commonMain.EvaluationEngine.kt Maven / Gradle / Ivy
package com.amplitude.experiment.evaluation
import kotlin.native.concurrent.SharedImmutable
interface EvaluationEngine {
fun evaluate(flags: List, user: SkylabUser?): Map
}
@SharedImmutable
private const val MAX_HASH_VALUE = 4294967295L
@SharedImmutable
private const val MAX_VARIANT_HASH_VALUE = MAX_HASH_VALUE.floorDiv(100)
internal data class EvaluationResult(val variant: Variant, val description: String) {
companion object {
const val DESC_MISSING_USER_FULLY_ROLLED_OUT = "missing-user-fully-rolled-out-variant"
const val DESC_MISSING_USER_DEFAULT_VARIANT = "missing-user-default-variant"
const val DESC_FULLY_ROLLED_OUT_VARIANT = "fully-rolled-out-variant"
const val DESC_DEFAULT_SEGMENT = "default-segment"
const val DESC_INCLUSION_LIST = "inclusion-list"
const val DESC_FLAG_DISABLED = "flag-disabled"
}
}
class EvaluationEngineImpl : EvaluationEngine {
override fun evaluate(flags: List, user: SkylabUser?): Map {
val result: MutableMap = mutableMapOf()
for (flag in flags) {
val evalResult = evaluateFlag(flag, user)
val flagResult = FlagResult(flag, evalResult)
result[flag.flagKey] = flagResult
}
return result
}
internal fun evaluateFlag(flag: FlagConfig, user: SkylabUser?): EvaluationResult {
var result = checkEnabled(flag) ?: checkEmptyUser(flag, user)
if (result != null) {
return result
}
if (user == null) {
throw RuntimeException("User should always be non-null at this point.")
}
val excludedVariantsForUser = getExclusions(flag, user)
result = checkInclusions(flag, user, excludedVariantsForUser)
if (result != null) {
return result
}
val bucketingValue = user.getBucketingValue(flag.bucketingKey)
// Now we have a bucketing value
result = checkSegmentRules(flag, user, bucketingValue, excludedVariantsForUser)
?: checkAllUsersRule(flag, user, bucketingValue, excludedVariantsForUser)
return result
}
private fun scaled(pct: Double, max: Long): Double {
// add 1 to max to allow for range [0, max+1) when comparing the upper bound (which uses <, not <=)
return pct * (max + 1)
}
private fun checkEnabled(flag: FlagConfig): EvaluationResult? {
return if (!flag.enabled) {
EvaluationResult(Variant(flag.defaultValue), EvaluationResult.DESC_FLAG_DISABLED)
} else null
}
private fun checkEmptyUser(flag: FlagConfig, user: SkylabUser?): EvaluationResult? {
// if the user is null, return a fully rolled out variant if any, otherwise return the default
if (user == null) {
val variant = getFullyRolledOutVariantIfPresent(flag.allUsersTargetingConfig.allocations, flag.variants)
return if (variant != null) {
EvaluationResult(variant, EvaluationResult.DESC_MISSING_USER_FULLY_ROLLED_OUT)
} else {
EvaluationResult(Variant(flag.defaultValue), EvaluationResult.DESC_MISSING_USER_DEFAULT_VARIANT)
}
}
return null
}
private fun checkSegmentRules(
flag: FlagConfig,
user: SkylabUser?,
bucketingValue: String?,
excludedVariantsForUser: Set
): EvaluationResult? {
// check custom target segments and see if a user falls in any of the target segments
if (flag.customSegmentTargetingConfigs != null && flag.customSegmentTargetingConfigs.isNotEmpty()) {
for (segTargetingConfig in flag.customSegmentTargetingConfigs) {
if (!segTargetingConfig.match(user)) {
continue
}
val resolvedBucketingValue = if (segTargetingConfig.bucketingKey.isNullOrEmpty()) {
bucketingValue
} else {
user?.getBucketingValue(segTargetingConfig.bucketingKey)
}
// user matches filters. Serve the variantKey based on the current segment's distribution
val variant = getVariantBasedOnRollout(
flag.variants,
segTargetingConfig.allocations,
flag.defaultValue,
excludedVariantsForUser,
flag.bucketingSalt,
resolvedBucketingValue,
)
return EvaluationResult(variant, segTargetingConfig.name)
}
}
return null
}
private fun checkAllUsersRule(
flag: FlagConfig,
user: SkylabUser?,
bucketingValue: String?,
excludedVariantsForUser: Set
): EvaluationResult {
// Optimization: we have already computed that the flag has been fully rolled out to a single variant. And we
// got to this point because there is no special allowlist/blocklist/custom-target-segment for this user
val fullyRolledOutVariant =
getFullyRolledOutVariantIfPresent(flag.allUsersTargetingConfig.allocations, flag.variants)
if (fullyRolledOutVariant != null) {
if (!excludedVariantsForUser.contains(fullyRolledOutVariant.key)) {
return EvaluationResult(fullyRolledOutVariant, EvaluationResult.DESC_FULLY_ROLLED_OUT_VARIANT)
}
}
val bucketingKey = flag.allUsersTargetingConfig.bucketingKey
val resolvedBucketingValue = if (bucketingKey.isNullOrEmpty()) {
bucketingValue
} else {
user?.getBucketingValue(bucketingKey)
}
// fall back to the all users target segment
val variant = getVariantBasedOnRollout(
flag.variants,
flag.allUsersTargetingConfig.allocations,
flag.defaultValue,
excludedVariantsForUser,
flag.bucketingSalt,
resolvedBucketingValue
)
return EvaluationResult(variant, EvaluationResult.DESC_DEFAULT_SEGMENT)
}
private fun getExclusions(flag: FlagConfig, user: SkylabUser): Set {
val excludedVariantsForUser: MutableSet = mutableSetOf()
// check variant exclusions and inclusions together. If a user is in both lists, the key won't be served as
// exclusion takes priority
if (flag.variantsExclusions == null) {
return excludedVariantsForUser
}
for (variant in flag.variants) {
val exclusions = flag.variantsExclusions[variant.key] ?: continue
if (exclusions.contains(user.userId) || exclusions.contains(user.deviceId)) {
// can't be served this key. Keep track to make sure the user doesn't get served this key later
excludedVariantsForUser.add(variant.key)
}
}
return excludedVariantsForUser
}
private fun checkInclusions(
flag: FlagConfig,
user: SkylabUser,
excludedVariantsForUser: Set
): EvaluationResult? {
if (flag.variantsInclusions == null) {
return null
}
for (variant in flag.variants) {
val inclusions = flag.variantsInclusions[variant.key] ?: continue
if (excludedVariantsForUser.contains(variant.key)) {
continue
}
if (inclusions.contains(user.userId) || inclusions.contains(user.deviceId)) {
// return the first match
return EvaluationResult(variant, EvaluationResult.DESC_INCLUSION_LIST)
}
}
return null
}
internal fun getHash(key: String): Long {
// hash32x86 returns a number that can't fit in a signed 32-bit java integer.
// Source: https://stackoverflow.com/a/24090718/2322146
val data = key.encodeToByteArray()
val value = Murmur3.hash32x86(data, data.size, 0)
return value.toLong() and 0xffffffffL
}
internal fun getVariantBasedOnRollout(
variants: List,
allocations: List,
defaultValue: String?,
excludedVariantsForUser: Set,
bucketingSalt: String?,
bucketingValue: String?,
): Variant {
if (bucketingValue.isNullOrEmpty()) {
return getFullyRolledOutVariantIfPresent(allocations, variants) ?: Variant(defaultValue)
}
val keyToHash = "$bucketingSalt/$bucketingValue"
val hash = getHash(keyToHash)
val bucket = hash % 100
val variantHash = hash.floorDiv(100)
var minBucket: Long
var maxBucket: Long = 0
for (allocation in allocations) {
minBucket = maxBucket
maxBucket += (allocation.percentage / 100).toLong()
if (bucket in minBucket until maxBucket) {
val distribution = allocation.getVariantDistribution(variants)
if (distribution.isEmpty()) {
continue
}
// rolled out, serve the appropriate variant
var upperBound: Double
for (slice in distribution) {
if (slice.pct <= 0) {
continue
}
upperBound = scaled(slice.cumulativePct, MAX_VARIANT_HASH_VALUE)
if (variantHash >= upperBound) {
continue
}
return if (excludedVariantsForUser.contains(slice.variant.key)) {
Variant(defaultValue)
} else slice.variant
}
}
}
return Variant(defaultValue)
}
private fun getFullyRolledOutVariantIfPresent(allocations: List, variants: List): Variant? {
val totalAllocationPercentage: Int = allocations.sumOf { it.percentage }
if (totalAllocationPercentage < 10000) {
return null
}
// If a flag is rolled out to 100% and there's only one variant, return the variant
if (variants.size == 1) {
return variants[0]
}
val weights: Map = allocations[0].weights
?: return null
var fullyRolledOutVariant: Variant? = null
var variantsWithWeights = 0
for (variant in variants) {
if ((weights[variant.key] ?: 0) > 0) {
fullyRolledOutVariant = variant
variantsWithWeights++
}
}
if (variantsWithWeights == 1) {
return fullyRolledOutVariant
}
return null
}
}
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