256 lines
8.0 KiB
JavaScript
256 lines
8.0 KiB
JavaScript
/**
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* Handles the loading of LTG files
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*
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* Copyright (C) 2012 Mike Gerwitz
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Affero General Public License for more details.
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*
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* You should have received a copy of the GNU Affero General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*
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*
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* LTG files contain the game tiles and additional metadata. Specifically, the
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* structure of the header is:
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* - name (string), 40 bytes
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* - author (string), 30 bytes
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* - description (string), 245 bytes
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* - id (string), 5 bytes, "LTG1\0"
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* - mask offset (32-bit integer), 4 bytes (little-endian)
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* ~ (see TLTGREC struct in LTANK.H of the original game sources)
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*
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* Immediately following the header is the game tile set (a bitmap), immediately
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* after which we find the mask bitmap (at the mask offset).
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*
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* In the original game (written in C), the loading of this file into the
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* necessary data structures was trivial and highly performant. In the case of
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* ECMAScript, we are left to string parsing. With the string in memory, we will
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* cut out the necessary segments.
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*
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* At that point, we can easily convert the binary bitmap data into usable
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* images by creating new Image objects in memory and assigning the `src'
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* attribute to "data:image/bmp;base64,B", where B is the base64-encoded bitmap.
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* (To help visualize the data, one can open the LTG file in his/her favorite
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* text editor and search for "BM" (0x424D), which is the header field used to
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* identify the file as a bitmap image.)
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*/
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/**
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* Loads tiles and metadata from LTG files
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*/
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ltjs.LtgLoader = Class( 'LtgLoader',
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{
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/** various data segment byte offsets and ranges **/
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'private const _POS_NAME': [ 0, 40 ],
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'private const _POS_AUTHOR': [ 40, 30 ],
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'private const _POS_DESC': [ 70, 315 ],
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'private const _POS_ID': [ 315, 320 ],
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'private const _POS_MOFF': [ 320, 324 ],
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/**
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* Beginning of game bitmap (one byte past the header)
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* @type {number}
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*/
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'private const _OFFSET_HEADER_END': 324,
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/**
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* Load LTG file from memory and return the raw data
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*
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* @param {string} ltg_data binary LTG data
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*
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* @return {Object} LTG metadata and bitmaps (sans mask offset)
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*/
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'public fromString': function( ltg_data )
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{
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var mask_offset = this._getMaskOffsetFromData( ltg_data );
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return {
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name: this._getNameFromData( ltg_data ),
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author: this._getAuthorFromData( ltg_data ),
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desc: this._getDescFromData( ltg_data ),
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id: this._getIdFromData( ltg_data ),
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tiles: this._getBitmapDataUrl(
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this._getGameBitmap( ltg_data, mask_offset )
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),
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mask: this._getBitmapDataUrl(
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this._getMaskBitmap( ltg_data, mask_offset )
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)
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};
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},
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/**
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* Retrieve the requested portion of the given data, optionally stripping
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* null bytes
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*
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* @param {string} ltg_data source LTG data
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* @param {string} sgmt name of segment to retrieve (constant)
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* @param {=boolean} stripnull whether to strip null bytes (default true)
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*
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* @return {string} requested segment
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*/
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'private _getDataSegment': function( ltg_data, sgmt, stripnull )
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{
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// strip null bytes by default
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stripnull = ( stripnull === undefined ) ? true : false;
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if ( typeof sgmt === 'string' )
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{
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sgmt = this.__self.$( sgmt );
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}
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return String.prototype.substr.apply( ltg_data, sgmt )
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.split( '\x00' )[ 0 ];
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},
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/**
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* Retrieve LTG name from given LTG data
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*
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* @param {string} ltg_data raw LTG data
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*
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* @return {string} LTG name, null bytes stripped
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*/
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'private _getNameFromData': function( ltg_data )
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{
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return this._getDataSegment( ltg_data, '_POS_NAME' );
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},
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/**
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* Retrieve author name from the given LTG data
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*
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* @param {string} ltg_data raw LTG data
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*
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* @return {string} LTG author, null bytes stripped
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*/
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'private _getAuthorFromData': function( ltg_data )
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{
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return this._getDataSegment( ltg_data, '_POS_AUTHOR' );
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},
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/**
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* Retrieve description from the given LTG data
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*
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* @param {string} ltg_data raw LTG data
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*
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* @return {string} LTG description, null bytes stripped
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*/
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'private _getDescFromData': function( ltg_data )
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{
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return this._getDataSegment( ltg_data, '_POS_DESC' );
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},
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/**
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* Retrieve id from the given LTG data
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*
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* @param {string} ltg_data raw LTG data
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*
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* @return {string} LTG id, null bytes stripped
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*/
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'private _getIdFromData': function( ltg_data )
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{
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return this._getDataSegment( ltg_data, '_POS_ID' );
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},
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/**
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* Retrieve mask bitmap offset (relative to beginning of file) from the
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* given LTG data
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*
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* The mask is stored as a 32-bit integer, little-endian.
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*
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* @param {string} ltg_data raw LTG data
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*
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* @return {number} LTG mask offset in bytes
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*/
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'private _getMaskOffsetFromData': function( ltg_data )
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{
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// grab the data and don't bother stripping off the null bytes (it would
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// function the same with them stripped, but let's avoid the confusion
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// since we are supposed to be working with a 32-bit value)
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var data = this._getDataSegment( ltg_data, '_POS_MOFF', false ),
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i = data.length,
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offset = 0;
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// convert the DWORD entry (little-endian format, 32-bit) into an
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// integer that we can work with
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while ( i-- )
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{
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offset += ( data.charCodeAt( i ) << ( 8 * i ) );
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}
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return offset;
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},
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/**
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* Return data URL for the given bitmap data
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*
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* The data URL may be used with any image element in place of an external
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* resource. It consists of a "data:" prefix, MIME type and the
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* base64-encoded data.
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*
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* @param {string} data binary bitmap data
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*
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* @return {string} data URL corresponding to the given bitmap data
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*/
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'private _getBitmapDataUrl': function( data )
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{
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return 'data:image/bmp;base64,' + btoa( data );
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},
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/**
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* Extracts the game bitmap from the given LTG data
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*
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* While the beginning offset of the game bitmap is static, the end is
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* determined by the mask offset. The game bitmap would be displayed
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* properly even if we read to the end of the file, but that is incorrect
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* and poor practice.
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*
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* @param {string} ltg_data raw LTG data
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* @param {number} mask_offset mask bitmap offset in bytes
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*
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* @return {string} game bitmap data
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*/
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'private _getGameBitmap': function( ltg_data, mask_offset )
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{
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var bmp_offset = this.__self.$( '_OFFSET_HEADER_END' );
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// return the bitmap data up until the mask offset
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return ltg_data.substr( bmp_offset, ( mask_offset - bmp_offset ) );
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},
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/**
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* Extracts the mask bitmap from the given LTG data
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*
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* The mask bitmap position must be provided and consists of the remainder
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* of the file.
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*
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* @param {string} ltg_data raw LTG data
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* @param {number} mask_offset mask bitmap offset in bytes
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*
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* @return {string} mask bitmap data
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*/
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'private _getMaskBitmap': function( ltg_data, mask_offset )
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{
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// the mask bitmap accounts for the remainder of the data
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return ltg_data.substr( mask_offset );
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}
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} );
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