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Fix SeekEnd direction and ReadInt24 sign extension - #131

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Fix SeekEnd direction and ReadInt24 sign extension#131
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@youdie006youdie006 commented Sep 1, 2026

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Two bugs in BinaryReader, both in the part of binary.go that no test or in-tree caller reaches.

1. Seek with io.SeekEnd moves the wrong way

binary.go:316 says // Seek complies with io.Seeker. For whence == 2 the guard is already
right — off must be in [-Len(), 0] — but the arithmetic subtracts it (binary.go:332):

r.pos=r.f.Len() -off

With a negative offset that lands past the end. Against bytes.Reader, the standard library's
io.Seeker, on an 8-byte buffer:

Seek( 0, SeekEnd) stdlib=8 parse=8
Seek( -1, SeekEnd) stdlib=7 parse=9 mismatch
Seek( -4, SeekEnd) stdlib=4 parse=12 mismatch
Seek( -8, SeekEnd) stdlib=0 parse=16 mismatch

and no error is returned, so reading the last four bytes silently yields zeros:

Seek(-4, io.SeekEnd) -> pos=12, err=<nil>
ReadUint32() -> 0x00000000 (want 0x05060708)

2. ReadInt24 does not sign-extend

binary.go:441:

func (r*BinaryReader) ReadInt24() int32 {
returnint32(r.ReadUint24())
}

ReadInt8/16/32/64 get the sign for free because the conversion keeps the width. There is no
int24, so the 24-bit value is zero-extended into an int32 and every negative number comes back
positive. The package's own writer is the oracle:

WriteInt24( -1) -> ff ff ff -> ReadInt24() = 16777215
WriteInt24( -2) -> ff ff fe -> ReadInt24() = 16777214
WriteInt24(-8388608) -> 80 00 00 -> ReadInt24() = 8388608
for contrast: Int8 -1 -> -1, Int16 -1 -> -1, Int32 -1 -> -1

Change

binary.go:332: - becomes +. binary.go:441: sign-extend from bit 23,
int32(r.ReadUint24()<<8) >> 8.

Why these survived

grep -c "Seek\|ReadInt24" *_test.go is 0 in every test file, and binary.go has no in-tree
caller outside its own definitions — so neither line has ever been executed by the suite.

Tests added to binary_test.go in the existing style: the four SeekEnd offsets checked against
their expected positions plus the read that follows, and a WriteInt24/ReadInt24 round trip over
-1, -2, -8388608, 0, 1, 8388607. Red with only binary.go reverted, green with the
change; go test -race -count=1 ./... passes across all packages and gofmt -l is clean on both
files.

Happy to split these into two PRs if you would rather review them separately — they are independent
apart from living in the same untested file.

Disclosure: prepared with AI assistance; I verified both reproductions, the stdlib differential and
the red/green runs myself.

@youdie006
Fix SeekEnd direction and ReadInt24 sign extension
5676369
Seek with io.SeekEnd computed Len()-off, so a negative offset moved past
the end instead of before it and later reads returned zeros. ReadInt24
converted a 24-bit value straight to int32, which cannot carry the sign
the way the int8/16/32/64 siblings do.
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function addCopyButtons() {
document.querySelectorAll('pre code').forEach(function(codeBlock) {
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var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '
Fix SeekEnd direction and ReadInt24 sign extension by youdie006 · Pull Request #131 · tdewolff/parse · GitHub
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Fix SeekEnd direction and ReadInt24 sign extension - #131
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youdie006 wants to merge 1 commit into
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youdie006:binaryreader-seekend-int24
Open
Fix SeekEnd direction and ReadInt24 sign extension#131
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
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@youdie006
@youdie006youdie006
commented
Sep 1, 2026

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Two bugs in BinaryReader, both in the part of binary.go that no test or in-tree caller reaches.
1. Seek with io.SeekEnd moves the wrong way
binary.go:316 says // Seek complies with io.Seeker. For whence == 2 the guard is already

right — off must be in [-Len(), 0] — but the arithmetic subtracts it (binary.go:332):
r.pos=r.f.Len() -off
With a negative offset that lands past the end. Against bytes.Reader, the standard library's
io.Seeker, on an 8-byte buffer:
Seek( 0, SeekEnd) stdlib=8 parse=8
Seek( -1, SeekEnd) stdlib=7 parse=9 mismatch
Seek( -4, SeekEnd) stdlib=4 parse=12 mismatch
Seek( -8, SeekEnd) stdlib=0 parse=16 mismatch

and no error is returned, so reading the last four bytes silently yields zeros:
Seek(-4, io.SeekEnd) -> pos=12, err=<nil>
ReadUint32() -> 0x00000000 (want 0x05060708)

2. ReadInt24 does not sign-extend
binary.go:441:
func (r*BinaryReader) ReadInt24() int32 {
returnint32(r.ReadUint24())
}
ReadInt8/16/32/64 get the sign for free because the conversion keeps the width. There is no
int24, so the 24-bit value is zero-extended into an int32 and every negative number comes back

positive. The package's own writer is the oracle:
WriteInt24( -1) -> ff ff ff -> ReadInt24() = 16777215
WriteInt24( -2) -> ff ff fe -> ReadInt24() = 16777214
WriteInt24(-8388608) -> 80 00 00 -> ReadInt24() = 8388608
for contrast: Int8 -1 -> -1, Int16 -1 -> -1, Int32 -1 -> -1

Change
binary.go:332: - becomes +. binary.go:441: sign-extend from bit 23,
int32(r.ReadUint24()<<8) >> 8.
Why these survived
grep -c "Seek\|ReadInt24" *_test.go is 0 in every test file, and binary.go has no in-tree

caller outside its own definitions — so neither line has ever been executed by the suite.
Tests added to binary_test.go in the existing style: the four SeekEnd offsets checked against

their expected positions plus the read that follows, and a WriteInt24/ReadInt24 round trip over
-1, -2, -8388608, 0, 1, 8388607. Red with only binary.go reverted, green with the

change; go test -race -count=1 ./... passes across all packages and gofmt -l is clean on both

files.
Happy to split these into two PRs if you would rather review them separately — they are independent

apart from living in the same untested file.
Disclosure: prepared with AI assistance; I verified both reproductions, the stdlib differential and

the red/green runs myself.
@youdie006
Fix SeekEnd direction and ReadInt24 sign extension
5676369
Seek with io.SeekEnd computed Len()-off, so a negative offset moved past
the end instead of before it and later reads returned zeros. ReadInt24
converted a 24-bit value straight to int32, which cannot carry the sign
the way the int8/16/32/64 siblings do.
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var style = document.createElement('style');
style.textContent = '
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color-scheme: dark light;
}
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try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Fix SeekEnd direction and ReadInt24 sign extension by youdie006 · Pull Request #131 · tdewolff/parse · GitHub
Skip to content
Fix SeekEnd direction and ReadInt24 sign extension - #131
Open
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
Open
Fix SeekEnd direction and ReadInt24 sign extension#131
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
Conversation
@youdie006
@youdie006youdie006
commented
Sep 1, 2026

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Two bugs in BinaryReader, both in the part of binary.go that no test or in-tree caller reaches.
1. Seek with io.SeekEnd moves the wrong way
binary.go:316 says // Seek complies with io.Seeker. For whence == 2 the guard is already

right — off must be in [-Len(), 0] — but the arithmetic subtracts it (binary.go:332):
r.pos=r.f.Len() -off
With a negative offset that lands past the end. Against bytes.Reader, the standard library's
io.Seeker, on an 8-byte buffer:
Seek( 0, SeekEnd) stdlib=8 parse=8
Seek( -1, SeekEnd) stdlib=7 parse=9 mismatch
Seek( -4, SeekEnd) stdlib=4 parse=12 mismatch
Seek( -8, SeekEnd) stdlib=0 parse=16 mismatch

and no error is returned, so reading the last four bytes silently yields zeros:
Seek(-4, io.SeekEnd) -> pos=12, err=<nil>
ReadUint32() -> 0x00000000 (want 0x05060708)

2. ReadInt24 does not sign-extend
binary.go:441:
func (r*BinaryReader) ReadInt24() int32 {
returnint32(r.ReadUint24())
}
ReadInt8/16/32/64 get the sign for free because the conversion keeps the width. There is no
int24, so the 24-bit value is zero-extended into an int32 and every negative number comes back

positive. The package's own writer is the oracle:
WriteInt24( -1) -> ff ff ff -> ReadInt24() = 16777215
WriteInt24( -2) -> ff ff fe -> ReadInt24() = 16777214
WriteInt24(-8388608) -> 80 00 00 -> ReadInt24() = 8388608
for contrast: Int8 -1 -> -1, Int16 -1 -> -1, Int32 -1 -> -1

Change
binary.go:332: - becomes +. binary.go:441: sign-extend from bit 23,
int32(r.ReadUint24()<<8) >> 8.
Why these survived
grep -c "Seek\|ReadInt24" *_test.go is 0 in every test file, and binary.go has no in-tree

caller outside its own definitions — so neither line has ever been executed by the suite.
Tests added to binary_test.go in the existing style: the four SeekEnd offsets checked against

their expected positions plus the read that follows, and a WriteInt24/ReadInt24 round trip over
-1, -2, -8388608, 0, 1, 8388607. Red with only binary.go reverted, green with the

change; go test -race -count=1 ./... passes across all packages and gofmt -l is clean on both

files.
Happy to split these into two PRs if you would rather review them separately — they are independent

apart from living in the same untested file.
Disclosure: prepared with AI assistance; I verified both reproductions, the stdlib differential and

the red/green runs myself.
@youdie006
Fix SeekEnd direction and ReadInt24 sign extension
5676369
Seek with io.SeekEnd computed Len()-off, so a negative offset moved past
the end instead of before it and later reads returned zeros. ReadInt24
converted a 24-bit value straight to int32, which cannot carry the sign
the way the int8/16/32/64 siblings do.
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
(function() {
var ref = document.referrer;
var terms = [];
if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {
var url = new URL(ref);
var q = url.searchParams.get('q') || url.searchParams.get('p');
if (q) {
terms = q.split(/\s+/).filter(function(t) { return t.length > 2; });
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if (terms.length === 0) return;
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mutations.forEach(function(m) {
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observer.observe(document.body, { childList: true, subtree: true });
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} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
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(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Fix SeekEnd direction and ReadInt24 sign extension by youdie006 · Pull Request #131 · tdewolff/parse · GitHub
Skip to content
Fix SeekEnd direction and ReadInt24 sign extension - #131
Open
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
Open
Fix SeekEnd direction and ReadInt24 sign extension#131
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
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@youdie006
@youdie006youdie006
commented
Sep 1, 2026

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Two bugs in BinaryReader, both in the part of binary.go that no test or in-tree caller reaches.
1. Seek with io.SeekEnd moves the wrong way
binary.go:316 says // Seek complies with io.Seeker. For whence == 2 the guard is already

right — off must be in [-Len(), 0] — but the arithmetic subtracts it (binary.go:332):
r.pos=r.f.Len() -off
With a negative offset that lands past the end. Against bytes.Reader, the standard library's
io.Seeker, on an 8-byte buffer:
Seek( 0, SeekEnd) stdlib=8 parse=8
Seek( -1, SeekEnd) stdlib=7 parse=9 mismatch
Seek( -4, SeekEnd) stdlib=4 parse=12 mismatch
Seek( -8, SeekEnd) stdlib=0 parse=16 mismatch

and no error is returned, so reading the last four bytes silently yields zeros:
Seek(-4, io.SeekEnd) -> pos=12, err=<nil>
ReadUint32() -> 0x00000000 (want 0x05060708)

2. ReadInt24 does not sign-extend
binary.go:441:
func (r*BinaryReader) ReadInt24() int32 {
returnint32(r.ReadUint24())
}
ReadInt8/16/32/64 get the sign for free because the conversion keeps the width. There is no
int24, so the 24-bit value is zero-extended into an int32 and every negative number comes back

positive. The package's own writer is the oracle:
WriteInt24( -1) -> ff ff ff -> ReadInt24() = 16777215
WriteInt24( -2) -> ff ff fe -> ReadInt24() = 16777214
WriteInt24(-8388608) -> 80 00 00 -> ReadInt24() = 8388608
for contrast: Int8 -1 -> -1, Int16 -1 -> -1, Int32 -1 -> -1

Change
binary.go:332: - becomes +. binary.go:441: sign-extend from bit 23,
int32(r.ReadUint24()<<8) >> 8.
Why these survived
grep -c "Seek\|ReadInt24" *_test.go is 0 in every test file, and binary.go has no in-tree

caller outside its own definitions — so neither line has ever been executed by the suite.
Tests added to binary_test.go in the existing style: the four SeekEnd offsets checked against

their expected positions plus the read that follows, and a WriteInt24/ReadInt24 round trip over
-1, -2, -8388608, 0, 1, 8388607. Red with only binary.go reverted, green with the

change; go test -race -count=1 ./... passes across all packages and gofmt -l is clean on both

files.
Happy to split these into two PRs if you would rather review them separately — they are independent

apart from living in the same untested file.
Disclosure: prepared with AI assistance; I verified both reproductions, the stdlib differential and

the red/green runs myself.
@youdie006
Fix SeekEnd direction and ReadInt24 sign extension
5676369
Seek with io.SeekEnd computed Len()-off, so a negative offset moved past
the end instead of before it and later reads returned zeros. ReadInt24
converted a 24-bit value straight to int32, which cannot carry the sign
the way the int8/16/32/64 siblings do.
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Strip utm_, fbclid, gclid, etc. from all links on page
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var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',
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})();
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try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
Fix SeekEnd direction and ReadInt24 sign extension by youdie006 · Pull Request #131 · tdewolff/parse · GitHub
Skip to content
Fix SeekEnd direction and ReadInt24 sign extension - #131
Open
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
Open
Fix SeekEnd direction and ReadInt24 sign extension#131
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
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@youdie006
@youdie006youdie006
commented
Sep 1, 2026

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Two bugs in BinaryReader, both in the part of binary.go that no test or in-tree caller reaches.
1. Seek with io.SeekEnd moves the wrong way
binary.go:316 says // Seek complies with io.Seeker. For whence == 2 the guard is already

right — off must be in [-Len(), 0] — but the arithmetic subtracts it (binary.go:332):
r.pos=r.f.Len() -off
With a negative offset that lands past the end. Against bytes.Reader, the standard library's
io.Seeker, on an 8-byte buffer:
Seek( 0, SeekEnd) stdlib=8 parse=8
Seek( -1, SeekEnd) stdlib=7 parse=9 mismatch
Seek( -4, SeekEnd) stdlib=4 parse=12 mismatch
Seek( -8, SeekEnd) stdlib=0 parse=16 mismatch

and no error is returned, so reading the last four bytes silently yields zeros:
Seek(-4, io.SeekEnd) -> pos=12, err=<nil>
ReadUint32() -> 0x00000000 (want 0x05060708)

2. ReadInt24 does not sign-extend
binary.go:441:
func (r*BinaryReader) ReadInt24() int32 {
returnint32(r.ReadUint24())
}
ReadInt8/16/32/64 get the sign for free because the conversion keeps the width. There is no
int24, so the 24-bit value is zero-extended into an int32 and every negative number comes back

positive. The package's own writer is the oracle:
WriteInt24( -1) -> ff ff ff -> ReadInt24() = 16777215
WriteInt24( -2) -> ff ff fe -> ReadInt24() = 16777214
WriteInt24(-8388608) -> 80 00 00 -> ReadInt24() = 8388608
for contrast: Int8 -1 -> -1, Int16 -1 -> -1, Int32 -1 -> -1

Change
binary.go:332: - becomes +. binary.go:441: sign-extend from bit 23,
int32(r.ReadUint24()<<8) >> 8.
Why these survived
grep -c "Seek\|ReadInt24" *_test.go is 0 in every test file, and binary.go has no in-tree

caller outside its own definitions — so neither line has ever been executed by the suite.
Tests added to binary_test.go in the existing style: the four SeekEnd offsets checked against

their expected positions plus the read that follows, and a WriteInt24/ReadInt24 round trip over
-1, -2, -8388608, 0, 1, 8388607. Red with only binary.go reverted, green with the

change; go test -race -count=1 ./... passes across all packages and gofmt -l is clean on both

files.
Happy to split these into two PRs if you would rather review them separately — they are independent

apart from living in the same untested file.
Disclosure: prepared with AI assistance; I verified both reproductions, the stdlib differential and

the red/green runs myself.
@youdie006
Fix SeekEnd direction and ReadInt24 sign extension
5676369
Seek with io.SeekEnd computed Len()-off, so a negative offset moved past
the end instead of before it and later reads returned zeros. ReadInt24
converted a 24-bit value straight to int32, which cannot carry the sign
the way the int8/16/32/64 siblings do.
Sign up for freeto join this conversation on GitHub.
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@youdie006

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
}
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var lastUrl = location.href;
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if (location.href !== lastUrl) {
lastUrl = location.href;
setTimeout(tryTheater, 500);
}
}, 1000);
// Also try on player load
var observer = new MutationObserver(tryTheater);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Fix SeekEnd direction and ReadInt24 sign extension by youdie006 · Pull Request #131 · tdewolff/parse · GitHub
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Fix SeekEnd direction and ReadInt24 sign extension - #131
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Fix SeekEnd direction and ReadInt24 sign extension#131
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Sep 1, 2026

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Two bugs in BinaryReader, both in the part of binary.go that no test or in-tree caller reaches.
1. Seek with io.SeekEnd moves the wrong way
binary.go:316 says // Seek complies with io.Seeker. For whence == 2 the guard is already

right — off must be in [-Len(), 0] — but the arithmetic subtracts it (binary.go:332):
r.pos=r.f.Len() -off
With a negative offset that lands past the end. Against bytes.Reader, the standard library's
io.Seeker, on an 8-byte buffer:
Seek( 0, SeekEnd) stdlib=8 parse=8
Seek( -1, SeekEnd) stdlib=7 parse=9 mismatch
Seek( -4, SeekEnd) stdlib=4 parse=12 mismatch
Seek( -8, SeekEnd) stdlib=0 parse=16 mismatch

and no error is returned, so reading the last four bytes silently yields zeros:
Seek(-4, io.SeekEnd) -> pos=12, err=<nil>
ReadUint32() -> 0x00000000 (want 0x05060708)

2. ReadInt24 does not sign-extend
binary.go:441:
func (r*BinaryReader) ReadInt24() int32 {
returnint32(r.ReadUint24())
}
ReadInt8/16/32/64 get the sign for free because the conversion keeps the width. There is no
int24, so the 24-bit value is zero-extended into an int32 and every negative number comes back

positive. The package's own writer is the oracle:
WriteInt24( -1) -> ff ff ff -> ReadInt24() = 16777215
WriteInt24( -2) -> ff ff fe -> ReadInt24() = 16777214
WriteInt24(-8388608) -> 80 00 00 -> ReadInt24() = 8388608
for contrast: Int8 -1 -> -1, Int16 -1 -> -1, Int32 -1 -> -1

Change
binary.go:332: - becomes +. binary.go:441: sign-extend from bit 23,
int32(r.ReadUint24()<<8) >> 8.
Why these survived
grep -c "Seek\|ReadInt24" *_test.go is 0 in every test file, and binary.go has no in-tree

caller outside its own definitions — so neither line has ever been executed by the suite.
Tests added to binary_test.go in the existing style: the four SeekEnd offsets checked against

their expected positions plus the read that follows, and a WriteInt24/ReadInt24 round trip over
-1, -2, -8388608, 0, 1, 8388607. Red with only binary.go reverted, green with the

change; go test -race -count=1 ./... passes across all packages and gofmt -l is clean on both

files.
Happy to split these into two PRs if you would rather review them separately — they are independent

apart from living in the same untested file.
Disclosure: prepared with AI assistance; I verified both reproductions, the stdlib differential and

the red/green runs myself.
@youdie006
Fix SeekEnd direction and ReadInt24 sign extension
5676369
Seek with io.SeekEnd computed Len()-off, so a negative offset moved past
the end instead of before it and later reads returned zeros. ReadInt24
converted a 24-bit value straight to int32, which cannot carry the sign
the way the int8/16/32/64 siblings do.
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var __m = "*";
var __re = new RegExp('^' + ".*" + '
Fix SeekEnd direction and ReadInt24 sign extension by youdie006 · Pull Request #131 · tdewolff/parse · GitHub
Skip to content
Fix SeekEnd direction and ReadInt24 sign extension - #131
Open
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
Open
Fix SeekEnd direction and ReadInt24 sign extension#131
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
Conversation
@youdie006
@youdie006youdie006
commented
Sep 1, 2026

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Two bugs in BinaryReader, both in the part of binary.go that no test or in-tree caller reaches.
1. Seek with io.SeekEnd moves the wrong way
binary.go:316 says // Seek complies with io.Seeker. For whence == 2 the guard is already

right — off must be in [-Len(), 0] — but the arithmetic subtracts it (binary.go:332):
r.pos=r.f.Len() -off
With a negative offset that lands past the end. Against bytes.Reader, the standard library's
io.Seeker, on an 8-byte buffer:
Seek( 0, SeekEnd) stdlib=8 parse=8
Seek( -1, SeekEnd) stdlib=7 parse=9 mismatch
Seek( -4, SeekEnd) stdlib=4 parse=12 mismatch
Seek( -8, SeekEnd) stdlib=0 parse=16 mismatch

and no error is returned, so reading the last four bytes silently yields zeros:
Seek(-4, io.SeekEnd) -> pos=12, err=<nil>
ReadUint32() -> 0x00000000 (want 0x05060708)

2. ReadInt24 does not sign-extend
binary.go:441:
func (r*BinaryReader) ReadInt24() int32 {
returnint32(r.ReadUint24())
}
ReadInt8/16/32/64 get the sign for free because the conversion keeps the width. There is no
int24, so the 24-bit value is zero-extended into an int32 and every negative number comes back

positive. The package's own writer is the oracle:
WriteInt24( -1) -> ff ff ff -> ReadInt24() = 16777215
WriteInt24( -2) -> ff ff fe -> ReadInt24() = 16777214
WriteInt24(-8388608) -> 80 00 00 -> ReadInt24() = 8388608
for contrast: Int8 -1 -> -1, Int16 -1 -> -1, Int32 -1 -> -1

Change
binary.go:332: - becomes +. binary.go:441: sign-extend from bit 23,
int32(r.ReadUint24()<<8) >> 8.
Why these survived
grep -c "Seek\|ReadInt24" *_test.go is 0 in every test file, and binary.go has no in-tree

caller outside its own definitions — so neither line has ever been executed by the suite.
Tests added to binary_test.go in the existing style: the four SeekEnd offsets checked against

their expected positions plus the read that follows, and a WriteInt24/ReadInt24 round trip over
-1, -2, -8388608, 0, 1, 8388607. Red with only binary.go reverted, green with the

change; go test -race -count=1 ./... passes across all packages and gofmt -l is clean on both

files.
Happy to split these into two PRs if you would rather review them separately — they are independent

apart from living in the same untested file.
Disclosure: prepared with AI assistance; I verified both reproductions, the stdlib differential and

the red/green runs myself.
@youdie006
Fix SeekEnd direction and ReadInt24 sign extension
5676369
Seek with io.SeekEnd computed Len()-off, so a negative offset moved past
the end instead of before it and later reads returned zeros. ReadInt24
converted a 24-bit value straight to int32, which cannot carry the sign
the way the int8/16/32/64 siblings do.
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Universal Dark Mode - works on any site
(function() {
var enabled = true;
function applyDarkMode() {
if (!enabled) return;
// Create style element if it doesn't exist
var style = document.getElementById('universal-dark-mode-style');
if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
}
// Dark mode CSS - inverts colors but preserves images/video
style.textContent = '
/* Invert everything except media */
html {
filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
}
/* Restore images, videos, iframes, canvas */
img, video, iframe, canvas, svg, picture, [style*="background-image"] {
filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
.no-dark-mode, .no-dark-mode *,
[data-theme="light"], [data-theme="light"],
.ace_editor, .ace_editor *,
.CodeMirror, .CodeMirror *,
.monaco-editor, .monaco-editor *,
.markdown-body pre, .markdown-body pre *,
.highlight, .highlight *,
pre code, pre code * {
filter: none !important;
}
/* Fix common UI elements */
.modal, .popup, .dropdown-menu, .tooltip, .popover {
filter: invert(1) hue-rotate(180deg) !important;
background: #2d2d44 !important;
border-color: #444 !important;
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::-webkit-scrollbar-thumb { background: #444 !important; }
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::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
';
}
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var style = document.getElementById('universal-dark-mode-style');
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e.preventDefault();
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console.log('[Universal Dark Mode] Disabled');
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if (enabled && !document.getElementById('universal-dark-mode-style')) {
applyDarkMode();
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observer.observe(document.head, { childList: true });
console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');
})();
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();
Fix SeekEnd direction and ReadInt24 sign extension by youdie006 · Pull Request #131 · tdewolff/parse · GitHub
Skip to content
Fix SeekEnd direction and ReadInt24 sign extension - #131
Open
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
Open
Fix SeekEnd direction and ReadInt24 sign extension#131
youdie006 wants to merge 1 commit into
tdewolff:masterfrom 
youdie006:binaryreader-seekend-int24
Conversation
@youdie006
@youdie006youdie006
commented
Sep 1, 2026

Copy link


Copy Markdown

Two bugs in BinaryReader, both in the part of binary.go that no test or in-tree caller reaches.
1. Seek with io.SeekEnd moves the wrong way
binary.go:316 says // Seek complies with io.Seeker. For whence == 2 the guard is already

right — off must be in [-Len(), 0] — but the arithmetic subtracts it (binary.go:332):
r.pos=r.f.Len() -off
With a negative offset that lands past the end. Against bytes.Reader, the standard library's
io.Seeker, on an 8-byte buffer:
Seek( 0, SeekEnd) stdlib=8 parse=8
Seek( -1, SeekEnd) stdlib=7 parse=9 mismatch
Seek( -4, SeekEnd) stdlib=4 parse=12 mismatch
Seek( -8, SeekEnd) stdlib=0 parse=16 mismatch

and no error is returned, so reading the last four bytes silently yields zeros:
Seek(-4, io.SeekEnd) -> pos=12, err=<nil>
ReadUint32() -> 0x00000000 (want 0x05060708)

2. ReadInt24 does not sign-extend
binary.go:441:
func (r*BinaryReader) ReadInt24() int32 {
returnint32(r.ReadUint24())
}
ReadInt8/16/32/64 get the sign for free because the conversion keeps the width. There is no
int24, so the 24-bit value is zero-extended into an int32 and every negative number comes back

positive. The package's own writer is the oracle:
WriteInt24( -1) -> ff ff ff -> ReadInt24() = 16777215
WriteInt24( -2) -> ff ff fe -> ReadInt24() = 16777214
WriteInt24(-8388608) -> 80 00 00 -> ReadInt24() = 8388608
for contrast: Int8 -1 -> -1, Int16 -1 -> -1, Int32 -1 -> -1

Change
binary.go:332: - becomes +. binary.go:441: sign-extend from bit 23,
int32(r.ReadUint24()<<8) >> 8.
Why these survived
grep -c "Seek\|ReadInt24" *_test.go is 0 in every test file, and binary.go has no in-tree

caller outside its own definitions — so neither line has ever been executed by the suite.
Tests added to binary_test.go in the existing style: the four SeekEnd offsets checked against

their expected positions plus the read that follows, and a WriteInt24/ReadInt24 round trip over
-1, -2, -8388608, 0, 1, 8388607. Red with only binary.go reverted, green with the

change; go test -race -count=1 ./... passes across all packages and gofmt -l is clean on both

files.
Happy to split these into two PRs if you would rather review them separately — they are independent

apart from living in the same untested file.
Disclosure: prepared with AI assistance; I verified both reproductions, the stdlib differential and

the red/green runs myself.
@youdie006
Fix SeekEnd direction and ReadInt24 sign extension
5676369
Seek with io.SeekEnd computed Len()-off, so a negative offset moved past
the end instead of before it and later reads returned zeros. ReadInt24
converted a 24-bit value straight to int32, which cannot carry the sign
the way the int8/16/32/64 siblings do.
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