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bae9f6d2 JC |
1 | package shadow |
2 | ||
3 | import ( | |
4 | "container/list" | |
5 | "sync" | |
6 | ) | |
7 | ||
8 | // OrderedValue is a struct that keeps track of a value in the order | |
9 | // it is set. Each time Value() is called, it will return the most recent | |
10 | // calls value then discard it. | |
11 | // | |
12 | // This is unlike Value that returns the same value once it is set. | |
13 | type OrderedValue struct { | |
14 | lock sync.Mutex | |
15 | values *list.List | |
16 | waiters *list.List | |
17 | } | |
18 | ||
19 | // Value returns the last value that was set, or blocks until one | |
20 | // is received. | |
21 | func (w *OrderedValue) Value() interface{} { | |
22 | w.lock.Lock() | |
23 | ||
24 | // If we have a pending value already, use it | |
25 | if w.values != nil && w.values.Len() > 0 { | |
26 | front := w.values.Front() | |
27 | w.values.Remove(front) | |
28 | w.lock.Unlock() | |
29 | return front.Value | |
30 | } | |
31 | ||
32 | // No pending value, create a waiter | |
33 | if w.waiters == nil { | |
34 | w.waiters = list.New() | |
35 | } | |
36 | ||
37 | var val Value | |
38 | w.waiters.PushBack(&val) | |
39 | w.lock.Unlock() | |
40 | ||
41 | // Return the value once we have it | |
42 | return val.Value() | |
43 | } | |
44 | ||
45 | // SetValue sets the latest value. | |
46 | func (w *OrderedValue) SetValue(v interface{}) { | |
47 | w.lock.Lock() | |
48 | defer w.lock.Unlock() | |
49 | ||
50 | // If we have a waiter, notify it | |
51 | if w.waiters != nil && w.waiters.Len() > 0 { | |
52 | front := w.waiters.Front() | |
53 | w.waiters.Remove(front) | |
54 | ||
55 | val := front.Value.(*Value) | |
56 | val.SetValue(v) | |
57 | return | |
58 | } | |
59 | ||
60 | // Add it to the list of values | |
61 | if w.values == nil { | |
62 | w.values = list.New() | |
63 | } | |
64 | ||
65 | w.values.PushBack(v) | |
66 | } |