'use strict';
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// NOTE: we only support baseline and progressive JPGs here
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// due to the structure of the loader class, we only get a buffer
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// with a maximum size of 4096 bytes. so if the SOF marker is outside
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// if this range we can't detect the file size correctly.
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function isJPG (buffer) { //, filepath
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var SOIMarker = buffer.toString('hex', 0, 2);
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return ('ffd8' === SOIMarker);
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}
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function extractSize (buffer, i) {
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return {
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'height' : buffer.readUInt16BE(i),
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'width' : buffer.readUInt16BE(i + 2)
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};
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}
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function validateBuffer (buffer, i) {
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// index should be within buffer limits
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if (i > buffer.length) {
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throw new TypeError('Corrupt JPG, exceeded buffer limits');
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}
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// Every JPEG block must begin with a 0xFF
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if (buffer[i] !== 0xFF) {
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throw new TypeError('Invalid JPG, marker table corrupted');
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}
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}
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function calculate (buffer) {
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// Skip 4 chars, they are for signature
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buffer = buffer.slice(4);
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var i, next;
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while (buffer.length) {
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// read length of the next block
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i = buffer.readUInt16BE(0);
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// ensure correct format
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validateBuffer(buffer, i);
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// 0xFFC0 is baseline standard(SOF)
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// 0xFFC1 is baseline optimized(SOF)
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// 0xFFC2 is progressive(SOF2)
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next = buffer[i + 1];
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if (next === 0xC0 || next === 0xC1 || next === 0xC2) {
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return extractSize(buffer, i + 5);
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}
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// move to the next block
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buffer = buffer.slice(i + 2);
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}
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throw new TypeError('Invalid JPG, no size found');
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}
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module.exports = {
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'detect': isJPG,
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'calculate': calculate
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};
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