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// Copyright 2019-2023 int08h LLC
//
// 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.
//! A fast and efficient 64-bit integer histogram with power-of-2 spaced buckets.
//!
//! * Fixed memory footprint (520 bytes) with no dynamic allocations
//! * Constant time record and retrieve operations that compile down to a few instructions
//! * `no_std` support
//! * Work in progress: Compact binary serialization
//!
//! # Example
//!
//! ```rust
//! extern crate b2histogram;
//!
//! use b2histogram::Base2Histogram;
//!
//! fn main() {
//! let mut hist = Base2Histogram::new();
//!
//! hist.record(0); // Record a single observation of '0'
//! hist.record(11); //
//! hist.record(11); // Two observations of '11'
//! hist.record_n(300_000, 6); // Six observations of 300,000
//!
//! // Retrieve counts directly
//! println!("Observations for 300,000: {}", hist.observations(300_000));
//!
//! // Retrieve the `Bucket` covering a given value
//! println!("Bucket corresponding to '11': {:?}", hist.bucket_for(11));
//!
//! // Iterate buckets that have observations
//! for bucket in hist.iter().filter(|b| b.count > 0) {
//! println!("({:5}, {:5}): {}", bucket.start, bucket.end, bucket.count);
//! }
//! }
//! ```
//!
//! # Recording Observations
//!
//! Use the `record()` and `record_n()` methods to add observations to the histogram.
//!
//! * [`record(value: u64)`](struct.Base2Histogram.html#method.record) to record a single
//! observation of `value`
//! * [`record_n(value: u64, count: u64)`](struct.Base2Histogram.html#method.record_n) to record
//! `count` observations of the same `value`
//!
//! # Retrieving Observations
//!
//! [`observations(value: u64)`](struct.Base2Histogram.html#method.observations) returns the
//! current count for the bucket that covers `value`.
//!
//! [`bucket_for(value: u64)`](struct.Base2Histogram.html#method.bucket_for) will return a
//! [`Bucket`](struct.Bucket.html) struct for the bucket corresponding to `value`. A
//! [`Bucket`](struct.Bucket.html) provides fields to access the lower and upper bounds of the
//! bucket along with the current count.
//!
//! # Bucket Ranges
//!
//! Buckets cover the range `[2^n, 2^(n+1)-1]` including their start and end values (closed-closed)
//! for all powers of 2 from 0 through 2^62. The bottom-most bucket records observations of
//! zero and the top-most bucket covers `[2^62, +infinity]`.
//!
//! # Overflow Behavior
//!
//! Bucket counts **saturate** (reach maximum value and stay there) instead of overflowing or
//! wrapping around.
//!
//! # Implementation Details
//!
//! ## Bucket Mask
//!
//! The `mask` field is a 64-bit bitmask with each bit corresponding to one of the 64 buckets.
//! If the bit is 1 then that bucket has one or more observations while a 0 value means the
//! bucket is has no observations.
//!
//! The bit position at index `i` (from least-significant-bit (LSB) to most-significant
//! (MSB)) corresponds to bucket `[2^(i-1), (2^i)-1)]`. In diagram form:
//!
//! ```text
//! MSB <------------------------ LSB
//! +--+--+--+-------+--+--+--+--+--+
//! |63|62|61| . . . | 4| 3| 2| 1| 0|
//! +--+--+--+-------+--+--+--+--+--+
//! ^ ^ ^ ^ ^ ^ ^
//! | | +------+ | | | +-------+
//! Values | ++ | +----+ | +----+ |
//! (2^62, +inf) --+ | Values | | | Values
//! | (8, 15) | | | (0, 0)
//! Values Values Values Values
//! (2^61, 2^62-1) (4, 7) (2, 3) (1, 1)
//! ```
//!
//! ## Counts Field
//!
//! The `counts` field is a simple `u64` array of 64 elements. There is one array entry for
//! each bucket and array index `i` corresponds to mask bit `i`.
#![no_std]
///
/// A compact and efficient integer histogram with fixed memory footprint,
/// constant runtime performance, and very compact binary serialization.
///
pub struct Base2Histogram {
counts: [u64; 64],
mask: u64,
}
/// A bucket maintains a `count` of observations between its `begin` and `end` endpoints.
///
/// Buckets include their endpoint values (known as a "closed-closed" interval). Each
/// bucket covers `[2^n, (2^(n+1))-1]` with the exception of the top-most bucket which
/// covers `[2^62, +infinity]`.
///
#[derive(Debug)]
pub struct Bucket {
/// Beginning of the range, inclusive
pub start: u64,
/// Maximum value of the range, inclusive
pub end: u64,
/// Number of observations in the bucket
pub count: u64,
}
impl Base2Histogram {
/// Create a new `Base2Histogram` instance
pub fn new() -> Self {
Base2Histogram {
counts: [0u64; 64],
mask: 0u64,
}
}
/// Record a single observation of `value`
#[inline]
pub fn record(&mut self, value: u64) {
self.record_n(value, 1);
}
/// Record `count` observations of `value`
#[inline]
pub fn record_n(&mut self, value: u64, count: u64) {
let idx = self.index_of(value);
self.counts[idx] = self.counts[idx].saturating_add(count);
self.mask |= 1 << (idx as u64);
}
/// Returns the number of observations recorded by the bucket containing `value`
///
/// To retrieve the number of observations along with its bucket bounds, see
/// [`bucket_for()`](struct.Base2Histogram.html#method.bucket_for).
#[inline]
pub fn observations(&self, value: u64) -> u64 {
let idx = self.index_of(value);
self.counts[idx]
}
/// Return the `Bucket` to which `value` belongs.
///
/// To retrieve only the number of observations see
/// [`observations()`](struct.Base2Histogram.html#method.observations).
#[inline]
pub fn bucket_for(&self, value: u64) -> Bucket {
let idx = self.index_of(value);
self.bucket_at(idx)
}
/// Returns the number of buckets with one or more observations
#[inline]
pub fn nonzero_buckets(&self) -> u32 {
u64::count_ones(self.mask)
}
/// Returns `true` if the bucket count corresponding to `value` is non-zero
#[inline]
pub fn has_counts(&self, value: u64) -> bool {
let idx = self.index_of(value) as u64;
self.mask & (1 << idx) != 0
}
/// Iterate through all 64 buckets of the histogram in order (0..63)
pub fn iter(&self) -> impl Iterator<Item=Bucket> + '_ {
let mut idx = 0;
core::iter::from_fn(move || {
if idx < 64 {
let bucket = self.bucket_at(idx);
idx += 1;
Some(bucket)
} else {
None
}
})
}
/// Returns the bucket index into `self.counts` for the `value`
#[inline]
fn index_of(&self, value: u64) -> usize {
match u64::leading_zeros(value) {
0 => 63 as usize,
clz => (64 - clz) as usize
}
}
/// Return the `Bucket` at the provided index (index values 0..63)
fn bucket_at(&self, idx: usize) -> Bucket {
if idx == 0 {
Bucket { start: 0, end: 0, count: self.counts[0] }
} else {
let shift = (idx - 1) as u32;
let begin = u64::saturating_pow(2, shift);
let end = u64::saturating_mul(begin, 2) - 1;
let count = self.counts[idx];
Bucket { start: begin, end, count }
}
}
}