rfc:cachediterable

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PHP RFC: ImmutableIterable (immutable, rewindable, memory-efficient, allows any key&repeating keys)

Introduction

Currently, PHP does not provide a built-in way to store the state of an arbitrary iterable for reuse later (when the iterable has arbitrary keys, or when keys might be repeated). It would be useful to do so for many use cases, such as:

  1. Creating a rewindable copy of a non-rewindable Traversable (e.g. a Generator) before passing that copy to a function that consumes an iterable/Traversable. (new CachedIterator(my_generator()))
  2. Generating an IteratorAggregate from a class still implementing Iterator (e.g. SplObjectStorage) so that code can independently iterate over the key-value sequences.
    (e.g. foreach ($immutableKeyValueSequence as $k1 => $v1) { foreach ($immutableKeyValueSequence as $k2 => $v2) { /* process pairs */ } })
  3. Providing internal or userland helpers such as iterable_flip(iterable $input), iterable_take(iterable $input, int $limit), iterable_chunk(iterable $input, int $chunk_size) that act on iterables with arbitrary key/value sequences and have return values including iterables with arbitrary key/value sequences
  4. Providing constant time access to both keys and values of arbitrary key-value sequences at any offset (for binary searching on keys and/or values, etc.)

Having this implemented as an internal class would also allow it to be much more efficient than a userland solution (in terms of time to create, time to iterate over the result, and total memory usage).

Proposal

Add a class ImmutableIterable that contains an immutable copy of the keys and values of the iterable it was constructed from. (references inside of arrays within those keys/values will remain modifiable references, and objects within those keys/values will remain mutable)

final class ImmutableIterable implements 
    IteratorAggregate,
    Countable,
    JsonSerializable 
{
    public function __construct(iterable $iterator) {}
    public function getIterator(): InternalIterator {}
    public function count(): int {}
    // [[$key1, $value1], [$key2, $value2]]
    public static function fromPairs(array $pairs): ImmutableIterable {}
    // [[$key1, $value1], [$key2, $value2]]
    public function toPairs(): array{}
    public function __serialize(): array {}  // [$k1, $v1, $k2, $v2,...]
    public function __unserialize(array $data): void {}
    public static function __set_state(array $array): ImmutableIterable {}
 
    // useful for converting iterables back to arrays for further processing
    public function keys(): array {}  // [$k1, $k2, ...]
    public function values(): array {}  // [$v1, $v2, ...]
    // useful to efficiently get offsets at the middle/end of a long iterable
    public function keyAt(int $offset): mixed {}
    public function valueAt(int $offset): mixed {}
 
    // '[["key1","value1"],["key2","value2"]]' instead of '{...}'
    public function jsonSerialize(): array {}
    // dynamic properties are forbidden
}

ImmutableIterables are IteratorAggregates, so foreach loops do not interfere with each other.

$x = new ImmutableIterable([0 => 100, 'key' => 'value']);
foreach ($x as $key1 => $value1) {
    echo "$key1 $value1:\n";
    foreach ($x as $key2 => $value2) {
        echo "- $key2 $value2\n";
    }
}
/*
0 100:
- 0 100
- key value
key value:
- 0 100
- key value
 */

ImmutableIterables can be used to cache and efficiently process results of Traversables

ImmutableIterables can be created from any iterable (arrays or Traversables). They eagerly evaluate the results of Traversables (e.g. Generators) and store an exact copy of the keys and values that can be processed in many ways.

In comparison to php's array/ArrayObject type:

  • Arrays can only store integers and strings
  • Arrays coerce stringified integers to integers, potentially causing unexpected Errors/notices (especially when strict_types=1)
  • Arrays cannot represent repeated keys
function my_generator() {
    yield from ['first array'];
    yield from ['repeated key is allowed'];
    yield '0' => 'string key is preserved';
    yield ['an array'] => null;  // any type can be used as keys
    echo "Finished iterating over the generator\n";
}
 
$x = new ImmutableIterable(my_generator());
foreach ($x as $k => $v) {
    printf("%s: %s\n", json_encode($k), json_encode($v));
}
/*
Finished iterating over the generator
0: 'first array'
0: 'repeated key is allowed'
'0': 'string key is preserved'
["an array"]: null
 */
 
printf("Keys: %s\n", json_encode($x->keys()));
printf("Values: %s\n", json_encode($x->values()));
/*
Keys: [0,0,"0",["an array"]]
Values: ["first array","repeated key is allowed","string key is preserved",null]
 */
printf("Last key: %s\n", json_encode($x->keyAt(count($x) - 1)));
// Last key: ["an array"]

ImmutableIterables are immutable

ImmutableIterable is a final class.

Dynamic properties are forbidden on ImmutableIterables.

The keys and values of the ImmutableIterable cannot be modified or appended to after an instance is constructed, though objects and references within those values can be modified.

This makes it useful for returning to wrap the keys and values that would be returned by a generator or single-use Iterator (it can't be modified after being constructed by other applications or libraries)

ImmutableIterables can be created from pairs

This can be done imperatively, to avoid the need to manually create a generator with the sequence of keys and values to pass to the constructor. The values of an iterable (array or Traversable) can be used.

$it = ImmutableIterable::fromPairs([['first', 'x'], [(object)['key' => 'value'], null]]);
foreach ($it as $key => $value) {
    printf("key=%s value=%s\n", json_encode($key), json_encode($value));
}
/*
key="first" value="x"
key={"key":"value"} value=null
*/
var_dump($it);
/*
object(ImmutableIterable)#2 (1) {
  [0]=>
  array(2) {
    [0]=>
    string(5) "first"
    [1]=>
    string(1) "x"
  }
}
*/
php > echo json_encode((array)$it), "\n";
[["first","x"],[{"key":"value},null]]

ImmutableIterables can also be converted back into pairs for further processing (e.g. using the wide array of helper methods php has for processing arrays):

php > $reversedIt = ImmutableIterable::fromPairs(array_reverse($it->toPairs()));
php > echo json_encode($reversedIt->toPairs());
[[{"key":"value"},null],["first","x"]]

Benchmarks

ImmutableIterables are memory-efficient

Similarly to how SplFixedArray is a memory-efficient way to store a list of values, ImmutableIterable is a memory-efficient way to eagerly evaluate and store a sequence of arbitrary keys and values.

<?php
 
function show_array_memory(int $n) {
    gc_collect_cycles();
    $before = memory_get_usage();
    $result = array_flip(range(10, 10 + $n - 1));  // create an **associative** array of size $n
    $after = memory_get_usage();
    printf("array memory:          (n=%5d) %7d bytes\n", count($result), $after - $before);
}
function show_cachediterable_memory(int $n) {
    gc_collect_cycles();
    $before = memory_get_usage();
    // create a ImmutableIterable from an **associative** array of size $n
    $result = new ImmutableIterable(array_flip(range(10, 10 + $n - 1)));
    $after = memory_get_usage();
    printf("ImmutableIterable memory: (n=%5d) %7d bytes\n", count($result), $after - $before);
}
foreach ([1, 8, 12, 16, 2**16] as $n) {
    show_array_memory($n);
    show_cachediterable_memory($n);
}
/*
array memory:             (n=    1)     376 bytes
ImmutableIterable memory: (n=    1)      88 bytes
array memory:             (n=    8)     376 bytes
ImmutableIterable memory: (n=    8)     312 bytes
array memory:             (n=   12)    1336 bytes
ImmutableIterable memory: (n=   12)     440 bytes
array memory:             (n=   16)    1336 bytes
ImmutableIterable memory: (n=   16)     568 bytes
array memory:             (n=65536) 4198480 bytes
ImmutableIterable memory: (n=65536) 2097232 bytes
 */

ImmutableIterables are much more efficient than a polyfill object

For a simple example, this uses much less time to construct. It is almost 6 times faster to iterate over and process results than a polyfill in that example, and uses half as much additional memory.

<?php
/*
Time to construct PolyfillImmutableIterator: 0.244787
Time to iterate: 0.183351, memory usage: 67117328
result:999000000
 
Time to construct         ImmutableIterable: 0.130534
Time to iterate: 0.021905, memory usage: 32002128
result:999000000
 */
 
/**
 * THIS IS AN INCOMPLETE POLYFILL THAT ONLY SUPPORTS ITERATION, AND DOES NOT INCLUDE ERROR HANDLING.
 *
 * Barely any of the functionality in the proposal is implemented.
 * This is just here to compare a fast (in terms of time to iterate) userland polyfill
 * against ImmutableIterable.
 *
 * Not an IteratorAggregate for simplicity.
 */
class PolyfillImmutableIterator implements Iterator {
    public $i = 0;
    public $count = 0;
    public $keys;
    public $values;
    public function __construct(iterable $data) {
        $keys = [];
        $values = [];
        foreach ($data as $key => $value) {
            $keys[] = $key;
            $values[] = $value;
        }
        $this->keys = $keys;
        $this->values = $values;
        $this->count = count($keys);
    }
    public function rewind() { $this->i = 0; }
    public function valid(): bool { return $this->i < $this->count; }
    public function key() { return $this->keys[$this->i]; }
    public function current() { return $this->values[$this->i]; }
    public function next(): void { $this->i++; }
}
 
function a_generator() {
    for ($i = 0; $i < 1000; $i++) {
        for ($j = 0; $j < 1000; $j++) {
            yield $j => $i;
        }
    }
}
 
function benchmark(string $class) {
    gc_collect_cycles();
    $memory_usage_1 = memory_get_usage();
    $t1 = microtime(true);
    $it = new $class(a_generator());
    $t2 = microtime(true);
    $total = 0;
    foreach ($it as $k => $v) {
        $total += $k + $v;
    }
    $t3 = microtime(true);
    gc_collect_cycles();
    $memory_usage_2 = memory_get_usage();
    printf("Time to construct %25s: %.6f\nTime to iterate: %.6f, memory usage: %d\nresult:%d\n\n",
        $class, $t2 - $t1, $t3 - $t2, $memory_usage_2 - $memory_usage_1, $total);
}
benchmark(PolyfillImmutableIterator::class);
benchmark(ImmutableIterable::class);

ImmutableIterables support constant-time access to keys and values

ImmutableIterables support constant-time access to keys and values, allowing the result to be used in a wide variety of ways in an application. For example, it is possible to do binary search on keys (and/or values) without using any additional time or memory to create a copy of the keys. (Same for values).

<?php
/**
 * @return int the offset of the first key in $it that is >= $target.
 * Returns count($it) if all keys are smaller than $target.
 */
function do_binary_search_on_key(ImmutableIterable $it, int $target) {
    $lowOffset = 0;
    $highOffset = count($it) - 1;
    while ($lowOffset <= $highOffset) {
        $mid = $lowOffset + (($highOffset - $lowOffset) >> 1);
        $key = $it->keyAt($mid);
        if ($key < $target) {
            echo "at offset $mid: $key <= $target\n";
            $lowOffset = $mid + 1;
        } else {
            echo "at offset $mid: $key > $target\n";
            $highOffset = $mid - 1;
        }
    }
    echo "offset $lowOffset has the first key ({$it->keyAt($lowOffset)}) >= $target " .
         ": associated value={$it->valueAt($lowOffset)}\n";
    return $lowOffset;
}
 
mt_srand(123);
$data = [];
$N = 1000;
for ($i = 0; $i < $N; $i++) {
    $data[mt_rand()] = "value$i";
}
ksort($data);
$it = new ImmutableIterable($data);
 
do_binary_search_on_key($it, mt_rand());
/*
at offset 499: 1039143806 > 457052171
at offset 249: 595271545 > 457052171
at offset 124: 262516026 <= 457052171
at offset 186: 438739745 <= 457052171
at offset 217: 511637778 > 457052171
at offset 201: 468958912 > 457052171
at offset 193: 442664110 <= 457052171
at offset 197: 455906707 <= 457052171
at offset 199: 462794419 > 457052171
at offset 198: 459587085 > 457052171
offset 198 has the first key (459587085) >= 457052171 : associated value=value530
 */

Backward Incompatible Changes

None, except that the class name ImmutableIterable will be declared by PHP and conflict with applications declaring the same class name in that namespace.

Proposed PHP Version(s)

8.1

Future Scope

  • This will enable adding internal iterable functions such as *take(iterable $input, int $limit): ImmutableIterable or *flip(iterable $input): ImmutableIterable or
  • More methods may be useful to add to ImmutableIterable, e.g. for returning a sorted copy, returning a slice(range of entries), returning a copy sorted by keys/values, quickly returning the index/corresponding value of the first occurrence of mixed $key etc.
  • This may or may not be useful for future data types, e.g. a MapObject (hash map on any key type) type and may potentially be useful for converting some existing internal/user-defined Iterable types to IteratorAggregate types.

Proposed Voting Choices

Yes/No, requiring a 2/3 majority.

References

Rejected Features

Rejected: ArrayAccess

From https://github.com/php/php-src/pull/6655#issuecomment-770444285

I think ArrayAccess would lead to more bugs in application code for those expecting $cachedIt[$i] to find the value corresponding to the first key occurrence of $i - adding keyAt(int $offset): mixed, valueAt(int $offset): mixed, keyIndex(mixed $key): int, valueIndex(mixed $value): int would be my preference for fetching values.

Rejected: Lazy Evaluation

ImmutableIterable evaluates the entire iterable in its constructor (eagerly instead of lazily) for the following reasons:

  • Exceptions will be thrown during construction instead of during iteration or call to count()/keyAt()/valueAt() - it would be unintuitive for those to throw SomeUserlandFrameworkException
  • This is easier to understand, debug, serialize, and represent
  • If the underlying iterable (e.g. a Generator) has side effects, having those side effects take place immediately instead of being interleaved with other parts of the program may be easier to reason about.
  • The majority of use cases of Traversables would iterate over the entire Traversable at some point.
  • Eagerly evaluating iterables reduces the memory needed by the implementation. The amount of memory needed to represent this is much lower (without the need to store the underlying iterable, potentially the most recent exception(s) thrown by the undlying iterable, etc).

The addition of an iterable library class that evaluates arguments on-demand is mentioned in the “future scope” section.

https://externals.io/message/114805#114798

2) Userland library/application authors that are interested in lazy generators could use or implement something such as https://github.com/nikic/iter instead. My opinion is that the standard library should provide something that is easy to understand, debug, serialize or represent, etc. I expect the inner iterable may be hidden entirely in a (lazy) CachedIterable from var_dump as an implementation detail.

3) It would be harder to understand why SomeFrameworkException is thrown in code unrelated to that framework when a lazy (instead of eager) iterable is passed to some function that accepts a generic iterable, and harder to write correct exception handling for it if done in a lazy generation style.

Many RFCs have been rejected due to being perceived as being likely to be misused in userland or to make code harder to understand.

4) It is possible to implement a lazy alternative to (ImmutableIterable) that only loads values as needed. However, I hadn't proposed it due to doubts that 2/3 of voters would consider it widely useful enough to be included in php rather than as a userland or PECL library.

CachedIterable should load from the underlying datastore lazily -- there is hardly any visible impact from the user if this happens, because for the most part it looks and behaves the same as it does today. The only visible changes are around loading data from the underlying iterable.

For example, if the user calls the count method on the CachedIterable, it would then load the remainder of the underlying data-store (and then drop its reference to it). If the user asks for valueAt($n) and it's beyond what's already loaded and we haven't finished consuming the underlying iterable, then it would load until $n is found or the end of the store is reached.

I understand your concerns with map, filter, etc. CachedIterable is different because it holds onto the data, can be iterated over more than once, including the two nested loop cases, even if it loads data from the underlying iterable on demand.

Thanks for explaining 4 months ago about my concern. I think I understand the main real impact of an eager iterable cache vs a lazy iterable cache from a functional point of view:

  • exceptions are thrown during construction vs during the first iteration
  • predictable performance also on the first iteration.

How did you gather the information that eager implementation is more valuable than lazy one? I'm mostly curious also how to assess this as technically to me it also looks the other way around. Maybe mention that in the RFC. I was even thinking that CachedIterable should be lazy and an EagerCachedIterable would be built upon that with more methods. Or have it in the same class with a constructor parameter.

One of the reasons was size/efficiency. Adding the functionality to support lazy evaluation would require extra properties to track internal state and extra checks at runtime, point to the original iterable and the functions being applied to that iterable - so an application that creates lots of small/empty cached iterables would have a higher memory usage.

Having a data structure that tries to do everything would do other things poorly (potentially not support serialization, use more memory than necessary, have unintuitive behaviors when attempting to var_export/var_dump it, surprisingly throw when being iterated over, etc)

Changelog

  • 0.2: Use optimized build with opcache enabled for benchmark timings
  • 0.3: Rename from CachedIterable to ImmutableKeyValueSequence (the lack of clarity about the functionality associated with the name CachedIterable being eagerly evalulated was mentioned *after* most of the responses to the straw poll were already submitted)
  • 0.3.1: Add __set_state
  • 0.4.0: Rename from ImmutableKeyValueSequence to ImmutableIterable
rfc/cachediterable.1623763922.txt.gz · Last modified: 2021/06/15 13:32 by tandre