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Optimize cross reference object offset validation by avoiding nested loop #935

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merged 2 commits into from
Nov 13, 2024
Merged

Optimize cross reference object offset validation by avoiding nested loop #935

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67a6401
Optimize cross reference object offset validation by avoiding nested …
madelson Nov 12, 2024
067d5d5
Address https://github.com/UglyToad/PdfPig/pull/935#discussion_r18395…
madelson Nov 13, 2024
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16 changes: 8 additions & 8 deletions src/UglyToad.PdfPig.Core/IndirectReference.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@
/// <summary>
/// Used to uniquely identify and refer to objects in the PDF file.
/// </summary>
public readonly struct IndirectReference
public readonly struct IndirectReference : IEquatable<IndirectReference>
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Seeing that dictionary insertion was the bottleneck initially made me think that Equals boxing might be the issue so I added IEquatable. However, this didn't make a noticeable difference in my particular case. Given how frequently this type is used as a dictionary key I think it is worth keeping (I notice that other PdfPig types do have this).

{
/// <summary>
/// A positive integer object number.
Expand All @@ -31,15 +31,15 @@ public IndirectReference(long objectNumber, int generation)
}

/// <inheritdoc />
public override bool Equals(object obj)
public bool Equals(IndirectReference other)
{
if (obj is IndirectReference reference)
{
return reference.ObjectNumber == ObjectNumber
&& reference.Generation == Generation;
}
return other.ObjectNumber == ObjectNumber && other.Generation == Generation;
}

return false;
/// <inheritdoc />
public override bool Equals(object obj)
{
return obj is IndirectReference other && Equals(other);
}

/// <inheritdoc />
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2 changes: 1 addition & 1 deletion src/UglyToad.PdfPig/CrossReference/CrossReferenceTable.cs
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Expand Up @@ -49,7 +49,7 @@ internal CrossReferenceTable(CrossReferenceType type, IReadOnlyDictionary<Indire
Trailer = trailer ?? throw new ArgumentNullException(nameof(trailer));
CrossReferenceOffsets = crossReferenceOffsets ?? throw new ArgumentNullException(nameof(crossReferenceOffsets));

var result = new Dictionary<IndirectReference, long>();
var result = new Dictionary<IndirectReference, long>(capacity: objectOffsets.Count);
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Fly-by optimization since we're just copying the dictionary.

foreach (var objectOffset in objectOffsets)
{
result[objectOffset.Key] = objectOffset.Value;
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13 changes: 7 additions & 6 deletions src/UglyToad.PdfPig/Parser/FileStructure/CrossReferenceObjectOffsetValidator.cs
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Expand Up @@ -24,11 +24,12 @@ public static bool ValidateCrossReferenceOffsets(IInputBytes bytes, CrossReferen
return true;
}

var builderOffsets = new Dictionary<IndirectReference, long>();

var bruteForceOffsets = BruteForceSearcher.GetObjectLocations(bytes);
if (bruteForceOffsets.Count > 0)
{
// Pre-allocate capacity for at least the bruteForceOffsets, since we'll be adding all of them
var builderOffsets = new Dictionary<IndirectReference, long>(bruteForceOffsets.Count);

// find all object streams
foreach (var entry in crossReferenceTable.ObjectOffsets)
{
Expand All @@ -39,11 +40,11 @@ public static bool ValidateCrossReferenceOffsets(IInputBytes bytes, CrossReferen
// TODO: more validation of streams.
builderOffsets[entry.Key] = entry.Value;
}
}

foreach (var item in bruteForceOffsets)
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Moving this loo outside the foreach loop that starts on 33 was the real win. As far as I can tell, the two loops are independent, and we're still maintaining the order such that bruteForceOffsets are added after object offsets and thus could override them in builderOffsets.

I don't see a reason why this needs to be an inner loop.

{
builderOffsets[item.Key] = item.Value;
}
foreach (var item in bruteForceOffsets)
{
builderOffsets[item.Key] = item.Value;
}

actualOffsets = builderOffsets;
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