// Merge updates the value of this Counter with the supplied
// update and returns an error in case of an integer overflow.
func (n Counter) Merge(o Mergable) (Mergable, error) {
m, ok := o.(Counter)
if !ok {
return n, util.Error("parameter is of wrong type")
}
if encoding.WillOverflow(int64(n), int64(m)) {
return n, util.Errorf("merge error: %d + %d overflows", n, m)
}
result := Counter(n + m)
return result, nil
}
// increment fetches the varint encoded int64 value specified by key
// and adds "inc" to it then re-encodes as varint. The newly incremented
// value is returned.
func increment(engine Engine, key Key, inc int64) (int64, error) {
// First retrieve existing value.
val, err := engine.get(key)
if err != nil {
return 0, err
}
var int64Val int64
// If the value exists, attempt to decode it as a varint.
if len(val) != 0 {
decoded, err := encoding.Decode(key, val)
if err != nil {
return 0, err
}
if _, ok := decoded.(int64); !ok {
return 0, util.Errorf("received value of wrong type %v", reflect.TypeOf(decoded))
}
int64Val = decoded.(int64)
}
// Check for overflow and underflow.
if encoding.WillOverflow(int64Val, inc) {
return 0, util.Errorf("key %q with value %d incremented by %d results in overflow", key, int64Val, inc)
}
if inc == 0 {
return int64Val, nil
}
r := int64Val + inc
encoded, err := encoding.Encode(key, r)
if err != nil {
return 0, util.Errorf("error encoding %d", r)
}
if err = engine.put(key, encoded); err != nil {
return 0, err
}
return r, nil
}
