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[Deflate] Reduce nesting in lengthEncode

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This should be a purely refactoring change with no effect on performance or functionality.
This commit is contained in:
Timofey Solomko
2021-10-18 18:30:15 +03:00
parent f2be0db44f
commit 113972bcce
1 changed files with 34 additions and 34 deletions
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Sources/Deflate/Deflate+Compress.swift
+34 -34
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@@ -159,46 +159,46 @@ extension Deflate: CompressionAlgorithm {
let threeByteCrc = CheckSums.crc32(data[inputIndex..<inputIndex + 3])
if let matchStartIndex = dictionary[threeByteCrc] {
// We need to update position of this match to keep distances as small as possible.
dictionary[threeByteCrc] = inputIndex
/// - Note: Minimum match length equals to three.
var matchLength = 3
/// Cyclic index which is used to compare bytes in match and in input.
var repeatIndex = matchStartIndex + matchLength
/// - Note: Maximum allowed distance equals to 32768.
let distance = inputIndex - matchStartIndex
// Again, the distance cannot be greater than 32768.
if distance <= 32768 {
while inputIndex + matchLength < data.count &&
data[inputIndex + matchLength] == data[repeatIndex] && matchLength < 258 {
matchLength += 1
repeatIndex += 1
if repeatIndex > inputIndex {
repeatIndex = matchStartIndex + 1
}
}
buffer.append(BLDCode.lengthDistance(UInt16(truncatingIfNeeded: matchLength),
UInt16(truncatingIfNeeded: distance)))
stats[Constants.lengthCode[matchLength - 3]] += 1 // Length symbol.
stats[286 + ((Constants.distanceBase.firstIndex { $0 > distance }) ?? 30) - 1] += 1 // Distance symbol.
inputIndex += matchLength
} else {
buffer.append(BLDCode.byte(byte))
stats[byte.toInt()] += 1
inputIndex += 1
}
} else {
// We need to remember where we met this three-byte sequence.
guard let matchStartIndex = dictionary[threeByteCrc] else {
// No match found.
// We need to save where we met this three-byte sequence.
dictionary[threeByteCrc] = inputIndex
buffer.append(BLDCode.byte(byte))
stats[byte.toInt()] += 1
inputIndex += 1
continue
}
// We need to update position of this match to keep distances as small as possible.
dictionary[threeByteCrc] = inputIndex
// Minimum match length equals to three.
var matchLength = 3
/// Cyclic index which is used to compare bytes in match and in input.
var repeatIndex = matchStartIndex + matchLength
// Maximum allowed distance equals to 32768.
let distance = inputIndex - matchStartIndex
guard distance <= 32768 else {
buffer.append(BLDCode.byte(byte))
stats[byte.toInt()] += 1
inputIndex += 1
continue
}
while inputIndex + matchLength < data.count &&
data[inputIndex + matchLength] == data[repeatIndex] && matchLength < 258 {
matchLength += 1
repeatIndex += 1
if repeatIndex > inputIndex {
repeatIndex = matchStartIndex + 1
}
}
buffer.append(BLDCode.lengthDistance(UInt16(truncatingIfNeeded: matchLength),
UInt16(truncatingIfNeeded: distance)))
stats[Constants.lengthCode[matchLength - 3]] += 1 // Length symbol.
stats[286 + ((Constants.distanceBase.firstIndex { $0 > distance }) ?? 30) - 1] += 1 // Distance symbol.
inputIndex += matchLength
}
// For last two bytes there certainly will be no match.