##qvm.go

package qvm

import (
    "encoding/binary"
    "fmt"
    "os"
)

const (
    VM_MAGIC_VER1 = 0x12721444
    VM_MAGIC_VER2 = 0x12721445
)

//QVMHeader represents the header for a QVMFile. Note that even if VM_MAGIC_VER1
//QVMHeader still contains a JumpTableLength field.
type QVMHeader struct {
    Magic uint32
    InstructionCount uint32
    CodeOffset uint32
    CodeLength uint32
    DataOffset uint32
    DataLength uint32
    LitLength uint32
    BssLength uint32
    JumpTableLength uint32
}

//Pretty simple, QVMFile has a header and the three sections normally embedded
//in a QVM file.
type QVMFile struct {
    Header QVMHeader
    Code []byte
    Data []byte
    Lit []byte
}

//Also simple. Takes an os.File and creates a QVMFile from it.
//Also performs a few sanity check to save the programmer from
//extra work and potentially malicious files.
func NewQVMFile(file *os.File) (*QVMFile, os.Error) {

    //Read the header...
    theFile := &QVMFile{QVMHeader{}, nil, nil, nil}
    if err := binary.Read(file, binary.LittleEndian, &theFile.Header); err != nil {
        return nil, err
    }

    //Run a couple sanity checks on the header...
    //First case: Unrecognized magic number
    if theFile.Header.Magic != VM_MAGIC_VER1 && theFile.Header.Magic != VM_MAGIC_VER2 {
        return nil, os.NewError(fmt.Sprintf("Unrecognized QVM version[Magic: 0x%08x]", theFile.Header.Magic))
    }

    //Second case: Magic is VM_MAGIC_VER1 but CodeOffset != 32
    if theFile.Header.Magic == VM_MAGIC_VER1 && theFile.Header.CodeOffset != 32 {
        return nil, os.NewError(fmt.Sprintf("Invalid code offset[%d] for magic(Ver1)[0x%08x]", theFile.Header.CodeOffset, theFile.Header.Magic))
    }

    //Third case: Magic is VM_MAGIC_VER2 but CodeOffset != 36
    if theFile.Header.Magic == VM_MAGIC_VER2 && theFile.Header.CodeOffset != 36 {
        return nil, os.NewError(fmt.Sprintf("Invalid code offset[%d] for magic(Ver2)[0x%08x]", theFile.Header.CodeOffset, theFile.Header.Magic))
    }

    //Fourth case: CodeOffset + CodeLength + (CodeLength % 4) != DataOffset
    if theFile.Header.CodeOffset + theFile.Header.CodeLength + (theFile.Header.CodeLength % 4) != theFile.Header.DataOffset {
        return nil, os.NewError(fmt.Sprintf("Invalid data offset[0x%x] or code length[0x%x]", theFile.Header.DataOffset, theFile.Header.CodeLength))
    }

    //Read the Code/Data/Lit sections...
    theFile.Code = make([]byte, theFile.Header.CodeLength)
    if _, err := file.ReadAt(theFile.Code, int64(theFile.Header.CodeOffset)); err != nil {
        return nil, err
    }

    theFile.Data = make([]byte, theFile.Header.DataLength)
    if _, err := file.ReadAt(theFile.Data, int64(theFile.Header.DataOffset)); err != nil {
        return nil, err
    }

    theFile.Lit = make([]byte, theFile.Header.LitLength)
    if _, err := file.ReadAt(theFile.Lit, int64(theFile.Header.DataOffset + theFile.Header.DataLength)); err != nil {
        return nil, err
    }

    return theFile, nil
}