package common

import (
	"bytes"
	"fmt"
	"io"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
)

// The maximum size of a single line returned by ReadLinesBlock.
const maxLineSize = 256 * 1024

// Default size in bytes of a single block returned by ReadLinesBlock.
const defaultBlockSize = 64 * 1024

// ReadLinesBlock reads a block of lines delimited by '\n' from tailBuf and r into dstBuf.
//
// Trailing chars after the last newline are put into tailBuf.
//
// Returns (dstBuf, tailBuf).
func ReadLinesBlock(r io.Reader, dstBuf, tailBuf []byte) ([]byte, []byte, error) {
	if cap(dstBuf) < defaultBlockSize {
		dstBuf = bytesutil.Resize(dstBuf, defaultBlockSize)
	}
	dstBuf = append(dstBuf[:0], tailBuf...)
again:
	n, err := r.Read(dstBuf[len(dstBuf):cap(dstBuf)])
	// Check for error only if zero bytes read from r, i.e. no forward progress made.
	// Otherwise process the read data.
	if n == 0 {
		if err == nil {
			return dstBuf, tailBuf, fmt.Errorf("no forward progress made")
		}
		return dstBuf, tailBuf, err
	}
	dstBuf = dstBuf[:len(dstBuf)+n]

	// Search for the last newline in dstBuf and put the rest into tailBuf.
	nn := bytes.LastIndexByte(dstBuf[len(dstBuf)-n:], '\n')
	if nn < 0 {
		// Didn't found at least a single line.
		if len(dstBuf) > maxLineSize {
			return dstBuf, tailBuf, fmt.Errorf("too long line: more than %d bytes", maxLineSize)
		}
		if cap(dstBuf) < 2*len(dstBuf) {
			// Increase dsbBuf capacity, so more data could be read into it.
			dstBufLen := len(dstBuf)
			dstBuf = bytesutil.Resize(dstBuf, 2*cap(dstBuf))
			dstBuf = dstBuf[:dstBufLen]
		}
		goto again
	}

	// Found at least a single line. Return it.
	nn += len(dstBuf) - n
	tailBuf = append(tailBuf[:0], dstBuf[nn+1:]...)
	dstBuf = dstBuf[:nn]
	return dstBuf, tailBuf, nil
}