perf_hooks: implement histogram based api

Add a sampling-based event loop delay monitor.

```js
const { monitorEventLoopDelay } = require('perf_hooks');

const h = monitorEventLoopDelay();

h.enable();

h.disable();

console.log(h.percentiles);
console.log(h.min);
console.log(h.max);
console.log(h.mean);
console.log(h.stddev);
console.log(h.percentile(50));
```

PR-URL: https://github.com/nodejs/node/pull/25378
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

6 files changed, 1547 insertions, 0 deletions

diff --git a/deps/histogram/LICENSE.txt b/deps/histogram/LICENSE.txt
new file mode 100644
index 0000000000..9b4e66ed7d
--- /dev/null
+++ b/deps/histogram/LICENSE.txt
@@ -0,0 +1,41 @@
+The code in this repository code was Written by Gil Tene, Michael Barker,
+and Matt Warren, and released to the public domain, as explained at
+http://creativecommons.org/publicdomain/zero/1.0/
+
+For users of this code who wish to consume it under the "BSD" license
+rather than under the public domain or CC0 contribution text mentioned
+above, the code found under this directory is *also* provided under the
+following license (commonly referred to as the BSD 2-Clause License). This
+license does not detract from the above stated release of the code into
+the public domain, and simply represents an additional license granted by
+the Author.
+
+-----------------------------------------------------------------------------
+** Beginning of "BSD 2-Clause License" text. **
+
+ Copyright (c) 2012, 2013, 2014 Gil Tene
+ Copyright (c) 2014 Michael Barker
+ Copyright (c) 2014 Matt Warren
+ All rights reserved.
+
+ Redistribution and use in source and binary forms, with or without
+ modification, are permitted provided that the following conditions are met:
+
+ 1. Redistributions of source code must retain the above copyright notice,
+    this list of conditions and the following disclaimer.
+
+ 2. Redistributions in binary form must reproduce the above copyright notice,
+    this list of conditions and the following disclaimer in the documentation
+    and/or other materials provided with the distribution.
+
+ THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/deps/histogram/README.md b/deps/histogram/README.md
new file mode 100644
index 0000000000..b6775fd6dc
--- /dev/null
+++ b/deps/histogram/README.md
@@ -0,0 +1,3 @@
+# HdrHistogram_c
+
+From: https://github.com/HdrHistogram/HdrHistogram_c
diff --git a/deps/histogram/histogram.gyp b/deps/histogram/histogram.gyp
new file mode 100644
index 0000000000..bcfa198f9d
--- /dev/null
+++ b/deps/histogram/histogram.gyp
@@ -0,0 +1,15 @@
+{
+  'targets': [
+    {
+      'target_name': 'histogram',
+      'type': 'static_library',
+      'include_dirs': ['src'],
+      'direct_dependent_settings': {
+        'include_dirs': [ 'src' ]
+      },
+      'sources': [
+        'src/hdr_histogram.c',
+      ]
+    }
+  ]
+}
diff --git a/deps/histogram/src/hdr_histogram.c b/deps/histogram/src/hdr_histogram.c
new file mode 100644
index 0000000000..1d7343442d
--- /dev/null
+++ b/deps/histogram/src/hdr_histogram.c
@@ -0,0 +1,1032 @@
+/**
+ * hdr_histogram.c
+ * Written by Michael Barker and released to the public domain,
+ * as explained at http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+#include <stdlib.h>
+#include <stdbool.h>
+#include <math.h>
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdint.h>
+#include <errno.h>
+#include <inttypes.h>
+
+#include "hdr_histogram.h"
+#include "hdr_tests.h"
+
+/*  ######   #######  ##     ## ##    ## ########  ######  */
+/* ##    ## ##     ## ##     ## ###   ##    ##    ##    ## */
+/* ##       ##     ## ##     ## ####  ##    ##    ##       */
+/* ##       ##     ## ##     ## ## ## ##    ##     ######  */
+/* ##       ##     ## ##     ## ##  ####    ##          ## */
+/* ##    ## ##     ## ##     ## ##   ###    ##    ##    ## */
+/*  ######   #######   #######  ##    ##    ##     ######  */
+
+static int32_t normalize_index(const struct hdr_histogram* h, int32_t index)
+{
+    int32_t normalized_index;
+    int32_t adjustment = 0;
+    if (h->normalizing_index_offset == 0)
+    {
+        return index;
+    }
+
+    normalized_index = index - h->normalizing_index_offset;
+
+    if (normalized_index < 0)
+    {
+        adjustment = h->counts_len;
+    }
+    else if (normalized_index >= h->counts_len)
+    {
+        adjustment = -h->counts_len;
+    }
+
+    return normalized_index + adjustment;
+}
+
+static int64_t counts_get_direct(const struct hdr_histogram* h, int32_t index)
+{
+    return h->counts[index];
+}
+
+static int64_t counts_get_normalised(const struct hdr_histogram* h, int32_t index)
+{
+    return counts_get_direct(h, normalize_index(h, index));
+}
+
+static void counts_inc_normalised(
+    struct hdr_histogram* h, int32_t index, int64_t value)
+{
+    int32_t normalised_index = normalize_index(h, index);
+    h->counts[normalised_index] += value;
+    h->total_count += value;
+}
+
+static void update_min_max(struct hdr_histogram* h, int64_t value)
+{
+    h->min_value = (value < h->min_value && value != 0) ? value : h->min_value;
+    h->max_value = (value > h->max_value) ? value : h->max_value;
+}
+
+/* ##     ## ######## #### ##       #### ######## ##    ## */
+/* ##     ##    ##     ##  ##        ##     ##     ##  ##  */
+/* ##     ##    ##     ##  ##        ##     ##      ####   */
+/* ##     ##    ##     ##  ##        ##     ##       ##    */
+/* ##     ##    ##     ##  ##        ##     ##       ##    */
+/* ##     ##    ##     ##  ##        ##     ##       ##    */
+/*  #######     ##    #### ######## ####    ##       ##    */
+
+static int64_t power(int64_t base, int64_t exp)
+{
+    int64_t result = 1;
+    while(exp)
+    {
+        result *= base; exp--;
+    }
+    return result;
+}
+
+#if defined(_MSC_VER)
+#pragma intrinsic(_BitScanReverse64)
+#endif
+
+static int32_t get_bucket_index(const struct hdr_histogram* h, int64_t value)
+{
+#if defined(_MSC_VER)
+    uint32_t leading_zero = 0;
+    _BitScanReverse64(&leading_zero, value | h->sub_bucket_mask);
+    int32_t pow2ceiling = 64 - (63 - leading_zero); /* smallest power of 2 containing value */
+#else
+    int32_t pow2ceiling = 64 - __builtin_clzll(value | h->sub_bucket_mask); /* smallest power of 2 containing value */
+#endif
+    return pow2ceiling - h->unit_magnitude - (h->sub_bucket_half_count_magnitude + 1);
+}
+
+static int32_t get_sub_bucket_index(int64_t value, int32_t bucket_index, int32_t unit_magnitude)
+{
+    return (int32_t)(value >> (bucket_index + unit_magnitude));
+}
+
+static int32_t counts_index(const struct hdr_histogram* h, int32_t bucket_index, int32_t sub_bucket_index)
+{
+    /* Calculate the index for the first entry in the bucket: */
+    /* (The following is the equivalent of ((bucket_index + 1) * subBucketHalfCount) ): */
+    int32_t bucket_base_index = (bucket_index + 1) << h->sub_bucket_half_count_magnitude;
+    /* Calculate the offset in the bucket: */
+    int32_t offset_in_bucket = sub_bucket_index - h->sub_bucket_half_count;
+    /* The following is the equivalent of ((sub_bucket_index  - subBucketHalfCount) + bucketBaseIndex; */
+    return bucket_base_index + offset_in_bucket;
+}
+
+static int64_t value_from_index(int32_t bucket_index, int32_t sub_bucket_index, int32_t unit_magnitude)
+{
+    return ((int64_t) sub_bucket_index) << (bucket_index + unit_magnitude);
+}
+
+int32_t counts_index_for(const struct hdr_histogram* h, int64_t value)
+{
+    int32_t bucket_index     = get_bucket_index(h, value);
+    int32_t sub_bucket_index = get_sub_bucket_index(value, bucket_index, h->unit_magnitude);
+
+    return counts_index(h, bucket_index, sub_bucket_index);
+}
+
+int64_t hdr_value_at_index(const struct hdr_histogram *h, int32_t index)
+{
+    int32_t bucket_index = (index >> h->sub_bucket_half_count_magnitude) - 1;
+    int32_t sub_bucket_index = (index & (h->sub_bucket_half_count - 1)) + h->sub_bucket_half_count;
+
+    if (bucket_index < 0)
+    {
+        sub_bucket_index -= h->sub_bucket_half_count;
+        bucket_index = 0;
+    }
+
+    return value_from_index(bucket_index, sub_bucket_index, h->unit_magnitude);
+}
+
+int64_t hdr_size_of_equivalent_value_range(const struct hdr_histogram* h, int64_t value)
+{
+    int32_t bucket_index     = get_bucket_index(h, value);
+    int32_t sub_bucket_index = get_sub_bucket_index(value, bucket_index, h->unit_magnitude);
+    int32_t adjusted_bucket  = (sub_bucket_index >= h->sub_bucket_count) ? (bucket_index + 1) : bucket_index;
+    return INT64_C(1) << (h->unit_magnitude + adjusted_bucket);
+}
+
+static int64_t lowest_equivalent_value(const struct hdr_histogram* h, int64_t value)
+{
+    int32_t bucket_index     = get_bucket_index(h, value);
+    int32_t sub_bucket_index = get_sub_bucket_index(value, bucket_index, h->unit_magnitude);
+    return value_from_index(bucket_index, sub_bucket_index, h->unit_magnitude);
+}
+
+int64_t hdr_next_non_equivalent_value(const struct hdr_histogram *h, int64_t value)
+{
+    return lowest_equivalent_value(h, value) + hdr_size_of_equivalent_value_range(h, value);
+}
+
+static int64_t highest_equivalent_value(const struct hdr_histogram* h, int64_t value)
+{
+    return hdr_next_non_equivalent_value(h, value) - 1;
+}
+
+int64_t hdr_median_equivalent_value(const struct hdr_histogram *h, int64_t value)
+{
+    return lowest_equivalent_value(h, value) + (hdr_size_of_equivalent_value_range(h, value) >> 1);
+}
+
+static int64_t non_zero_min(const struct hdr_histogram* h)
+{
+    if (INT64_MAX == h->min_value)
+    {
+        return INT64_MAX;
+    }
+
+    return lowest_equivalent_value(h, h->min_value);
+}
+
+void hdr_reset_internal_counters(struct hdr_histogram* h)
+{
+    int min_non_zero_index = -1;
+    int max_index = -1;
+    int64_t observed_total_count = 0;
+    int i;
+
+    for (i = 0; i < h->counts_len; i++)
+    {
+        int64_t count_at_index;
+
+        if ((count_at_index = counts_get_direct(h, i)) > 0)
+        {
+            observed_total_count += count_at_index;
+            max_index = i;
+            if (min_non_zero_index == -1 && i != 0)
+            {
+                min_non_zero_index = i;
+            }
+        }
+    }
+
+    if (max_index == -1)
+    {
+        h->max_value = 0;
+    }
+    else
+    {
+        int64_t max_value = hdr_value_at_index(h, max_index);
+        h->max_value = highest_equivalent_value(h, max_value);
+    }
+
+    if (min_non_zero_index == -1)
+    {
+        h->min_value = INT64_MAX;
+    }
+    else
+    {
+        h->min_value = hdr_value_at_index(h, min_non_zero_index);
+    }
+
+    h->total_count = observed_total_count;
+}
+
+static int32_t buckets_needed_to_cover_value(int64_t value, int32_t sub_bucket_count, int32_t unit_magnitude)
+{
+    int64_t smallest_untrackable_value = ((int64_t) sub_bucket_count) << unit_magnitude;
+    int32_t buckets_needed = 1;
+    while (smallest_untrackable_value <= value)
+    {
+        if (smallest_untrackable_value > INT64_MAX / 2)
+        {
+            return buckets_needed + 1;
+        }
+        smallest_untrackable_value <<= 1;
+        buckets_needed++;
+    }
+
+    return buckets_needed;
+}
+
+/* ##     ## ######## ##     ##  #######  ########  ##    ## */
+/* ###   ### ##       ###   ### ##     ## ##     ##  ##  ##  */
+/* #### #### ##       #### #### ##     ## ##     ##   ####   */
+/* ## ### ## ######   ## ### ## ##     ## ########     ##    */
+/* ##     ## ##       ##     ## ##     ## ##   ##      ##    */
+/* ##     ## ##       ##     ## ##     ## ##    ##     ##    */
+/* ##     ## ######## ##     ##  #######  ##     ##    ##    */
+
+int hdr_calculate_bucket_config(
+        int64_t lowest_trackable_value,
+        int64_t highest_trackable_value,
+        int significant_figures,
+        struct hdr_histogram_bucket_config* cfg)
+{
+    int32_t sub_bucket_count_magnitude;
+    int64_t largest_value_with_single_unit_resolution;
+
+    if (lowest_trackable_value < 1 ||
+            significant_figures < 1 || 5 < significant_figures)
+    {
+        return EINVAL;
+    }
+    else if (lowest_trackable_value * 2 > highest_trackable_value)
+    {
+        return EINVAL;
+    }
+
+    cfg->lowest_trackable_value = lowest_trackable_value;
+    cfg->significant_figures = significant_figures;
+    cfg->highest_trackable_value = highest_trackable_value;
+
+    largest_value_with_single_unit_resolution = 2 * power(10, significant_figures);
+    sub_bucket_count_magnitude = (int32_t) ceil(log((double)largest_value_with_single_unit_resolution) / log(2));
+    cfg->sub_bucket_half_count_magnitude = ((sub_bucket_count_magnitude > 1) ? sub_bucket_count_magnitude : 1) - 1;
+
+    cfg->unit_magnitude = (int32_t) floor(log((double)lowest_trackable_value) / log(2));
+
+    cfg->sub_bucket_count      = (int32_t) pow(2, (cfg->sub_bucket_half_count_magnitude + 1));
+    cfg->sub_bucket_half_count = cfg->sub_bucket_count / 2;
+    cfg->sub_bucket_mask       = ((int64_t) cfg->sub_bucket_count - 1) << cfg->unit_magnitude;
+
+    if (cfg->unit_magnitude + cfg->sub_bucket_half_count_magnitude > 61)
+    {
+        return EINVAL;
+    }
+
+    cfg->bucket_count = buckets_needed_to_cover_value(highest_trackable_value, cfg->sub_bucket_count, (int32_t)cfg->unit_magnitude);
+    cfg->counts_len = (cfg->bucket_count + 1) * (cfg->sub_bucket_count / 2);
+
+    return 0;
+}
+
+void hdr_init_preallocated(struct hdr_histogram* h, struct hdr_histogram_bucket_config* cfg)
+{
+    h->lowest_trackable_value          = cfg->lowest_trackable_value;
+    h->highest_trackable_value         = cfg->highest_trackable_value;
+    h->unit_magnitude                  = (int32_t)cfg->unit_magnitude;
+    h->significant_figures             = (int32_t)cfg->significant_figures;
+    h->sub_bucket_half_count_magnitude = cfg->sub_bucket_half_count_magnitude;
+    h->sub_bucket_half_count           = cfg->sub_bucket_half_count;
+    h->sub_bucket_mask                 = cfg->sub_bucket_mask;
+    h->sub_bucket_count                = cfg->sub_bucket_count;
+    h->min_value                       = INT64_MAX;
+    h->max_value                       = 0;
+    h->normalizing_index_offset        = 0;
+    h->conversion_ratio                = 1.0;
+    h->bucket_count                    = cfg->bucket_count;
+    h->counts_len                      = cfg->counts_len;
+    h->total_count                     = 0;
+}
+
+int hdr_init(
+        int64_t lowest_trackable_value,
+        int64_t highest_trackable_value,
+        int significant_figures,
+        struct hdr_histogram** result)
+{
+    int64_t* counts;
+    struct hdr_histogram_bucket_config cfg;
+    struct hdr_histogram* histogram;
+
+    int r = hdr_calculate_bucket_config(lowest_trackable_value, highest_trackable_value, significant_figures, &cfg);
+    if (r)
+    {
+        return r;
+    }
+
+    counts = calloc((size_t) cfg.counts_len, sizeof(int64_t));
+    histogram = calloc(1, sizeof(struct hdr_histogram));
+
+    if (!counts || !histogram)
+    {
+        return ENOMEM;
+    }
+
+    histogram->counts = counts;
+
+    hdr_init_preallocated(histogram, &cfg);
+    *result = histogram;
+
+    return 0;
+}
+
+void hdr_close(struct hdr_histogram* h)
+{
+    free(h->counts);
+    free(h);
+}
+
+int hdr_alloc(int64_t highest_trackable_value, int significant_figures, struct hdr_histogram** result)
+{
+    return hdr_init(1, highest_trackable_value, significant_figures, result);
+}
+
+/* reset a histogram to zero. */
+void hdr_reset(struct hdr_histogram *h)
+{
+     h->total_count=0;
+     h->min_value = INT64_MAX;
+     h->max_value = 0;
+     memset(h->counts, 0, (sizeof(int64_t) * h->counts_len));
+}
+
+size_t hdr_get_memory_size(struct hdr_histogram *h)
+{
+    return sizeof(struct hdr_histogram) + h->counts_len * sizeof(int64_t);
+}
+
+/* ##     ## ########  ########     ###    ######## ########  ######  */
+/* ##     ## ##     ## ##     ##   ## ##      ##    ##       ##    ## */
+/* ##     ## ##     ## ##     ##  ##   ##     ##    ##       ##       */
+/* ##     ## ########  ##     ## ##     ##    ##    ######    ######  */
+/* ##     ## ##        ##     ## #########    ##    ##             ## */
+/* ##     ## ##        ##     ## ##     ##    ##    ##       ##    ## */
+/*  #######  ##        ########  ##     ##    ##    ########  ######  */
+
+
+bool hdr_record_value(struct hdr_histogram* h, int64_t value)
+{
+    return hdr_record_values(h, value, 1);
+}
+
+bool hdr_record_values(struct hdr_histogram* h, int64_t value, int64_t count)
+{
+    int32_t counts_index;
+
+    if (value < 0)
+    {
+        return false;
+    }
+
+    counts_index = counts_index_for(h, value);
+
+    if (counts_index < 0 || h->counts_len <= counts_index)
+    {
+        return false;
+    }
+
+    counts_inc_normalised(h, counts_index, count);
+    update_min_max(h, value);
+
+    return true;
+}
+
+bool hdr_record_corrected_value(struct hdr_histogram* h, int64_t value, int64_t expected_interval)
+{
+    return hdr_record_corrected_values(h, value, 1, expected_interval);
+}
+
+
+bool hdr_record_corrected_values(struct hdr_histogram* h, int64_t value, int64_t count, int64_t expected_interval)
+{
+    int64_t missing_value;
+
+    if (!hdr_record_values(h, value, count))
+    {
+        return false;
+    }
+
+    if (expected_interval <= 0 || value <= expected_interval)
+    {
+        return true;
+    }
+
+    missing_value = value - expected_interval;
+    for (; missing_value >= expected_interval; missing_value -= expected_interval)
+    {
+        if (!hdr_record_values(h, missing_value, count))
+        {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+int64_t hdr_add(struct hdr_histogram* h, const struct hdr_histogram* from)
+{
+    struct hdr_iter iter;
+    int64_t dropped = 0;
+    hdr_iter_recorded_init(&iter, from);
+
+    while (hdr_iter_next(&iter))
+    {
+        int64_t value = iter.value;
+        int64_t count = iter.count;
+
+        if (!hdr_record_values(h, value, count))
+        {
+            dropped += count;
+        }
+    }
+
+    return dropped;
+}
+
+int64_t hdr_add_while_correcting_for_coordinated_omission(
+        struct hdr_histogram* h, struct hdr_histogram* from, int64_t expected_interval)
+{
+    struct hdr_iter iter;
+    int64_t dropped = 0;
+    hdr_iter_recorded_init(&iter, from);
+
+    while (hdr_iter_next(&iter))
+    {
+        int64_t value = iter.value;
+        int64_t count = iter.count;
+
+        if (!hdr_record_corrected_values(h, value, count, expected_interval))
+        {
+            dropped += count;
+        }
+    }
+
+    return dropped;
+}
+
+
+
+/* ##     ##    ###    ##       ##     ## ########  ######  */
+/* ##     ##   ## ##   ##       ##     ## ##       ##    ## */
+/* ##     ##  ##   ##  ##       ##     ## ##       ##       */
+/* ##     ## ##     ## ##       ##     ## ######    ######  */
+/*  ##   ##  ######### ##       ##     ## ##             ## */
+/*   ## ##   ##     ## ##       ##     ## ##       ##    ## */
+/*    ###    ##     ## ########  #######  ########  ######  */
+
+
+int64_t hdr_max(const struct hdr_histogram* h)
+{
+    if (0 == h->max_value)
+    {
+        return 0;
+    }
+
+    return highest_equivalent_value(h, h->max_value);
+}
+
+int64_t hdr_min(const struct hdr_histogram* h)
+{
+    if (0 < hdr_count_at_index(h, 0))
+    {
+        return 0;
+    }
+
+    return non_zero_min(h);
+}
+
+int64_t hdr_value_at_percentile(const struct hdr_histogram* h, double percentile)
+{
+    struct hdr_iter iter;
+    int64_t total = 0;
+    double requested_percentile = percentile < 100.0 ? percentile : 100.0;
+    int64_t count_at_percentile =
+        (int64_t) (((requested_percentile / 100) * h->total_count) + 0.5);
+    count_at_percentile = count_at_percentile > 1 ? count_at_percentile : 1;
+
+    hdr_iter_init(&iter, h);
+
+    while (hdr_iter_next(&iter))
+    {
+        total += iter.count;
+
+        if (total >= count_at_percentile)
+        {
+            int64_t value_from_index = iter.value;
+            return highest_equivalent_value(h, value_from_index);
+        }
+    }
+
+    return 0;
+}
+
+double hdr_mean(const struct hdr_histogram* h)
+{
+    struct hdr_iter iter;
+    int64_t total = 0;
+
+    hdr_iter_init(&iter, h);
+
+    while (hdr_iter_next(&iter))
+    {
+        if (0 != iter.count)
+        {
+            total += iter.count * hdr_median_equivalent_value(h, iter.value);
+        }
+    }
+
+    return (total * 1.0) / h->total_count;
+}
+
+double hdr_stddev(const struct hdr_histogram* h)
+{
+    double mean = hdr_mean(h);
+    double geometric_dev_total = 0.0;
+
+    struct hdr_iter iter;
+    hdr_iter_init(&iter, h);
+
+    while (hdr_iter_next(&iter))
+    {
+        if (0 != iter.count)
+        {
+            double dev = (hdr_median_equivalent_value(h, iter.value) * 1.0) - mean;
+            geometric_dev_total += (dev * dev) * iter.count;
+        }
+    }
+
+    return sqrt(geometric_dev_total / h->total_count);
+}
+
+bool hdr_values_are_equivalent(const struct hdr_histogram* h, int64_t a, int64_t b)
+{
+    return lowest_equivalent_value(h, a) == lowest_equivalent_value(h, b);
+}
+
+int64_t hdr_lowest_equivalent_value(const struct hdr_histogram* h, int64_t value)
+{
+    return lowest_equivalent_value(h, value);
+}
+
+int64_t hdr_count_at_value(const struct hdr_histogram* h, int64_t value)
+{
+    return counts_get_normalised(h, counts_index_for(h, value));
+}
+
+int64_t hdr_count_at_index(const struct hdr_histogram* h, int32_t index)
+{
+    return counts_get_normalised(h, index);
+}
+
+
+/* #### ######## ######## ########     ###    ########  #######  ########   ######  */
+/*  ##     ##    ##       ##     ##   ## ##      ##    ##     ## ##     ## ##    ## */
+/*  ##     ##    ##       ##     ##  ##   ##     ##    ##     ## ##     ## ##       */
+/*  ##     ##    ######   ########  ##     ##    ##    ##     ## ########   ######  */
+/*  ##     ##    ##       ##   ##   #########    ##    ##     ## ##   ##         ## */
+/*  ##     ##    ##       ##    ##  ##     ##    ##    ##     ## ##    ##  ##    ## */
+/* ####    ##    ######## ##     ## ##     ##    ##     #######  ##     ##  ######  */
+
+
+static bool has_buckets(struct hdr_iter* iter)
+{
+    return iter->counts_index < iter->h->counts_len;
+}
+
+static bool has_next(struct hdr_iter* iter)
+{
+    return iter->cumulative_count < iter->total_count;
+}
+
+static bool move_next(struct hdr_iter* iter)
+{
+    iter->counts_index++;
+
+    if (!has_buckets(iter))
+    {
+        return false;
+    }
+
+    iter->count = counts_get_normalised(iter->h, iter->counts_index);
+    iter->cumulative_count += iter->count;
+
+    iter->value = hdr_value_at_index(iter->h, iter->counts_index);
+    iter->highest_equivalent_value = highest_equivalent_value(iter->h, iter->value);
+    iter->lowest_equivalent_value = lowest_equivalent_value(iter->h, iter->value);
+    iter->median_equivalent_value = hdr_median_equivalent_value(iter->h, iter->value);
+
+    return true;
+}
+
+static int64_t peek_next_value_from_index(struct hdr_iter* iter)
+{
+    return hdr_value_at_index(iter->h, iter->counts_index + 1);
+}
+
+static bool next_value_greater_than_reporting_level_upper_bound(
+    struct hdr_iter *iter, int64_t reporting_level_upper_bound)
+{
+    if (iter->counts_index >= iter->h->counts_len)
+    {
+        return false;
+    }
+
+    return peek_next_value_from_index(iter) > reporting_level_upper_bound;
+}
+
+static bool _basic_iter_next(struct hdr_iter *iter)
+{
+    if (!has_next(iter) || iter->counts_index >= iter->h->counts_len)
+    {
+        return false;
+    }
+
+    move_next(iter);
+
+    return true;
+}
+
+static void _update_iterated_values(struct hdr_iter* iter, int64_t new_value_iterated_to)
+{
+    iter->value_iterated_from = iter->value_iterated_to;
+    iter->value_iterated_to = new_value_iterated_to;
+}
+
+static bool _all_values_iter_next(struct hdr_iter* iter)
+{
+    bool result = move_next(iter);
+
+    if (result)
+    {
+        _update_iterated_values(iter, iter->value);
+    }
+
+    return result;
+}
+
+void hdr_iter_init(struct hdr_iter* iter, const struct hdr_histogram* h)
+{
+    iter->h = h;
+
+    iter->counts_index = -1;
+    iter->total_count = h->total_count;
+    iter->count = 0;
+    iter->cumulative_count = 0;
+    iter->value = 0;
+    iter->highest_equivalent_value = 0;
+    iter->value_iterated_from = 0;
+    iter->value_iterated_to = 0;
+
+    iter->_next_fp = _all_values_iter_next;
+}
+
+bool hdr_iter_next(struct hdr_iter* iter)
+{
+    return iter->_next_fp(iter);
+}
+
+/* ########  ######## ########   ######  ######## ##    ## ######## #### ##       ########  ######  */
+/* ##     ## ##       ##     ## ##    ## ##       ###   ##    ##     ##  ##       ##       ##    ## */
+/* ##     ## ##       ##     ## ##       ##       ####  ##    ##     ##  ##       ##       ##       */
+/* ########  ######   ########  ##       ######   ## ## ##    ##     ##  ##       ######    ######  */
+/* ##        ##       ##   ##   ##       ##       ##  ####    ##     ##  ##       ##             ## */
+/* ##        ##       ##    ##  ##    ## ##       ##   ###    ##     ##  ##       ##       ##    ## */
+/* ##        ######## ##     ##  ######  ######## ##    ##    ##    #### ######## ########  ######  */
+
+static bool _percentile_iter_next(struct hdr_iter* iter)
+{
+    int64_t temp, half_distance, percentile_reporting_ticks;
+
+    struct hdr_iter_percentiles* percentiles = &iter->specifics.percentiles;
+
+    if (!has_next(iter))
+    {
+        if (percentiles->seen_last_value)
+        {
+            return false;
+        }
+
+        percentiles->seen_last_value = true;
+        percentiles->percentile = 100.0;
+
+        return true;
+    }
+
+    if (iter->counts_index == -1 && !_basic_iter_next(iter))
+    {
+        return false;
+    }
+
+    do
+    {
+        double current_percentile = (100.0 * (double) iter->cumulative_count) / iter->h->total_count;
+        if (iter->count != 0 &&
+                percentiles->percentile_to_iterate_to <= current_percentile)
+        {
+            _update_iterated_values(iter, highest_equivalent_value(iter->h, iter->value));
+
+            percentiles->percentile = percentiles->percentile_to_iterate_to;
+            temp = (int64_t)(log(100 / (100.0 - (percentiles->percentile_to_iterate_to))) / log(2)) + 1;
+            half_distance = (int64_t) pow(2, (double) temp);
+            percentile_reporting_ticks = percentiles->ticks_per_half_distance * half_distance;
+            percentiles->percentile_to_iterate_to += 100.0 / percentile_reporting_ticks;
+
+            return true;
+        }
+    }
+    while (_basic_iter_next(iter));
+
+    return true;
+}
+
+void hdr_iter_percentile_init(struct hdr_iter* iter, const struct hdr_histogram* h, int32_t ticks_per_half_distance)
+{
+    iter->h = h;
+
+    hdr_iter_init(iter, h);
+
+    iter->specifics.percentiles.seen_last_value          = false;
+    iter->specifics.percentiles.ticks_per_half_distance  = ticks_per_half_distance;
+    iter->specifics.percentiles.percentile_to_iterate_to = 0.0;
+    iter->specifics.percentiles.percentile               = 0.0;
+
+    iter->_next_fp = _percentile_iter_next;
+}
+
+static void format_line_string(char* str, size_t len, int significant_figures, format_type format)
+{
+#if defined(_MSC_VER)
+#define snprintf _snprintf
+#pragma warning(push)
+#pragma warning(disable: 4996)
+#endif
+    const char* format_str = "%s%d%s";
+
+    switch (format)
+    {
+        case CSV:
+            snprintf(str, len, format_str, "%.", significant_figures, "f,%f,%d,%.2f\n");
+            break;
+        case CLASSIC:
+            snprintf(str, len, format_str, "%12.", significant_figures, "f %12f %12d %12.2f\n");
+            break;
+        default:
+            snprintf(str, len, format_str, "%12.", significant_figures, "f %12f %12d %12.2f\n");
+    }
+#if defined(_MSC_VER)
+#undef snprintf
+#pragma warning(pop)
+#endif
+}
+
+
+/* ########  ########  ######   #######  ########  ########  ######## ########   */
+/* ##     ## ##       ##    ## ##     ## ##     ## ##     ## ##       ##     ##  */
+/* ##     ## ##       ##       ##     ## ##     ## ##     ## ##       ##     ##  */
+/* ########  ######   ##       ##     ## ########  ##     ## ######   ##     ##  */
+/* ##   ##   ##       ##       ##     ## ##   ##   ##     ## ##       ##     ##  */
+/* ##    ##  ##       ##    ## ##     ## ##    ##  ##     ## ##       ##     ##  */
+/* ##     ## ########  ######   #######  ##     ## ########  ######## ########   */
+
+
+static bool _recorded_iter_next(struct hdr_iter* iter)
+{
+    while (_basic_iter_next(iter))
+    {
+        if (iter->count != 0)
+        {
+            _update_iterated_values(iter, iter->value);
+
+            iter->specifics.recorded.count_added_in_this_iteration_step = iter->count;
+            return true;
+        }
+    }
+
+    return false;
+}
+
+void hdr_iter_recorded_init(struct hdr_iter* iter, const struct hdr_histogram* h)
+{
+    hdr_iter_init(iter, h);
+
+    iter->specifics.recorded.count_added_in_this_iteration_step = 0;
+
+    iter->_next_fp = _recorded_iter_next;
+}
+
+/* ##       #### ##    ## ########    ###    ########  */
+/* ##        ##  ###   ## ##         ## ##   ##     ## */
+/* ##        ##  ####  ## ##        ##   ##  ##     ## */
+/* ##        ##  ## ## ## ######   ##     ## ########  */
+/* ##        ##  ##  #### ##       ######### ##   ##   */
+/* ##        ##  ##   ### ##       ##     ## ##    ##  */
+/* ######## #### ##    ## ######## ##     ## ##     ## */
+
+
+static bool _iter_linear_next(struct hdr_iter* iter)
+{
+    struct hdr_iter_linear* linear = &iter->specifics.linear;
+
+    linear->count_added_in_this_iteration_step = 0;
+
+    if (has_next(iter) ||
+        next_value_greater_than_reporting_level_upper_bound(
+            iter, linear->next_value_reporting_level_lowest_equivalent))
+    {
+        do
+        {
+            if (iter->value >= linear->next_value_reporting_level_lowest_equivalent)
+            {
+                _update_iterated_values(iter, linear->next_value_reporting_level);
+
+                linear->next_value_reporting_level += linear->value_units_per_bucket;
+                linear->next_value_reporting_level_lowest_equivalent =
+                    lowest_equivalent_value(iter->h, linear->next_value_reporting_level);
+
+                return true;
+            }
+
+            if (!move_next(iter))
+            {
+                return true;
+            }
+
+            linear->count_added_in_this_iteration_step += iter->count;
+        }
+        while (true);
+    }
+
+    return false;
+}
+
+
+void hdr_iter_linear_init(struct hdr_iter* iter, const struct hdr_histogram* h, int64_t value_units_per_bucket)
+{
+    hdr_iter_init(iter, h);
+
+    iter->specifics.linear.count_added_in_this_iteration_step = 0;
+    iter->specifics.linear.value_units_per_bucket = value_units_per_bucket;
+    iter->specifics.linear.next_value_reporting_level = value_units_per_bucket;
+    iter->specifics.linear.next_value_reporting_level_lowest_equivalent = lowest_equivalent_value(h, value_units_per_bucket);
+
+    iter->_next_fp = _iter_linear_next;
+}
+
+/* ##        #######   ######      ###    ########  #### ######## ##     ## ##     ## ####  ######  */
+/* ##       ##     ## ##    ##    ## ##   ##     ##  ##     ##    ##     ## ###   ###  ##  ##    ## */
+/* ##       ##     ## ##         ##   ##  ##     ##  ##     ##    ##     ## #### ####  ##  ##       */
+/* ##       ##     ## ##   #### ##     ## ########   ##     ##    ######### ## ### ##  ##  ##       */
+/* ##       ##     ## ##    ##  ######### ##   ##    ##     ##    ##     ## ##     ##  ##  ##       */
+/* ##       ##     ## ##    ##  ##     ## ##    ##   ##     ##    ##     ## ##     ##  ##  ##    ## */
+/* ########  #######   ######   ##     ## ##     ## ####    ##    ##     ## ##     ## ####  ######  */
+
+static bool _log_iter_next(struct hdr_iter *iter)
+{
+    struct hdr_iter_log* logarithmic = &iter->specifics.log;
+
+    logarithmic->count_added_in_this_iteration_step = 0;
+
+    if (has_next(iter) ||
+        next_value_greater_than_reporting_level_upper_bound(
+            iter, logarithmic->next_value_reporting_level_lowest_equivalent))
+    {
+        do
+        {
+            if (iter->value >= logarithmic->next_value_reporting_level_lowest_equivalent)
+            {
+                _update_iterated_values(iter, logarithmic->next_value_reporting_level);
+
+                logarithmic->next_value_reporting_level *= (int64_t)logarithmic->log_base;
+                logarithmic->next_value_reporting_level_lowest_equivalent = lowest_equivalent_value(iter->h, logarithmic->next_value_reporting_level);
+
+                return true;
+            }
+
+            if (!move_next(iter))
+            {
+                return true;
+            }
+
+            logarithmic->count_added_in_this_iteration_step += iter->count;
+        }
+        while (true);
+    }
+
+    return false;
+}
+
+void hdr_iter_log_init(
+        struct hdr_iter* iter,
+        const struct hdr_histogram* h,
+        int64_t value_units_first_bucket,
+        double log_base)
+{
+    hdr_iter_init(iter, h);
+    iter->specifics.log.count_added_in_this_iteration_step = 0;
+    iter->specifics.log.log_base = log_base;
+    iter->specifics.log.next_value_reporting_level = value_units_first_bucket;
+    iter->specifics.log.next_value_reporting_level_lowest_equivalent = lowest_equivalent_value(h, value_units_first_bucket);
+
+    iter->_next_fp = _log_iter_next;
+}
+
+/* Printing. */
+
+static const char* format_head_string(format_type format)
+{
+    switch (format)
+    {
+        case CSV:
+            return "%s,%s,%s,%s\n";
+        case CLASSIC:
+            return "%12s %12s %12s %12s\n\n";
+        default:
+            return "%12s %12s %12s %12s\n\n";
+    }
+}
+
+static const char CLASSIC_FOOTER[] =
+    "#[Mean    = %12.3f, StdDeviation   = %12.3f]\n"
+    "#[Max     = %12.3f, Total count    = %12" PRIu64 "]\n"
+    "#[Buckets = %12d, SubBuckets     = %12d]\n";
+
+int hdr_percentiles_print(
+        struct hdr_histogram* h, FILE* stream, int32_t ticks_per_half_distance,
+        double value_scale, format_type format)
+{
+    char line_format[25];
+    const char* head_format;
+    int rc = 0;
+    struct hdr_iter iter;
+    struct hdr_iter_percentiles * percentiles;
+
+    format_line_string(line_format, 25, h->significant_figures, format);
+    head_format = format_head_string(format);
+
+    hdr_iter_percentile_init(&iter, h, ticks_per_half_distance);
+
+    if (fprintf(
+            stream, head_format,
+            "Value", "Percentile", "TotalCount", "1/(1-Percentile)") < 0)
+    {
+        rc = EIO;
+        goto cleanup;
+    }
+
+    percentiles = &iter.specifics.percentiles;
+    while (hdr_iter_next(&iter))
+    {
+        double  value               = iter.highest_equivalent_value / value_scale;
+        double  percentile          = percentiles->percentile / 100.0;
+        int64_t total_count         = iter.cumulative_count;
+        double  inverted_percentile = (1.0 / (1.0 - percentile));
+
+        if (fprintf(
+                stream, line_format, value, percentile, total_count, inverted_percentile) < 0)
+        {
+            rc = EIO;
+            goto cleanup;
+        }
+    }
+
+    if (CLASSIC == format)
+    {
+        double mean   = hdr_mean(h)   / value_scale;
+        double stddev = hdr_stddev(h) / value_scale;
+        double max    = hdr_max(h)    / value_scale;
+
+        if (fprintf(
+                stream, CLASSIC_FOOTER,  mean, stddev, max,
+                h->total_count, h->bucket_count, h->sub_bucket_count) < 0)
+        {
+            rc = EIO;
+            goto cleanup;
+        }
+    }
+
+    cleanup:
+    return rc;
+}
diff --git a/deps/histogram/src/hdr_histogram.h b/deps/histogram/src/hdr_histogram.h
new file mode 100644
index 0000000000..4a0f4606b5
--- /dev/null
+++ b/deps/histogram/src/hdr_histogram.h
@@ -0,0 +1,434 @@
+/**
+ * hdr_histogram.h
+ * Written by Michael Barker and released to the public domain,
+ * as explained at http://creativecommons.org/publicdomain/zero/1.0/
+ *
+ * The source for the hdr_histogram utilises a few C99 constructs, specifically
+ * the use of stdint/stdbool and inline variable declaration.
+ */
+
+#ifndef HDR_HISTOGRAM_H
+#define HDR_HISTOGRAM_H 1
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stdio.h>
+
+struct hdr_histogram
+{
+    int64_t lowest_trackable_value;
+    int64_t highest_trackable_value;
+    int32_t unit_magnitude;
+    int32_t significant_figures;
+    int32_t sub_bucket_half_count_magnitude;
+    int32_t sub_bucket_half_count;
+    int64_t sub_bucket_mask;
+    int32_t sub_bucket_count;
+    int32_t bucket_count;
+    int64_t min_value;
+    int64_t max_value;
+    int32_t normalizing_index_offset;
+    double conversion_ratio;
+    int32_t counts_len;
+    int64_t total_count;
+    int64_t* counts;
+};
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Allocate the memory and initialise the hdr_histogram.
+ *
+ * Due to the size of the histogram being the result of some reasonably
+ * involved math on the input parameters this function it is tricky to stack allocate.
+ * The histogram should be released with hdr_close
+ *
+ * @param lowest_trackable_value The smallest possible value to be put into the
+ * histogram.
+ * @param highest_trackable_value The largest possible value to be put into the
+ * histogram.
+ * @param significant_figures The level of precision for this histogram, i.e. the number
+ * of figures in a decimal number that will be maintained.  E.g. a value of 3 will mean
+ * the results from the histogram will be accurate up to the first three digits.  Must
+ * be a value between 1 and 5 (inclusive).
+ * @param result Output parameter to capture allocated histogram.
+ * @return 0 on success, EINVAL if lowest_trackable_value is < 1 or the
+ * significant_figure value is outside of the allowed range, ENOMEM if malloc
+ * failed.
+ */
+int hdr_init(
+    int64_t lowest_trackable_value,
+    int64_t highest_trackable_value,
+    int significant_figures,
+    struct hdr_histogram** result);
+
+/**
+ * Free the memory and close the hdr_histogram.
+ *
+ * @param h The histogram you want to close.
+ */
+void hdr_close(struct hdr_histogram* h);
+
+/**
+ * Allocate the memory and initialise the hdr_histogram.  This is the equivalent of calling
+ * hdr_init(1, highest_trackable_value, significant_figures, result);
+ *
+ * @deprecated use hdr_init.
+ */
+int hdr_alloc(int64_t highest_trackable_value, int significant_figures, struct hdr_histogram** result);
+
+
+/**
+ * Reset a histogram to zero - empty out a histogram and re-initialise it
+ *
+ * If you want to re-use an existing histogram, but reset everything back to zero, this
+ * is the routine to use.
+ *
+ * @param h The histogram you want to reset to empty.
+ *
+ */
+void hdr_reset(struct hdr_histogram* h);
+
+/**
+ * Get the memory size of the hdr_histogram.
+ *
+ * @param h "This" pointer
+ * @return The amount of memory used by the hdr_histogram in bytes
+ */
+size_t hdr_get_memory_size(struct hdr_histogram* h);
+
+/**
+ * Records a value in the histogram, will round this value of to a precision at or better
+ * than the significant_figure specified at construction time.
+ *
+ * @param h "This" pointer
+ * @param value Value to add to the histogram
+ * @return false if the value is larger than the highest_trackable_value and can't be recorded,
+ * true otherwise.
+ */
+bool hdr_record_value(struct hdr_histogram* h, int64_t value);
+
+/**
+ * Records count values in the histogram, will round this value of to a
+ * precision at or better than the significant_figure specified at construction
+ * time.
+ *
+ * @param h "This" pointer
+ * @param value Value to add to the histogram
+ * @param count Number of 'value's to add to the histogram
+ * @return false if any value is larger than the highest_trackable_value and can't be recorded,
+ * true otherwise.
+ */
+bool hdr_record_values(struct hdr_histogram* h, int64_t value, int64_t count);
+
+
+/**
+ * Record a value in the histogram and backfill based on an expected interval.
+ *
+ * Records a value in the histogram, will round this value of to a precision at or better
+ * than the significant_figure specified at contruction time.  This is specifically used
+ * for recording latency.  If the value is larger than the expected_interval then the
+ * latency recording system has experienced co-ordinated omission.  This method fills in the
+ * values that would have occured had the client providing the load not been blocked.
+
+ * @param h "This" pointer
+ * @param value Value to add to the histogram
+ * @param expected_interval The delay between recording values.
+ * @return false if the value is larger than the highest_trackable_value and can't be recorded,
+ * true otherwise.
+ */
+bool hdr_record_corrected_value(struct hdr_histogram* h, int64_t value, int64_t expexcted_interval);
+/**
+ * Record a value in the histogram 'count' times.  Applies the same correcting logic
+ * as 'hdr_record_corrected_value'.
+ *
+ * @param h "This" pointer
+ * @param value Value to add to the histogram
+ * @param count Number of 'value's to add to the histogram
+ * @param expected_interval The delay between recording values.
+ * @return false if the value is larger than the highest_trackable_value and can't be recorded,
+ * true otherwise.
+ */
+bool hdr_record_corrected_values(struct hdr_histogram* h, int64_t value, int64_t count, int64_t expected_interval);
+
+/**
+ * Adds all of the values from 'from' to 'this' histogram.  Will return the
+ * number of values that are dropped when copying.  Values will be dropped
+ * if they around outside of h.lowest_trackable_value and
+ * h.highest_trackable_value.
+ *
+ * @param h "This" pointer
+ * @param from Histogram to copy values from.
+ * @return The number of values dropped when copying.
+ */
+int64_t hdr_add(struct hdr_histogram* h, const struct hdr_histogram* from);
+
+/**
+ * Adds all of the values from 'from' to 'this' histogram.  Will return the
+ * number of values that are dropped when copying.  Values will be dropped
+ * if they around outside of h.lowest_trackable_value and
+ * h.highest_trackable_value.
+ *
+ * @param h "This" pointer
+ * @param from Histogram to copy values from.
+ * @return The number of values dropped when copying.
+ */
+int64_t hdr_add_while_correcting_for_coordinated_omission(
+    struct hdr_histogram* h, struct hdr_histogram* from, int64_t expected_interval);
+
+/**
+ * Get minimum value from the histogram.  Will return 2^63-1 if the histogram
+ * is empty.
+ *
+ * @param h "This" pointer
+ */
+int64_t hdr_min(const struct hdr_histogram* h);
+
+/**
+ * Get maximum value from the histogram.  Will return 0 if the histogram
+ * is empty.
+ *
+ * @param h "This" pointer
+ */
+int64_t hdr_max(const struct hdr_histogram* h);
+
+/**
+ * Get the value at a specific percentile.
+ *
+ * @param h "This" pointer.
+ * @param percentile The percentile to get the value for
+ */
+int64_t hdr_value_at_percentile(const struct hdr_histogram* h, double percentile);
+
+/**
+ * Gets the standard deviation for the values in the histogram.
+ *
+ * @param h "This" pointer
+ * @return The standard deviation
+ */
+double hdr_stddev(const struct hdr_histogram* h);
+
+/**
+ * Gets the mean for the values in the histogram.
+ *
+ * @param h "This" pointer
+ * @return The mean
+ */
+double hdr_mean(const struct hdr_histogram* h);
+
+/**
+ * Determine if two values are equivalent with the histogram's resolution.
+ * Where "equivalent" means that value samples recorded for any two
+ * equivalent values are counted in a common total count.
+ *
+ * @param h "This" pointer
+ * @param a first value to compare
+ * @param b second value to compare
+ * @return 'true' if values are equivalent with the histogram's resolution.
+ */
+bool hdr_values_are_equivalent(const struct hdr_histogram* h, int64_t a, int64_t b);
+
+/**
+ * Get the lowest value that is equivalent to the given value within the histogram's resolution.
+ * Where "equivalent" means that value samples recorded for any two
+ * equivalent values are counted in a common total count.
+ *
+ * @param h "This" pointer
+ * @param value The given value
+ * @return The lowest value that is equivalent to the given value within the histogram's resolution.
+ */
+int64_t hdr_lowest_equivalent_value(const struct hdr_histogram* h, int64_t value);
+
+/**
+ * Get the count of recorded values at a specific value
+ * (to within the histogram resolution at the value level).
+ *
+ * @param h "This" pointer
+ * @param value The value for which to provide the recorded count
+ * @return The total count of values recorded in the histogram within the value range that is
+ * {@literal >=} lowestEquivalentValue(<i>value</i>) and {@literal <=} highestEquivalentValue(<i>value</i>)
+ */
+int64_t hdr_count_at_value(const struct hdr_histogram* h, int64_t value);
+
+int64_t hdr_count_at_index(const struct hdr_histogram* h, int32_t index);
+
+int64_t hdr_value_at_index(const struct hdr_histogram* h, int32_t index);
+
+struct hdr_iter_percentiles
+{
+    bool seen_last_value;
+    int32_t ticks_per_half_distance;
+    double percentile_to_iterate_to;
+    double percentile;
+};
+
+struct hdr_iter_recorded
+{
+    int64_t count_added_in_this_iteration_step;
+};
+
+struct hdr_iter_linear
+{
+    int64_t value_units_per_bucket;
+    int64_t count_added_in_this_iteration_step;
+    int64_t next_value_reporting_level;
+    int64_t next_value_reporting_level_lowest_equivalent;
+};
+
+struct hdr_iter_log
+{
+    double log_base;
+    int64_t count_added_in_this_iteration_step;
+    int64_t next_value_reporting_level;
+    int64_t next_value_reporting_level_lowest_equivalent;
+};
+
+/**
+ * The basic iterator.  This is a generic structure
+ * that supports all of the types of iteration.  Use
+ * the appropriate initialiser to get the desired
+ * iteration.
+ *
+ * @
+ */
+struct hdr_iter
+{
+    const struct hdr_histogram* h;
+    /** raw index into the counts array */
+    int32_t counts_index;
+    /** snapshot of the length at the time the iterator is created */
+    int32_t total_count;
+    /** value directly from array for the current counts_index */
+    int64_t count;
+    /** sum of all of the counts up to and including the count at this index */
+    int64_t cumulative_count;
+    /** The current value based on counts_index */
+    int64_t value;
+    int64_t highest_equivalent_value;
+    int64_t lowest_equivalent_value;
+    int64_t median_equivalent_value;
+    int64_t value_iterated_from;
+    int64_t value_iterated_to;
+
+    union
+    {
+        struct hdr_iter_percentiles percentiles;
+        struct hdr_iter_recorded recorded;
+        struct hdr_iter_linear linear;
+        struct hdr_iter_log log;
+    } specifics;
+
+    bool (* _next_fp)(struct hdr_iter* iter);
+
+};
+
+/**
+ * Initalises the basic iterator.
+ *
+ * @param itr 'This' pointer
+ * @param h The histogram to iterate over
+ */
+void hdr_iter_init(struct hdr_iter* iter, const struct hdr_histogram* h);
+
+/**
+ * Initialise the iterator for use with percentiles.
+ */
+void hdr_iter_percentile_init(struct hdr_iter* iter, const struct hdr_histogram* h, int32_t ticks_per_half_distance);
+
+/**
+ * Initialise the iterator for use with recorded values.
+ */
+void hdr_iter_recorded_init(struct hdr_iter* iter, const struct hdr_histogram* h);
+
+/**
+ * Initialise the iterator for use with linear values.
+ */
+void hdr_iter_linear_init(
+    struct hdr_iter* iter,
+    const struct hdr_histogram* h,
+    int64_t value_units_per_bucket);
+
+/**
+ * Initialise the iterator for use with logarithmic values
+ */
+void hdr_iter_log_init(
+    struct hdr_iter* iter,
+    const struct hdr_histogram* h,
+    int64_t value_units_first_bucket,
+    double log_base);
+
+/**
+ * Iterate to the next value for the iterator.  If there are no more values
+ * available return faluse.
+ *
+ * @param itr 'This' pointer
+ * @return 'false' if there are no values remaining for this iterator.
+ */
+bool hdr_iter_next(struct hdr_iter* iter);
+
+typedef enum
+{
+    CLASSIC,
+    CSV
+} format_type;
+
+/**
+ * Print out a percentile based histogram to the supplied stream.  Note that
+ * this call will not flush the FILE, this is left up to the user.
+ *
+ * @param h 'This' pointer
+ * @param stream The FILE to write the output to
+ * @param ticks_per_half_distance The number of iteration steps per half-distance to 100%
+ * @param value_scale Scale the output values by this amount
+ * @param format_type Format to use, e.g. CSV.
+ * @return 0 on success, error code on failure.  EIO if an error occurs writing
+ * the output.
+ */
+int hdr_percentiles_print(
+    struct hdr_histogram* h, FILE* stream, int32_t ticks_per_half_distance,
+    double value_scale, format_type format);
+
+/**
+* Internal allocation methods, used by hdr_dbl_histogram.
+*/
+struct hdr_histogram_bucket_config
+{
+    int64_t lowest_trackable_value;
+    int64_t highest_trackable_value;
+    int64_t unit_magnitude;
+    int64_t significant_figures;
+    int32_t sub_bucket_half_count_magnitude;
+    int32_t sub_bucket_half_count;
+    int64_t sub_bucket_mask;
+    int32_t sub_bucket_count;
+    int32_t bucket_count;
+    int32_t counts_len;
+};
+
+int hdr_calculate_bucket_config(
+    int64_t lowest_trackable_value,
+    int64_t highest_trackable_value,
+    int significant_figures,
+    struct hdr_histogram_bucket_config* cfg);
+
+void hdr_init_preallocated(struct hdr_histogram* h, struct hdr_histogram_bucket_config* cfg);
+
+int64_t hdr_size_of_equivalent_value_range(const struct hdr_histogram* h, int64_t value);
+
+int64_t hdr_next_non_equivalent_value(const struct hdr_histogram* h, int64_t value);
+
+int64_t hdr_median_equivalent_value(const struct hdr_histogram* h, int64_t value);
+
+/**
+ * Used to reset counters after importing data manuallying into the histogram, used by the logging code
+ * and other custom serialisation tools.
+ */
+void hdr_reset_internal_counters(struct hdr_histogram* h);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
\ No newline at end of file
diff --git a/deps/histogram/src/hdr_tests.h b/deps/histogram/src/hdr_tests.h
new file mode 100644
index 0000000000..c016d3a6de
--- /dev/null
+++ b/deps/histogram/src/hdr_tests.h
@@ -0,0 +1,22 @@
+#ifndef HDR_TESTS_H
+#define HDR_TESTS_H
+
+/* These are functions used in tests and are not intended for normal usage. */
+
+#include "hdr_histogram.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int32_t counts_index_for(const struct hdr_histogram* h, int64_t value);
+int hdr_encode_compressed(struct hdr_histogram* h, uint8_t** compressed_histogram, size_t* compressed_len);
+int hdr_decode_compressed(uint8_t* buffer, size_t length, struct hdr_histogram** histogram);
+void hdr_base64_decode_block(const char* input, uint8_t* output);
+void hdr_base64_encode_block(const uint8_t* input, char* output);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif