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

Intended as an alternative to FoundationDB's skip list.

Hardware for all benchmarks is an AMD Ryzen 9 7900 with (2x32GB) 5600MT/s CL28-34-34-89 1.35V RAM.

```
$ clang++ --version

Ubuntu clang version 20.0.0 (++20241119082716+7e85cb8a8a9d-1~exp1~20241119082825.551)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-20/bin
```

# Microbenchmark

## Skip list

|               ns/op |                op/s |    err% |          ins/op |          cyc/op |    IPC |         bra/op |   miss% |     total | benchmark
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|              169.16 |        5,911,582.44 |    0.0% |        3,014.03 |          855.12 |  3.525 |         504.59 |    0.0% |      2.02 | `point reads`
|              167.17 |        5,981,796.19 |    0.0% |        2,954.16 |          845.14 |  3.495 |         490.17 |    0.0% |      2.00 | `prefix reads`
|              250.44 |        3,992,954.35 |    0.1% |        3,592.41 |        1,265.18 |  2.839 |         629.31 |    0.0% |      2.99 | `range reads`
|              467.10 |        2,140,846.36 |    0.0% |        4,450.57 |        2,488.36 |  1.789 |         707.92 |    2.1% |      5.62 | `point writes`
|              465.18 |        2,149,723.11 |    0.2% |        4,410.22 |        2,474.92 |  1.782 |         694.74 |    2.1% |      5.55 | `prefix writes`
|              297.45 |        3,361,954.05 |    0.1% |        2,315.38 |        1,581.64 |  1.464 |         396.69 |    3.3% |      3.57 | `range writes`
|              476.56 |        2,098,370.82 |    1.0% |        6,999.33 |        2,492.26 |  2.808 |       1,251.74 |    1.3% |      0.06 | `monotonic increasing point writes`
|          129,455.00 |            7,724.69 |    1.0% |      807,446.67 |      698,559.40 |  1.156 |     144,584.60 |    0.8% |      0.01 | `worst case for radix tree`
|               44.67 |       22,384,996.63 |    0.5% |          902.00 |          235.18 |  3.835 |         132.00 |    0.0% |      0.01 | `create and destroy`

## Radix tree (this implementation)


|               ns/op |                op/s |    err% |          ins/op |          cyc/op |    IPC |         bra/op |   miss% |     total | benchmark
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|               14.11 |       70,857,435.19 |    0.1% |          247.13 |           71.03 |  3.479 |          32.64 |    0.8% |      0.17 | `point reads`
|               15.63 |       63,997,306.79 |    0.0% |          299.99 |           78.59 |  3.817 |          42.50 |    0.4% |      0.19 | `prefix reads`
|               36.24 |       27,590,266.59 |    0.1% |          782.70 |          182.21 |  4.296 |         106.65 |    0.2% |      0.43 | `range reads`
|               22.72 |       44,004,627.40 |    0.1% |          376.04 |          114.33 |  3.289 |          49.97 |    0.8% |      0.27 | `point writes`
|               40.83 |       24,494,110.04 |    0.0% |          666.07 |          205.35 |  3.244 |         101.33 |    0.3% |      0.49 | `prefix writes`
|               43.45 |       23,016,324.00 |    0.0% |          732.33 |          218.41 |  3.353 |         111.64 |    0.1% |      0.53 | `range writes`
|               81.46 |       12,276,650.63 |    3.6% |        1,458.85 |          411.52 |  3.545 |         280.42 |    0.1% |      0.01 | `monotonic increasing point writes`
|          314,217.00 |            3,182.51 |    1.2% |    4,043,063.50 |    1,593,715.00 |  2.537 |     714,828.00 |    0.1% |      0.01 | `worst case for radix tree`
|              106.79 |        9,364,602.60 |    0.5% |        2,046.00 |          539.75 |  3.791 |         329.00 |    0.0% |      0.01 | `create and destroy`

# "Real data" test

Point queries only. 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: 4.62434 seconds, 364.633 MB/s, Add: 3.90399 seconds, 147.371 MB/s, Gc ratio: 33.6898%, Peak idle memory: 5.61007e+06
```

## radix tree

```
Check: 0.956689 seconds, 1762.52 MB/s, Add: 1.35744 seconds, 423.84 MB/s, Gc ratio: 35.0946%, Peak idle memory: 2.32922e+06
```

## 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: 0.799863 seconds, 2108.09 MB/s, Add: 0.667736 seconds, 861.621 MB/s, Gc ratio: 35.0666%, Peak idle memory: 0
```