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
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|              175.88 |        5,685,575.81 |    0.0% |        3,014.03 |          883.13 |  3.413 |         504.59 |    0.0% |      2.10 | `point reads`
|              173.35 |        5,768,718.52 |    0.0% |        2,954.16 |          869.59 |  3.397 |         490.17 |    0.0% |      2.07 | `prefix reads`
|              251.74 |        3,972,274.68 |    0.1% |        3,592.41 |        1,265.71 |  2.838 |         629.31 |    0.0% |      3.01 | `range reads`
|              472.13 |        2,118,079.83 |    0.2% |        4,450.57 |        2,377.84 |  1.872 |         707.92 |    2.3% |      5.60 | `point writes`
|              471.34 |        2,121,609.91 |    0.0% |        4,410.22 |        2,372.55 |  1.859 |         694.74 |    2.3% |      5.61 | `prefix writes`
|              306.69 |        3,260,650.68 |    0.0% |        2,315.38 |        1,550.33 |  1.493 |         396.69 |    3.3% |      3.68 | `range writes`
|              502.70 |        1,989,277.15 |    0.9% |        6,999.33 |        2,527.47 |  2.769 |       1,251.74 |    1.3% |      0.06 | `monotonic increasing point writes`
|          138,097.67 |            7,241.25 |    0.7% |      807,445.67 |      699,899.00 |  1.154 |     144,584.50 |    0.8% |      0.01 | `worst case for radix tree`
|               46.29 |       21,605,126.00 |    1.0% |          902.00 |          230.73 |  3.909 |         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
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|               14.04 |       71,216,855.01 |    0.1% |          245.98 |           70.38 |  3.495 |          31.49 |    0.6% |      0.17 | `point reads`
|               14.57 |       68,630,592.89 |    0.1% |          297.83 |           72.93 |  4.084 |          40.34 |    0.5% |      0.17 | `prefix reads`
|               35.28 |       28,347,588.39 |    0.2% |          783.70 |          176.57 |  4.438 |         107.65 |    0.2% |      0.42 | `range reads`
|               20.64 |       48,453,280.08 |    0.0% |          378.97 |          103.31 |  3.668 |          49.92 |    0.7% |      0.25 | `point writes`
|               39.14 |       25,551,096.17 |    0.0% |          672.00 |          196.08 |  3.427 |         101.28 |    0.3% |      0.47 | `prefix writes`
|               39.88 |       25,075,121.85 |    0.0% |          738.26 |          199.71 |  3.697 |         111.59 |    0.1% |      0.49 | `range writes`
|               79.36 |       12,601,312.65 |    0.5% |        1,447.65 |          398.77 |  3.630 |         275.61 |    0.1% |      0.01 | `monotonic increasing point writes`
|          307,037.33 |            3,256.93 |    0.3% |    4,043,060.50 |    1,549,199.33 |  2.610 |     714,828.00 |    0.1% |      0.01 | `worst case for radix tree`
|               97.00 |       10,309,246.47 |    0.2% |        1,986.00 |          487.06 |  4.078 |         315.00 |    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: 4.66163 seconds, 361.716 MB/s, Add: 3.97534 seconds, 144.726 MB/s, Gc ratio: 33.3998%, Peak idle memory: 5.61007e+06
```

## radix tree

```
Check: 1.01776 seconds, 1656.77 MB/s, Add: 1.2718 seconds, 452.38 MB/s, Gc ratio: 38.626%, Peak idle memory: 2.05667e+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: 0.859188 seconds, 1962.54 MB/s, Add: 0.714174 seconds, 805.596 MB/s, Gc ratio: 34.9552%, Peak idle memory: 0
```