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Use unchecked math in encodeInteger #328

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Merged
Lukasa merged 2 commits into from
Feb 25, 2022
Merged

Use unchecked math in encodeInteger #328

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4e18e3c
Use unchecked math in encodeInteger
Lukasa Feb 22, 2022
ce06262
Merge branch 'main' into cb-unchecked-encode
Lukasa Feb 25, 2022
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13 changes: 8 additions & 5 deletions Sources/NIOHPACK/IntegerCoding.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -31,7 +31,8 @@ func encodeInteger(_ value: UInt, to buffer: inout ByteBuffer,

let start = buffer.writerIndex

let k = (1 << prefix) - 1
// The prefix is always hard-coded, and must fit within 8, so unchecked math here is definitely safe.
let k = (1 &<< prefix) &- 1
var initialByte = prefixBits

if value < k {
Expand All @@ -48,15 +49,17 @@ func encodeInteger(_ value: UInt, to buffer: inout ByteBuffer,

// deduct the initial [prefix] bits from the value, then encode it seven bits at a time into
// the remaining bytes.
var n = value - UInt(k)
// We can safely use unchecked subtraction here: we know that `k` is zero or greater, and that `value` is
// either the same value or greater. As a result, this can be unchecked: it's always safe.
var n = value &- UInt(k)
while n >= 128 {
let nextByte = (1 << 7) | UInt8(n & 0x7f)
let nextByte = (1 << 7) | UInt8(truncatingIfNeeded: n & 0x7f)
buffer.writeInteger(nextByte)
n >>= 7
}

buffer.writeInteger(UInt8(n))
return buffer.writerIndex - start
buffer.writeInteger(UInt8(truncatingIfNeeded: n))
return buffer.writerIndex &- start
}

fileprivate let valueMask: UInt8 = 127
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