// Copyright 2020 The Gitea Authors. All rights reserved.
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file.

// +build !gogit

package git

import (
	"bufio"
	"bytes"
	"math"
	"strconv"
)

// ReadBatchLine reads the header line from cat-file --batch
// We expect:
// <sha> SP <type> SP <size> LF
func ReadBatchLine(rd *bufio.Reader) (sha []byte, typ string, size int64, err error) {
	sha, err = rd.ReadBytes(' ')
	if err != nil {
		return
	}
	sha = sha[:len(sha)-1]

	typ, err = rd.ReadString(' ')
	if err != nil {
		return
	}
	typ = typ[:len(typ)-1]

	var sizeStr string
	sizeStr, err = rd.ReadString('\n')
	if err != nil {
		return
	}

	size, err = strconv.ParseInt(sizeStr[:len(sizeStr)-1], 10, 64)
	return
}

// ReadTagObjectID reads a tag object ID hash from a cat-file --batch stream, throwing away the rest of the stream.
func ReadTagObjectID(rd *bufio.Reader, size int64) (string, error) {
	id := ""
	var n int64
headerLoop:
	for {
		line, err := rd.ReadBytes('\n')
		if err != nil {
			return "", err
		}
		n += int64(len(line))
		idx := bytes.Index(line, []byte{' '})
		if idx < 0 {
			continue
		}

		if string(line[:idx]) == "object" {
			id = string(line[idx+1 : len(line)-1])
			break headerLoop
		}
	}

	// Discard the rest of the tag
	discard := size - n
	for discard > math.MaxInt32 {
		_, err := rd.Discard(math.MaxInt32)
		if err != nil {
			return id, err
		}
		discard -= math.MaxInt32
	}
	_, err := rd.Discard(int(discard))
	return id, err
}

// ReadTreeID reads a tree ID from a cat-file --batch stream, throwing away the rest of the stream.
func ReadTreeID(rd *bufio.Reader, size int64) (string, error) {
	id := ""
	var n int64
headerLoop:
	for {
		line, err := rd.ReadBytes('\n')
		if err != nil {
			return "", err
		}
		n += int64(len(line))
		idx := bytes.Index(line, []byte{' '})
		if idx < 0 {
			continue
		}

		if string(line[:idx]) == "tree" {
			id = string(line[idx+1 : len(line)-1])
			break headerLoop
		}
	}

	// Discard the rest of the commit
	discard := size - n
	for discard > math.MaxInt32 {
		_, err := rd.Discard(math.MaxInt32)
		if err != nil {
			return id, err
		}
		discard -= math.MaxInt32
	}
	_, err := rd.Discard(int(discard))
	return id, err
}

// git tree files are a list:
// <mode-in-ascii> SP <fname> NUL <20-byte SHA>
//
// Unfortunately this 20-byte notation is somewhat in conflict to all other git tools
// Therefore we need some method to convert these 20-byte SHAs to a 40-byte SHA

// constant hextable to help quickly convert between 20byte and 40byte hashes
const hextable = "0123456789abcdef"

// to40ByteSHA converts a 20-byte SHA in a 40-byte slice into a 40-byte sha in place
// without allocations. This is at least 100x quicker that hex.EncodeToString
// NB This requires that sha is a 40-byte slice
func to40ByteSHA(sha []byte) []byte {
	for i := 19; i >= 0; i-- {
		v := sha[i]
		vhi, vlo := v>>4, v&0x0f
		shi, slo := hextable[vhi], hextable[vlo]
		sha[i*2], sha[i*2+1] = shi, slo
	}
	return sha
}

// ParseTreeLineSkipMode reads an entry from a tree in a cat-file --batch stream
// This simply skips the mode - saving a substantial amount of time and carefully avoids allocations - except where fnameBuf is too small.
// It is recommended therefore to pass in an fnameBuf large enough to avoid almost all allocations
//
// Each line is composed of:
// <mode-in-ascii-dropping-initial-zeros> SP <fname> NUL <20-byte SHA>
//
// We don't attempt to convert the 20-byte SHA to 40-byte SHA to save a lot of time
func ParseTreeLineSkipMode(rd *bufio.Reader, fnameBuf, shaBuf []byte) (fname, sha []byte, n int, err error) {
	var readBytes []byte
	// Skip the Mode
	readBytes, err = rd.ReadSlice(' ') // NB: DOES NOT ALLOCATE SIMPLY RETURNS SLICE WITHIN READER BUFFER
	if err != nil {
		return
	}
	n += len(readBytes)

	// Deal with the fname
	readBytes, err = rd.ReadSlice('\x00')
	copy(fnameBuf, readBytes)
	if len(fnameBuf) > len(readBytes) {
		fnameBuf = fnameBuf[:len(readBytes)] // cut the buf the correct size
	} else {
		fnameBuf = append(fnameBuf, readBytes[len(fnameBuf):]...) // extend the buf and copy in the missing bits
	}
	for err == bufio.ErrBufferFull { // Then we need to read more
		readBytes, err = rd.ReadSlice('\x00')
		fnameBuf = append(fnameBuf, readBytes...) // there is little point attempting to avoid allocations here so just extend
	}
	n += len(fnameBuf)
	if err != nil {
		return
	}
	fnameBuf = fnameBuf[:len(fnameBuf)-1] // Drop the terminal NUL
	fname = fnameBuf                      // set the returnable fname to the slice

	// Now deal with the 20-byte SHA
	idx := 0
	for idx < 20 {
		read := 0
		read, err = rd.Read(shaBuf[idx:20])
		n += read
		if err != nil {
			return
		}
		idx += read
	}
	sha = shaBuf
	return
}

// ParseTreeLine reads an entry from a tree in a cat-file --batch stream
// This carefully avoids allocations - except where fnameBuf is too small.
// It is recommended therefore to pass in an fnameBuf large enough to avoid almost all allocations
//
// Each line is composed of:
// <mode-in-ascii-dropping-initial-zeros> SP <fname> NUL <20-byte SHA>
//
// We don't attempt to convert the 20-byte SHA to 40-byte SHA to save a lot of time
func ParseTreeLine(rd *bufio.Reader, modeBuf, fnameBuf, shaBuf []byte) (mode, fname, sha []byte, n int, err error) {
	var readBytes []byte

	// Read the Mode
	readBytes, err = rd.ReadSlice(' ')
	if err != nil {
		return
	}
	n += len(readBytes)
	copy(modeBuf, readBytes)
	if len(modeBuf) > len(readBytes) {
		modeBuf = modeBuf[:len(readBytes)]
	} else {
		modeBuf = append(modeBuf, readBytes[len(modeBuf):]...)

	}
	mode = modeBuf[:len(modeBuf)-1] // Drop the SP

	// Deal with the fname
	readBytes, err = rd.ReadSlice('\x00')
	copy(fnameBuf, readBytes)
	if len(fnameBuf) > len(readBytes) {
		fnameBuf = fnameBuf[:len(readBytes)]
	} else {
		fnameBuf = append(fnameBuf, readBytes[len(fnameBuf):]...)
	}
	for err == bufio.ErrBufferFull {
		readBytes, err = rd.ReadSlice('\x00')
		fnameBuf = append(fnameBuf, readBytes...)
	}
	n += len(fnameBuf)
	if err != nil {
		return
	}
	fnameBuf = fnameBuf[:len(fnameBuf)-1]
	fname = fnameBuf

	// Deal with the 20-byte SHA
	idx := 0
	for idx < 20 {
		read := 0
		read, err = rd.Read(shaBuf[idx:20])
		n += read
		if err != nil {
			return
		}
		idx += read
	}
	sha = shaBuf
	return
}
