CODEWITHSUNDEEP
node.jsbuffer
Joss Bird
Joss Bird

Reputation: 311

Experiencing weird behaviour with Buffer.compare / Buffer.from

I'm trying to modify buffers, however when modifying them I wish them to be in utf-8 so I attempt to do this via myBuffer.toString('utf8') however if I make no changes and attempt to convert it back via Buffer.from(myBuffer, 'utf8'), I am presented with a completely new buffer on occasions.

These occasions seem to be when parsing an image file, instead of a html file.

My next step was to accept a bug or erroneous behaviour by comparing the two buffers using the following code:

//myBuffer is the buffer is wish to attempt to modify
let testParse = Buffer.from(myBuffer.toString('utf8'), 'utf8');
let editable  = Buffer.compare(myBuffer, testParse);
console.log(myBuffer);
console.log(testParse);
console.log(editable);

The following snippet however refuses to work and editable is always -1 here is an example output:

<Buffer 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52 00 00 01 10 00 00 00 5c 08 02 00 00 00 29 85 7d e1 00 00 15 31 49 44 41 54 78 01 ed 5d 05 94 db c8 b2 ... >
<Buffer ef bf bd 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52 00 00 01 10 00 00 00 5c 08 02 00 00 00 29 ef bf bd 7d ef bf bd 00 00 15 31 49 44 41 54 78 01 ef ... >
-1

As you can see the buffers are completely different however returns -1

another example where the buffers are both equal:

<Buffer 3c 21 64 6f 63 74 79 70 65 20 68 74 6d 6c 3e 3c 68 74 6d 6c 20 69 74 65 6d 73 63 6f 70 65 3d 22 22 20 69 74 65 6d 74 79 70 65 3d 22 68 74 74 70 3a 2f ... >
<Buffer 3c 21 64 6f 63 74 79 70 65 20 68 74 6d 6c 3e 3c 68 74 6d 6c 20 69 74 65 6d 73 63 6f 70 65 3d 22 22 20 69 74 65 6d 74 79 70 65 3d 22 68 74 74 70 3a 2f ... >
-1

As you can see both buffers are equal and -1 is still returned.

So my question is, what am I doing wrong so that the buffers cannot be compared properly? Any suggestions/criticism are welcome.

Upvotes: 0

Views: 175

Answers (1)

EMX
EMX

Reputation: 6219

You have to compare in the same encoding :

//:Buffer Comparison
const fs = require('fs')
fs.readFile(__dirname+"/test.jpg",(e,buffer)=>{
 let testParse = Buffer.from(buffer.toString('utf8'), 'utf8');
 let editable  = Buffer.compare(buffer, testParse);
 console.log("----: wrong method :----")
 console.log(buffer);
 console.log(testParse);
 console.log(editable);
 // You have to compare them in the same encoding :
 console.log("----: right method :----")
 let goodParse = Buffer.from(buffer.toString('utf8'));
 let editable2  = goodParse.compare(Buffer.from(buffer.toString('utf8')));
 console.log(buffer);
 console.log(goodParse);
 console.log(editable2);
})

As you can see, we load an image as a buffer, then it is parsed into utf8, so if you modify it, and then want to compare it to the original buffer. Since the modified was parsed to utf8 the original must also be parsed to utf8 in the moment of the comparison.

I hope you understand that explanation.

Console output:

----: wrong method :----
<Buffer ff d8 ff e0 00 10 4a 46 49 46 00 01 01 00 00 01 00 01 00 00 ff db 00 43 00 08 06 06 07 06 05 08 07 07 07 09 09 08 0a 0c 14 0d 0c 0b 0b 0c 19 12 13 0f ... >
<Buffer ef bf bd ef bf bd ef bf bd ef bf bd 00 10 4a 46 49 46 00 01 01 00 00 01 00 01 00 00 ef bf bd ef bf bd 00 43 00 08 06 06 07 06 05 08 07 07 07 09 09 08 ... >
1
----: right method :----
<Buffer ff d8 ff e0 00 10 4a 46 49 46 00 01 01 00 00 01 00 01 00 00 ff db 00 43 00 08 06 06 07 06 05 08 07 07 07 09 09 08 0a 0c 14 0d 0c 0b 0b 0c 19 12 13 0f ... >
<Buffer ef bf bd ef bf bd ef bf bd ef bf bd 00 10 4a 46 49 46 00 01 01 00 00 01 00 01 00 00 ef bf bd ef bf bd 00 43 00 08 06 06 07 06 05 08 07 07 07 09 09 08 ... >
0

Upvotes: 3

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