syncthing/lib/protocol/vector_compare.go

90 lines
1.8 KiB
Go
Raw Normal View History

2015-03-20 08:58:32 +00:00
// Copyright (C) 2015 The Protocol Authors.
package protocol
// Ordering represents the relationship between two Vectors.
type Ordering int
const (
Equal Ordering = iota
Greater
Lesser
ConcurrentLesser
ConcurrentGreater
)
// There's really no such thing as "concurrent lesser" and "concurrent
// greater" in version vectors, just "concurrent". But it's useful to be able
// to get a strict ordering between versions for stable sorts and so on, so we
// return both variants. The convenience method Concurrent() can be used to
// check for either case.
// Compare returns the Ordering that describes a's relation to b.
func (a Vector) Compare(b Vector) Ordering {
var ai, bi int // index into a and b
var av, bv Counter // value at current index
result := Equal
for ai < len(a) || bi < len(b) {
var aMissing, bMissing bool
if ai < len(a) {
av = a[ai]
} else {
av = Counter{}
aMissing = true
}
if bi < len(b) {
bv = b[bi]
} else {
bv = Counter{}
bMissing = true
}
switch {
case av.ID == bv.ID:
// We have a counter value for each side
if av.Value > bv.Value {
if result == Lesser {
return ConcurrentLesser
}
result = Greater
} else if av.Value < bv.Value {
if result == Greater {
return ConcurrentGreater
}
result = Lesser
}
case !aMissing && av.ID < bv.ID || bMissing:
// Value is missing on the b side
if av.Value > 0 {
if result == Lesser {
return ConcurrentLesser
}
result = Greater
}
case !bMissing && bv.ID < av.ID || aMissing:
// Value is missing on the a side
if bv.Value > 0 {
if result == Greater {
return ConcurrentGreater
}
result = Lesser
}
}
if ai < len(a) && (av.ID <= bv.ID || bMissing) {
ai++
}
if bi < len(b) && (bv.ID <= av.ID || aMissing) {
bi++
}
}
return result
}