Algorithms & Data Structures
the bound is a claim, and it is checkable
Concept

Previous occurrence chain — where it appears

An array holding, for each row of a document index, the previous row belonging to the same document. It exists so that "is this the first row of its document in this range" is one comparison rather than a search — and a walk that visits rows left to right can answer the same question from what it has already reported.

Named by 8 essays across 4 fields — each of them below, with the objects they name alongside it.

range consideredminimum atC[at] < lonew document[476, 489)483 · document 4yesyes[476, 483)477 · document 1yesyes[476, 477)476 · document 0yesyes[478, 483)482 · document 2yesyes[478, 482)478 · document 0nono[484, 489)486 · document 3yesyes[484, 486)484 · document 0nono[487, 489)487 · document 2nono8 steps, 5 documents, and the two columns never differrows 476–489 · 8 documents8 steps

The array the walk never reads

Document listing compares a chain entry against the start of a range. The comparison is true exactly when the document has not been reported yet — which the walk already knows, because it just wrote it down.

indexes · Index range consideredminimum atC[at] < lonew document[476, 489)483 · document 4yesyes[476, 483)477 · document 1yesyes[476, 477)476 · document 0yesyes[478, 483)482 · document 2yesyes[478, 482)478 · document 0nono[484, 489)486 · document 3yesyes[484, 486)484 · document 0nono[487, 489)487 · document 2nono8 steps, 5 documents, and the two columns never differrows 476–489 · 8 documents8 steps

A document already in the answer

The same walk, with the chain's test replaced by a lookup in the answer so far. It reports the same documents at the same cost, and two exchanged lines make it lose nine of seventeen without failing.

structures · Index tree · suffix array2,363,886tree · text656,635tree · document array1,050,616tree · previous-occurrence chain2,363,886tree · range minimum4,727,772succinct · suffix array2,363,886succinct · text656,635succinct · document array1,050,616succinct · previous-occurrence chain2,363,886succinct · range minimum1,483,854chainless · suffix array2,363,886chainless · text656,635chainless · document array1,050,616chainless · range minimum1,483,854chainless · reported bitmap256131,327 characters · 256 documents6 parts

What the chain cost

The chain of previous occurrences is one row number per row — exactly as wide as the suffix array it sits beside, and the largest single part of a document-listing apparatus. It is now absent, and what replaces it is one bit per document.

space · Index every occurrence5610 documentsrange minimum and chain32510 documentsrange minimum, no chain9610 documentsone descent over D3810 documentsthe document arraythe document array, the chain and a range minimumthe document array, a range minimum and one bit per documentthe document array, in a wavelet tree56 occurrences in 10 documentsoperations inside the structure

The tree answers the question

The distinct documents in a range of rows are the distinct symbols of the document array in that range. A wavelet tree enumerates those in one descent, so the range minimum, the chain, the bitmap and the recursion all go at once.

indexes · Index a segment tree over the chain2.70x8,142,274 bitsa succinct range minimum over the chain1.62x4,898,356 bitsno chain at all0.84x2,534,726 bitsthe dashed rule is the index itself: a suffix array and the text131,327 characters · 256 documents2.70x → 0.84x

The apparatus that is smaller than its index

Answering "which documents hold this" at a price proportional to the answer used to cost 2.70 times the index it sits beside. Two changes later it costs 0.84, and the largest thing left is an array that says which document each row belongs to.

practice · Index range minimum and chain838,797255.0%range minimum, no chain285,46086.8%one descent over D119,97236.5%the document array, the chain and a range minimumthe document array, a range minimum and one bit per documentthe document array, in a wavelet tree64 documents · 16,447 characters14.3% of the published apparatus

The last array in the apparatus

The document-listing apparatus began as three arrays beside a suffix array. Two of them turned out to be machinery for reading the third, and both have gone. What is left is the document array, and it is the only one of the three that was information.

structures · Index range minimum and chain838,797255.0%range minimum, no chain285,46086.8%one descent over D119,97236.5%the document array, the chain and a range minimumthe document array, a range minimum and one bit per documentthe document array, in a wavelet tree64 documents · 16,447 characters14.3% of the published apparatus

The apparatus, three times smaller again

Eight hundred and thirty-nine thousand bits became two hundred and eighty-five thousand, and now a hundred and twenty thousand. The listing apparatus is fourteen per cent of what it was and holds one array, which is the only part of it that was ever information.

space · Index 01,024120 phrases · 1,024 characters60 drawn, every one pointing right

Every copy points right

A greedy self-referential parse chooses each phrase's source from text already produced, so an occurrence copied from another lies strictly to its right. That one fact removes a visited set from a propagation, exactly as a left-first walk removed an array one strand ago.

structures · Parse

Named alongside it

The objects these essays reach for when they reach for this one.

Document arrayDocument listingIndex sizeRange minimumOutput-sensitiveDocument collectionSuffix arrayWavelet treeBitmapInvariantSpaceSuccinct

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