package streams

import (
	"io"
)

type MetadataInjector struct {
	io.Reader
	MetadataInterval int
	blockOffset      int
	Metadata         map[string]string
	metadataBuf      []byte
}

func NewMetadataInjector(r io.Reader, metadataInterval int) *MetadataInjector {
	return &MetadataInjector{
		Reader:           r,
		MetadataInterval: metadataInterval,
	}
}

func (mi *MetadataInjector) Read(data []byte) (n int, err error) {
	if mi.metadataBuf != nil && len(mi.metadataBuf) > 0 {
		bytesToRead := len(data)
		if bytesToRead < len(mi.metadataBuf) {
			// only read as much as possible
			copy(data, mi.metadataBuf[0:bytesToRead])
			n = bytesToRead
			mi.metadataBuf = mi.metadataBuf[bytesToRead:]
			return
		}
		// read everything
		copy(data, mi.metadataBuf)
		n = len(mi.metadataBuf)
		mi.metadataBuf = nil
		return
	}

	bytesToRead := mi.MetadataInterval - mi.blockOffset
	if bytesToRead > len(data) {
		bytesToRead = len(data)
	}
	if bytesToRead > 0 {
		n, err = mi.Reader.Read(data[0:bytesToRead])
		if err != nil {
			return
		}
		mi.blockOffset += n
	}
	if mi.blockOffset == mi.MetadataInterval {
		mi.generateMetadataBuf() // will be read in on next Read call
		mi.blockOffset = 0
	} else if mi.blockOffset > mi.MetadataInterval {
		panic("block offset higher than metadata interval, logical error")
	}
	return
}