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// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include <cassert>
namespace vespalib::bits {
//-----------------------------------------------------------------------------
/**
* Mix the prefix of one number with the suffix of another.
*
* @return mixed value
* @param prefix the most significant bits are taken from this value
* @param suffix the least significant bits are taken from this value
* @param prefix_bits how many bits to take from the prefix
**/
uint32_t mix(uint32_t prefix, uint32_t suffix, uint32_t prefix_bits) {
if (prefix_bits == 0) {
return suffix;
}
if (prefix_bits >= 32) {
return prefix;
}
uint32_t suffix_mask = (1u << (32u - prefix_bits)) - 1u;
uint32_t prefix_mask = (0u - 1u) - suffix_mask;
return (prefix & prefix_mask) | (suffix & suffix_mask);
}
//-----------------------------------------------------------------------------
/**
* Find the number of leading zero bits in the given number.
*
* @return the number of leading zeros (0-32)
* @param value the value to inspect.
**/
uint32_t leading_zeros(uint32_t value) {
if (value == 0) {
return 32;
}
uint32_t n = 0;
if (value <= 0x0000ffff) { n += 16; value <<= 16; }
if (value <= 0x00FFffff) { n += 8; value <<= 8; }
if (value <= 0x0FFFffff) { n += 4; value <<= 4; }
if (value <= 0x3FFFffff) { n += 2; value <<= 2; }
if (value <= 0x7FFFffff) { n += 1; }
return n;
}
//-----------------------------------------------------------------------------
/**
* Split the inclusive range [min, max] into two separate adjacent
* ranges such that the highest differing bit between min and max will
* be 0 for both endpoints in the first range and 1 in the second.
* The split ranges will be [min, first_max] and [last_min, max] after
* calling this function. A higher return value indicate a coarser
* split. Note that when this function returns 0, you end up with 2
* copies of the same identity range.
*
* @param min original minimum endpoint
* @param max original maximum endpoint
* @param first_max maximum endpoint of first subrange
* @param last_min miniumum endpoint of last subrange
* @return the number of bits in min and max not part of a common prefix
**/
uint32_t split_range(uint32_t min, uint32_t max,
uint32_t &first_max, uint32_t &last_min)
{
assert(max >= min);
uint32_t prefix = leading_zeros(min ^ max);
first_max = mix(min, 0xFFFFffff, prefix + 1);
last_min = mix(max, 0x00000000, prefix + 1);
return (32 - prefix);
}
//-----------------------------------------------------------------------------
}
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