// checkExistingFile checks if a harvester has to be started for a already known file
// For existing files the following options exist:
// * Last reading position is 0, no harvester has to be started as old harvester probably still busy
// * The old known modification time is older then the current one. Start at last known position
// * The new file is not the same as the old file, means file was renamed
// ** New file is actually really a new file, start a new harvester
// ** Renamed file has a state, continue there
func (p ProspectorLog) checkExistingFile(h *harvester.Harvester, newFile *input.File, oldFile *input.File) {

	logp.Debug("prospector", "Update existing file for harvesting: %s", h.Path)

	// We assume it is the same file, but it wasn't
	if !oldFile.IsSameFile(newFile) {

		logp.Debug("prospector", "File previously found: %s", h.Path)

		if previousFile, err := p.getPreviousFile(h.Path, h.Stat.Fileinfo); err == nil {
			p.continueExistingFile(h, previousFile)
		} else {
			// File is not the same file we saw previously, it must have rotated and is a new file
			logp.Debug("prospector", "Launching harvester on rotated file: %s", h.Path)

			// Forget about the previous harvester and let it continue on the old file - so start a new channel to use with the new harvester
			h.Stat.Ignore()

			// Start a new harvester on the path
			h.Start()
		}

		// Keep the old file in missingFiles so we don't rescan it if it was renamed and we've not yet reached the new filename
		// We only need to keep it for the remainder of this iteration then we can assume it was deleted and forget about it
		p.missingFiles[h.Path] = oldFile.FileInfo

	} else if h.Stat.Finished() && oldFile.FileInfo.ModTime() != h.Stat.Fileinfo.ModTime() {
		// Resume harvesting of an old file we've stopped harvesting from
		// Start a harvester on the path; a file was just modified and it doesn't have a harvester
		// The offset to continue from will be stored in the harvester channel - so take that to use and also clear the channel
		p.resumeHarvesting(h, <-h.Stat.Return)
	} else {
		logp.Debug("prospector", "Not harvesting, file didn't change: %s", h.Path)
	}
}

// Continue reading on an existing file.
// The given file was renamed from another file we know -> The same harvester channel is linked as the old file
// The file param is only used for logging
func (p *ProspectorLog) continueExistingFile(h *harvester.Harvester, previousFile string) {
	logp.Debug("prospector", "Launching harvester on renamed file. File rename was detected: %s -> %s", previousFile, h.Path)

	lastinfo := p.harvesterStats[previousFile]
	h.Stat.Continue(&lastinfo)

	// Update state because of file rotation
	h.UpdateState()
}

// Start / resume harvester with a predefined offset
func (p *ProspectorLog) resumeHarvesting(h *harvester.Harvester, offset int64) {

	logp.Debug("prospector", "Start / resuming harvester of file: %s", h.Path)
	h.SetOffset(offset)
	h.Start()
}