A data structure for optimistic concurrency control on ranges of bitwise-lexicographically-ordered keys.

Intended to replace FoundationDB's skip list.

# FoundationDB's benchmark

## Skip list

```
New conflict set: 2.404 sec
                  0.520 Mtransactions/sec
                  2.080 Mkeys/sec
Detect only:      2.266 sec
                  0.552 Mtransactions/sec
                  2.207 Mkeys/sec
Skiplist only:    1.594 sec
                  0.784 Mtransactions/sec
                  3.137 Mkeys/sec
Performance counters:
               Build: 0.071
                 Add: 0.0641
              Detect: 2.27
              D.Sort: 0.44
           D.Combine: 0.018
         D.CheckRead: 0.855
   D.CheckIntraBatch: 0.00903
        D.MergeWrite: 0.739
      D.RemoveBefore: 0.201
```

## Radix tree (this implementation)

```
New conflict set: 1.743 sec
                  0.717 Mtransactions/sec
                  2.869 Mkeys/sec
Detect only:      1.611 sec
                  0.776 Mtransactions/sec
                  3.103 Mkeys/sec
Skiplist only:    0.919 sec
                  1.360 Mtransactions/sec
                  5.440 Mkeys/sec
Performance counters:
               Build: 0.0657
                 Add: 0.0628
              Detect: 1.61
              D.Sort: 0.442
           D.Combine: 0.0178
         D.CheckRead: 0.395
   D.CheckIntraBatch: 0.00776
        D.MergeWrite: 0.524
      D.RemoveBefore: 0.221
```

# Our benchmark

## Skip list

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|              270.07 |        3,702,706.03 |    0.4% |      0.01 | `point reads`
|              285.76 |        3,499,437.03 |    1.5% |      0.01 | `prefix reads`
|              532.54 |        1,877,794.90 |    0.7% |      0.01 | `range reads`
|              528.50 |        1,892,132.94 |    0.7% |      0.01 | `point writes`
|              516.53 |        1,935,978.22 |    0.9% |      0.01 | `prefix writes`
|              303.34 |        3,296,630.84 |    3.6% |      0.05 | `range writes`
|              502.88 |        1,988,553.24 |    2.0% |      0.01 | `monotonic increasing point writes`

## Radix tree (this implementation)

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|               14.52 |       68,850,842.99 |    1.2% |      0.01 | `point reads`
|               60.89 |       16,422,538.22 |    1.5% |      0.01 | `prefix reads`
|              226.89 |        4,407,362.98 |    0.5% |      0.01 | `range reads`
|               22.99 |       43,498,198.49 |    0.2% |      0.01 | `point writes`
|               50.51 |       19,799,864.54 |    1.0% |      0.01 | `prefix writes`
|               82.50 |       12,121,212.12 |    2.6% |      0.03 | `range writes`
|              119.94 |        8,337,354.54 |    2.1% |      0.01 | `monotonic increasing point writes`

# "Real data" test

Point queries only, best of three runs. Gc ratio is the ratio of time spent doing garbage collection to time spent adding writes or doing garbage collection. Lower is better.

## skip list

```
Check: 12.7863 seconds, 292.384 MB/s, Add: 19.8276 seconds, 35.4071 MB/s, Gc ratio: 23.5314%
```

## radix tree

```
Check: 3.60187 seconds, 1037.94 MB/s, Add: 3.03958 seconds, 230.966 MB/s, Gc ratio: 52.3876%
```

## hash table

(The hash table implementation doesn't work on range queries, and its purpose is to provide an idea of how fast point queries can be)

```
Check: 2.15925 seconds, 1731.4 MB/s, Add: 1.08519 seconds, 646.926 MB/s, Gc ratio: 52.1526%
```