaPath).
      Str("fullPath", r.check.fullPath).
      Logger()

    if r.check.err != nil {
      sl.Error().Err(r.check.err).Send()
    } else if r.check.match {
      sl.Debug().Msg("match")
    } else {
      syncUrls = append(syncUrls, metaUrls[r.check.metaUrl])
    }
  }

  logArray("syncUrls", syncUrls)
  for _, fullUrl := range(syncUrls) {
    go ctx.fetch(fullUrl)
  }

  // build list of changed files
  changed := make([]string, 0, len(syncUrls))

  // read sync results
  for range(syncUrls) {
    r := <-ctx.ch
    // build sublogger
    sl := log.With().Str("url", r.fetch.src).Logger()

    if r.fetch.err != nil {
      sl.Error().Err(r.fetch.err).Send()
    } else if !r.fetch.modified {
      sl.Debug().Msg("not modified")
    } else {
      sl.Debug().Msg("update")

      // build request headers
      headers := map[string]string {
        "if-none-match": r.fetch.headers.Get("etag"),
        "if-modified-since": r.fetch.headers.Get("last-modified"),
      }

      // save headers to cache
      if err := cache.Set(r.fetch.src, headers); err != nil {
        sl.Error().Err(r.fetch.err).Msg("cache.Set")
      } else {
        // append to list of changed files
        changed = append(changed, filepath.Base(r.fetch.src))
      }
    }
  }

  // log changed files
  logArray("changed", changed)

  // return success
  return changed
}