src/iso9660.js

// Source: https://wiki.osdev.org/ISO_9660

// Limitations:
// - can only generate iso files
// - only supports a single directory, no file system hierarchy
// - root directory entry is limited to 2 KiB (~42 files)
// - filenames are normalised to 8.3 length and [A-Z0-9_.]

import { dbg_assert } from "./log.js";

const BLOCK_SIZE = 2 * 1024; // 0x800

const FILE_FLAGS_HIDDEN = 1 << 0;
const FILE_FLAGS_DIRECTORY = 1 << 1;
const FILE_FLAGS_ASSOCIATED_FILE = 1 << 2;
const FILE_FLAGS_HAS_EXTENDED_ATTRIBUTE_RECORD = 1 << 3;
const FILE_FLAGS_HAS_PERMISSIONS = 1 << 4;
const FILE_FLAGS_NOT_FINAL = 1 << 7;

/**
 * @param {Array.<{ name: string, contents: Uint8Array}>} files
 */
export function generate(files)
{
    const te = new TextEncoder();
    const date = new Date;

    const write8 = (b, v) => { b.buffer[b.offset++] = v; };
    const write_le16 = (b, v) => { b.buffer[b.offset++] = v; b.buffer[b.offset++] = v >> 8; };
    const write_le32 = (b, v) => { b.buffer[b.offset++] = v; b.buffer[b.offset++] = v >> 8; b.buffer[b.offset++] = v >> 16; b.buffer[b.offset++] = v >> 24; };
    const write_be16 = (b, v) => { b.buffer[b.offset++] = v >> 8; b.buffer[b.offset++] = v; };
    const write_be32 = (b, v) => { b.buffer[b.offset++] = v >> 24; b.buffer[b.offset++] = v >> 16; b.buffer[b.offset++] = v >> 8; b.buffer[b.offset++] = v; };
    const write_lebe16 = (b, v) => { write_le16(b, v); write_be16(b, v); };
    const write_lebe32 = (b, v) => { write_le32(b, v); write_be32(b, v); };
    const fill = (b, len, v) => { b.buffer.fill(v, b.offset, b.offset += len); };
    const write_ascii = (b, v) => { b.offset += te.encodeInto(v, b.buffer.subarray(b.offset)).written; };
    const write_padded_ascii = (b, len, v) => { b.offset += te.encodeInto(v.padEnd(len), b.buffer.subarray(b.offset)).written; };
    const write_dummy_date_ascii = b => { fill(b, 16, 0x20); write8(b, 0); };
    const write_date_compact = b => {
        write8(b, date.getUTCFullYear() - 1900);
        write8(b, 1 + date.getUTCMonth());
        write8(b, date.getUTCDate());
        write8(b, date.getUTCHours());
        write8(b, date.getUTCMinutes());
        write8(b, date.getUTCSeconds());
        write8(b, 0);
    };
    const skip = (b, len) => { b.offset += len; };

    const write_record = (b, name, flags, is_special, lba, len) => {
        if(!is_special) name = sanitise_filename(name) + ";1";
        // write name first and get its length
        const START = buffer.offset;
        const NAME_OFFSET = 33;
        const name_len = te.encodeInto(name, b.buffer.subarray(b.offset + NAME_OFFSET)).written;
        const pad = (name_len & 1) ? 0 : 1;
        const len_field = 33 + name_len + pad;
        dbg_assert(len_field < 256);
        write8(buffer, len_field);      // Length of directory record
        write8(buffer, 0);              // Extended Attribute Record length
        write_lebe32(buffer, lba);      // Location of extent (LBA)
        write_lebe32(buffer, len);      // Data length (size of extent)
        write_date_compact(buffer);
        write8(buffer, flags);
        write8(buffer, 0);              // File unit size for files recorded in interleaved mode, zero otherwise
        write8(buffer, 0);              // Interleave gap size for files recorded in interleaved mode, zero otherwise
        write_lebe16(buffer, 1);        // Volume sequence number - the volume that this extent is recorded on
        write8(buffer, name_len);       // length of file name
        dbg_assert(buffer.offset === START + NAME_OFFSET);
        skip(buffer, name_len + pad);   // File name: was already written
        dbg_assert(buffer.offset === START + len_field);
    };
    const write_special_directory_record = (b, name, lba, len) => write_record(b, name, FILE_FLAGS_DIRECTORY, true, lba, len);
    const write_file_record = (b, name, lba, len) => write_record(b, name, 0, false, lba, len);

    function round_byte_size_to_block_size(n)
    {
        return 1 + Math.floor((n - 1) / BLOCK_SIZE);
    }
    dbg_assert(round_byte_size_to_block_size(0) === 0);
    dbg_assert(round_byte_size_to_block_size(1) === 1);
    dbg_assert(round_byte_size_to_block_size(BLOCK_SIZE - 1) === 1);
    dbg_assert(round_byte_size_to_block_size(BLOCK_SIZE) === 1);
    dbg_assert(round_byte_size_to_block_size(BLOCK_SIZE + 1) === 2);
    dbg_assert(round_byte_size_to_block_size(2 * BLOCK_SIZE) === 2);
    dbg_assert(round_byte_size_to_block_size(2 * BLOCK_SIZE + 1) === 3);
    dbg_assert(round_byte_size_to_block_size(10 * BLOCK_SIZE + 1) === 11);

    function to_msdos_filename(name)
    {
        const dot = name.lastIndexOf(".");
        if(dot === -1) return name.substr(0, 8);
        return name.substr(0, Math.min(8, dot)) + "." + name.substr(dot + 1, 3);
    }

    dbg_assert(to_msdos_filename("abcdefghijkl.qwerty") === "abcdefgh.qwe");
    dbg_assert(to_msdos_filename("abcdefghijkl") === "abcdefgh");

    function sanitise_filename(name)
    {
        return to_msdos_filename(name.toUpperCase().replace(/[^A-Z0-9_.]/g, ""));
    }

    // layout:
    // (lba = one block of BLOCK_SIZE bytes)
    // LBA   | contents
    // ------+--------
    // 0..15 | System Area (could be used for mbr, but not used by us)
    //    16 | Primary Volume Descriptor
    //    17 | Volume Descriptor Set Terminator
    //    18 | empty
    //    19 | Little Endian Path Table
    //    20 | empty
    //    21 | Big Endian Path Table
    //    22 | empty
    //    23 | Root directory
    // 24..n | File contents
    const SYSTEM_AREA_SIZE = 16 * BLOCK_SIZE;
    const PRIMARY_VOLUME_LBA = 16;
    const VOLUME_SET_TERMINATOR_LBA = 17;
    const LE_PATH_TABLE_LBA = 19;
    const BE_PATH_TABLE_LBA = 21;
    const ROOT_DIRECTORY_LBA = 23;
    const LE_PATH_TABLE_SIZE = BLOCK_SIZE;
    const BE_PATH_TABLE_SIZE = BLOCK_SIZE;
    const ROOT_DIRECTORY_SIZE = BLOCK_SIZE;

    let next_file_lba = 24;
    files = files.map(({ name, contents }) => {
        const lba = next_file_lba;
        next_file_lba += round_byte_size_to_block_size(contents.length);
        name = to_msdos_filename(name);
        return { name, contents, lba };
    });

    const N_LBAS = next_file_lba;
    const total_size = N_LBAS * BLOCK_SIZE;

    const buffer = {
        buffer: new Uint8Array(total_size),
        offset: SYSTEM_AREA_SIZE,
    };

    // LBA 16: Primary Volume Descriptor
    dbg_assert(buffer.offset === PRIMARY_VOLUME_LBA * BLOCK_SIZE);
    write8(buffer, 0x01);                    // Volume Descriptor type: Primary Volume Descriptor
    write_ascii(buffer, "CD001");            // Always CD001
    write8(buffer, 0x01);                    // Version
    write8(buffer, 0x00);                    // unused
    write_padded_ascii(buffer, 32, "V86");   // System Identifier
    write_padded_ascii(buffer, 32, "CDROM"); // Identification of this volume
    skip(buffer, 8);                         // unused
    write_lebe32(buffer, N_LBAS);
    skip(buffer, 32);                        // unused
    dbg_assert(buffer.offset === 0x8000 + 120);

    write_lebe16(buffer, 1); // Volume Set Size
    write_lebe16(buffer, 1); // Volume Sequence Number
    dbg_assert(buffer.offset === 0x8080);

    write_lebe16(buffer, BLOCK_SIZE);

    write_lebe32(buffer, 10);               // Path Table Size
    write_le32(buffer, LE_PATH_TABLE_LBA);  // Location of Type-L Path Table
    write_le32(buffer, 0);                  // Location of the Optional Type-L Path Table
    write_be32(buffer, BE_PATH_TABLE_LBA);  // Location of Type-M Path Table
    write_be32(buffer, 0);                  // Location of the Optional Type-M Path Table
    dbg_assert(buffer.offset === 0x8000 + 156);

    // Directory entry for the root directory
    write_special_directory_record(buffer, "\x00", ROOT_DIRECTORY_LBA, 0x800);
    dbg_assert(buffer.offset === 0x8000 + 190);

    fill(buffer, 128, 0x20); // Volume Set Identifier
    fill(buffer, 128, 0x20); // Publisher Identifier
    fill(buffer, 128, 0x20); // Data Preparer Identifier
    fill(buffer, 128, 0x20); // Application Identifier
    fill(buffer,  37, 0x20); // Copyright File Identifier
    fill(buffer,  37, 0x20); // Abstract File Identifier
    fill(buffer,  37, 0x20); // Bibliographic File Identifier

    dbg_assert(buffer.offset === 0x8000 + 813);

    write_dummy_date_ascii(buffer); // Volume Creation Date and Time
    write_dummy_date_ascii(buffer); // Volume Modification Date and Time
    write_dummy_date_ascii(buffer); // Volume Expiration Date and Time
    write_dummy_date_ascii(buffer); // Volume Effective Date and Time

    write8(buffer, 0x01); // File Structure Version
    dbg_assert(buffer.offset === 0x8000 + 882);

    write8(buffer, 0x00); // Unused
    skip(buffer, 512);    // Application Used
    skip(buffer, 653);    // Reserved

    // LBA 17: Volume Descriptor Set Terminator
    dbg_assert(buffer.offset === VOLUME_SET_TERMINATOR_LBA * BLOCK_SIZE);
    write8(buffer, 0xFF);           // 0xFF: Volume Descriptor Set Terminator
    write_ascii(buffer, "CD001");   // Always CD001
    write8(buffer, 0x01);           // Version

    // LBA 19: Little Endian Path Table
    buffer.offset = LE_PATH_TABLE_LBA * BLOCK_SIZE;
    write8(buffer, 0x01);                   // Length of Directory Identifier
    write8(buffer, 0x00);                   // Extended Attribute Record Length
    write_le32(buffer, ROOT_DIRECTORY_LBA); // Location of Extent (LBA)
    write_le16(buffer, 1);                  // Directory number of parent directory
    write_ascii(buffer, "\x00");            // file name
    dbg_assert(buffer.offset < LE_PATH_TABLE_LBA * BLOCK_SIZE + LE_PATH_TABLE_SIZE);

    // LBA 21: Big Endian Path Table
    buffer.offset = BE_PATH_TABLE_LBA * BLOCK_SIZE;
    write8(buffer, 0x01);                   // Length of Directory Identifier
    write8(buffer, 0x00);                   // Extended Attribute Record Length
    write_be32(buffer, ROOT_DIRECTORY_LBA); // Location of Extent (LBA)
    write_be16(buffer, 1);                  // Directory number of parent directory
    write_ascii(buffer, "\x00");            // file name
    dbg_assert(buffer.offset < BE_PATH_TABLE_LBA * BLOCK_SIZE + BE_PATH_TABLE_SIZE);

    // LBA 23: root directory
    buffer.offset = ROOT_DIRECTORY_LBA * BLOCK_SIZE;
    write_special_directory_record(buffer, "\x00", ROOT_DIRECTORY_LBA, 0x800);  // "."
    write_special_directory_record(buffer, "\x01", ROOT_DIRECTORY_LBA, 0x800);  // ".."
    for(const { name, contents, lba } of files)
    {
        write_file_record(buffer, name, lba, contents.length);
    }
    // TODO: this assertion can fail if too many files are used as input
    // ROOT_DIRECTORY_SIZE should be choosen dynamically
    dbg_assert(buffer.offset < ROOT_DIRECTORY_LBA * BLOCK_SIZE + ROOT_DIRECTORY_SIZE);

    // file contents
    for(let { contents, lba } of files)
    {
        buffer.buffer.set(contents, lba * BLOCK_SIZE);
    }

    return buffer.buffer;
}

/**
 * @param {Uint8Array} buffer
 */
export function is_probably_iso9660_file(buffer)
{
    return (
        buffer.length >= 17 * BLOCK_SIZE &&
        buffer[BLOCK_SIZE + 0] ===  1 && // Primary Volume Descriptor
        buffer[BLOCK_SIZE + 1] === 67 && // "C"
        buffer[BLOCK_SIZE + 2] === 68 && // "D"
        buffer[BLOCK_SIZE + 3] === 48 && // "0"
        buffer[BLOCK_SIZE + 4] === 48 && // "0"
        buffer[BLOCK_SIZE + 5] === 49    // "1"
    );
}