| // Copyright 2020 The LUCI Authors. |
| // |
| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| |
| package common |
| |
| import ( |
| "hash/fnv" |
| "time" |
| ) |
| |
| // DistributeOffset deterministically chooses an offset across keys |
| // in [1..pollInterval) range aimining for uniform distribution across all keys. |
| // |
| // A key is simply a concatenation of key parts with '\0' filler in between. |
| func DistributeOffset(pollInterval time.Duration, keyParts ...string) time.Duration { |
| // Basic idea: interval/N*random(0..N), but deterministic on kind+luciProject. |
| // Use fast hash function, as we don't need strong collision resistance. |
| h := fnv.New32a() |
| for _, part := range keyParts { |
| h.Write([]byte(part)) |
| h.Write([]byte{0}) |
| } |
| r := h.Sum32() |
| |
| i := int64(pollInterval) |
| // Avoid losing precision for low pollInterval values by first shifting them |
| // the more significant bits. |
| shifted := 0 |
| for i < (int64(1) << 55) { |
| i = i << 7 |
| shifted += 7 |
| } |
| // i = i / N * r, where N = 2^32 since r is (0..2^32-1). |
| i = (i >> 32) * int64(r) |
| return time.Duration(i >> shifted) |
| } |
