90 lines
1.8 KiB
Go
90 lines
1.8 KiB
Go
// Copyright (C) 2015 The Protocol Authors.
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package protocol
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// Ordering represents the relationship between two Vectors.
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type Ordering int
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const (
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Equal Ordering = iota
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Greater
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Lesser
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ConcurrentLesser
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ConcurrentGreater
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)
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// There's really no such thing as "concurrent lesser" and "concurrent
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// greater" in version vectors, just "concurrent". But it's useful to be able
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// to get a strict ordering between versions for stable sorts and so on, so we
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// return both variants. The convenience method Concurrent() can be used to
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// check for either case.
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// Compare returns the Ordering that describes a's relation to b.
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func (a Vector) Compare(b Vector) Ordering {
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var ai, bi int // index into a and b
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var av, bv Counter // value at current index
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result := Equal
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for ai < len(a) || bi < len(b) {
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var aMissing, bMissing bool
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if ai < len(a) {
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av = a[ai]
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} else {
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av = Counter{}
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aMissing = true
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}
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if bi < len(b) {
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bv = b[bi]
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} else {
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bv = Counter{}
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bMissing = true
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}
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switch {
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case av.ID == bv.ID:
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// We have a counter value for each side
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if av.Value > bv.Value {
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if result == Lesser {
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return ConcurrentLesser
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}
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result = Greater
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} else if av.Value < bv.Value {
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if result == Greater {
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return ConcurrentGreater
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}
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result = Lesser
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}
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case !aMissing && av.ID < bv.ID || bMissing:
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// Value is missing on the b side
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if av.Value > 0 {
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if result == Lesser {
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return ConcurrentLesser
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}
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result = Greater
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}
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case !bMissing && bv.ID < av.ID || aMissing:
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// Value is missing on the a side
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if bv.Value > 0 {
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if result == Greater {
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return ConcurrentGreater
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}
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result = Lesser
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}
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}
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if ai < len(a) && (av.ID <= bv.ID || bMissing) {
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ai++
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}
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if bi < len(b) && (bv.ID <= av.ID || aMissing) {
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bi++
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}
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}
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return result
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}
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