// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements ExportData.

package types

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"os"
	"strconv"
	"strings"
)

func readGopackHeader(buf *bufio.Reader) (name string, size int, err error) {
	// See $GOROOT/include/ar.h.
	hdr := make([]byte, 16+12+6+6+8+10+2)
	_, err = io.ReadFull(buf, hdr)
	if err != nil {
		return
	}
	if trace {
		fmt.Printf("header: %s", hdr)
	}
	s := strings.TrimSpace(string(hdr[16+12+6+6+8:][:10]))
	size, err = strconv.Atoi(s)
	if err != nil || hdr[len(hdr)-2] != '`' || hdr[len(hdr)-1] != '\n' {
		err = errors.New("invalid archive header")
		return
	}
	name = strings.TrimSpace(string(hdr[:16]))
	return
}

type dataReader struct {
	*bufio.Reader
	io.Closer
}

// ExportData returns a readCloser positioned at the beginning of the
// export data section of the given object/archive file, or an error.
// It is the caller's responsibility to close the readCloser.
//
func ExportData(filename string) (rc io.ReadCloser, err error) {
	file, err := os.Open(filename)
	if err != nil {
		return
	}

	defer func() {
		if err != nil {
			file.Close()
			// Add file name to error.
			err = fmt.Errorf("reading export data: %s: %v", filename, err)
		}
	}()

	buf := bufio.NewReader(file)

	// Read first line to make sure this is an object file.
	line, err := buf.ReadSlice('\n')
	if err != nil {
		return
	}
	if string(line) == "!<arch>\n" {
		// Archive file.  Scan to __.PKGDEF, which should
		// be second archive entry.
		var name string
		var size int

		// First entry should be __.SYMDEF.
		// Read and discard.
		if name, size, err = readGopackHeader(buf); err != nil {
			return
		}
		if name != "__.SYMDEF" {
			err = errors.New("go archive does not begin with __.SYMDEF")
			return
		}
		const block = 4096
		tmp := make([]byte, block)
		for size > 0 {
			n := size
			if n > block {
				n = block
			}
			_, err = io.ReadFull(buf, tmp[:n])
			if err != nil {
				return
			}
			size -= n
		}

		// Second entry should be __.PKGDEF.
		if name, size, err = readGopackHeader(buf); err != nil {
			return
		}
		if name != "__.PKGDEF" {
			err = errors.New("go archive is missing __.PKGDEF")
			return
		}

		// Read first line of __.PKGDEF data, so that line
		// is once again the first line of the input.
		line, err = buf.ReadSlice('\n')
		if err != nil {
			return
		}
	}

	// Now at __.PKGDEF in archive or still at beginning of file.
	// Either way, line should begin with "go object ".
	if !strings.HasPrefix(string(line), "go object ") {
		err = errors.New("not a go object file")
		return
	}

	// Skip over object header to export data.
	// Begins after first line with $$.
	for line[0] != '$' {
		line, err = buf.ReadSlice('\n')
		if err != nil {
			return
		}
	}

	rc = &dataReader{buf, file}
	return
}