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New APIs: ByteString.toIndex() and ByteString.toFraction()
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The first one may be useful with hashing to put byte strings in
partitioning buckets for scaling. For example, to divide a dataset
into 32 partitions, hash the key then use toIndex(32) to map the
key to its partition.

The second one may be useful with dynamic experiments and A/B
tests. For example, to assign a control group to 5% of customers
call toFraction() and put the user in the control group if the
result is less than 0.05.
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@squarejesse
squarejesse committed Jun 1, 2020
1 parent 3181015 commit b2ea609
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93 changes: 93 additions & 0 deletions okio/src/commonMain/kotlin/okio/ByteString.kt
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Expand Up @@ -20,6 +20,7 @@ import kotlin.jvm.JvmField
import kotlin.jvm.JvmName
import kotlin.jvm.JvmOverloads
import kotlin.jvm.JvmStatic
import kotlin.math.min

/**
* An immutable sequence of bytes.
Expand Down Expand Up @@ -137,6 +138,98 @@ internal constructor(data: ByteArray) : Comparable<ByteString> {

override fun compareTo(other: ByteString): Int

/**
* Projects this value to the range `[0..size)` using linear interpolation. This is equivalent to
* a sorted partitioning of all possible byte strings across [size] equally-sized buckets and
* returning the index of the bucket that this byte string fits in.
*
* For example, the byte string `8000` is the median of all 2-element byte strings, and calling
* `toIndex(100)` on it returns 50. Some other examples:
*
* | Byte String (hex) | `toIndex(100)` | `toIndex(256)` | `toIndex(Int.MAX_VALUE)` |
* | :----------------- | -------------: | -------------: | -----------------------: |
* | (empty) | 0 | 0 | 0 |
* | 00 | 0 | 0 | 0 |
* | 0000 | 0 | 0 | 0 |
* | 000000 | 0 | 0 | 0 |
* | 0000000001 | 0 | 0 | 0 |
* | 00000001 | 0 | 0 | 0 |
* | 00000002 | 0 | 0 | 0 |
* | 00000003 | 0 | 0 | 1 |
* | 01 | 0 | 1 | 8388607 |
* | 02 | 0 | 2 | 16777215 |
* | 03 | 1 | 3 | 25165823 |
* | 80 | 50 | 128 | 1073741823 |
* | 8000 | 50 | 128 | 1073741823 |
* | 80000000 | 50 | 128 | 1073741823 |
* | 81 | 50 | 129 | 1082130431 |
* | 81ffffff | 50 | 129 | 1090519038 |
* | 82 | 50 | 130 | 1090519039 |
* | 83 | 51 | 131 | 1098907647 |
* | ff | 99 | 255 | 2139095039 |
* | ffff | 99 | 255 | 2147450879 |
* | ffffffff | 99 | 255 | 2147483646 |
* | ffffffffffff | 99 | 255 | 2147483646 |
*
* This interprets the bytes in this byte string as **unsigned**. This behavior is consistent with
* [compareTo]. The returned value is also consistent with [compareTo] though the dynamic range
* is compressed. For two byte strings `a` and `b`, if `a < b`, then
* `a.toIndex(n) <= b.toIndex(n)` for all sizes `n`.
*
* This examines at most the first 4 bytes of this byte string. Data beyond the first 4 bytes is
* not used to compute the result.
*
* @param size a positive integer.
* @return a value that is greater than or equal to `0` and less than [size].
*/
fun toIndex(size: Int): Int

/**
* Projects this value to the range `[0.0..1.0)` using linear interpolation. This is equivalent to
* sorting all possible byte strings and returning the fraction that precede this byte string.
*
* For example, the byte string `8000` is the median of all 2-element byte strings, and calling
* `toFraction()` on it returns 0.5. Some other examples:
*
* | Byte String (hex) | `toFraction()` |
* | :----------------- | :----------------- |
* | (empty) | 0.0 |
* | 00 | 0.0 |
* | 0000 | 0.0 |
* | 000000 | 0.0 |
* | 00000000000001 | 0.0 |
* | 00000000000007 | 0.0 |
* | 00000000000008 | 0.0000000000000001 |
* | 0000000001 | 0.0000000000009094 |
* | 00000001 | 0.0000000002328306 |
* | 01 | 0.00390625 |
* | 02 | 0.0078125 |
* | 03 | 0.01171875 |
* | 80 | 0.5 |
* | 8000 | 0.5 |
* | 80000000000000 | 0.5 |
* | 81 | 0.50390625 |
* | 81ffffff | 0.5078124997671694 |
* | 82 | 0.5078125 |
* | 83 | 0.51171875 |
* | ff | 0.99609375 |
* | ffff | 0.9999847412109375 |
* | ffffffff | 0.9999999997671694 |
* | ffffffffffff | 0.9999999999999964 |
* | ffffffffffffff | 0.9999999999999999 |
*
* This interprets the bytes in this byte string as **unsigned**. This behavior is consistent with
* [compareTo]. The returned value is also consistent with [compareTo] though the dynamic range
* is compressed. For two byte strings `a` and `b`, if `a < b`, then
* `a.toFraction() <= b.toFraction()`.
*
* This examines at most the first 7 bytes of this byte string. Data beyond the first 7 bytes is
* not used to compute the result.
*
* @return a value that is greater than or equal to `0.0` and less than `1.0`.
*/
fun toFraction(): Double

/**
* Returns a human-readable string that describes the contents of this byte string. Typically this
* is a string like `[text=Hello]` or `[hex=0000ffff]`.
Expand Down
32 changes: 32 additions & 0 deletions okio/src/commonMain/kotlin/okio/internal/ByteString.kt
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Expand Up @@ -30,6 +30,7 @@ import okio.isIsoControl
import okio.processUtf8CodePoints
import okio.shr
import okio.toUtf8String
import kotlin.math.min

// TODO Kotlin's expect classes can't have default implementations, so platform implementations
// have to call these functions. Remove all this nonsense when expect class allow actual code.
Expand Down Expand Up @@ -248,6 +249,37 @@ internal inline fun ByteString.commonCompareTo(other: ByteString): Int {
return if (sizeA < sizeB) -1 else 1
}

@Suppress("NOTHING_TO_INLINE")
internal inline fun ByteString.commonToIndex(size: Int): Int {
require(size > 0)
var numerator = 0L
var denominator = 1L
for (i in 0 until min(4, this.size)) {
numerator = (numerator shl 8) + (get(i) and 0xff)
denominator = (denominator shl 8)
}
return (size * numerator / denominator).toInt()
}

@Suppress("NOTHING_TO_INLINE")
internal inline fun ByteString.commonToFraction(): Double {
var numerator = 0L
var denominator = 1L
for (i in 0 until min(7, size)) {
numerator = (numerator shl 8) + (get(i) and 0xff)
denominator = (denominator shl 8)
}

// Double wants 53 bits of precision but we have 56. Discard 3 bits of precision. Without this
// it's possible that this method returns 1.0 for byte strings like "ffffffffffffff".
if (size >= 7) {
numerator = numerator shr 3
denominator = denominator shr 3
}

return numerator.toDouble() / denominator
}

@Suppress("NOTHING_TO_INLINE")
internal inline fun commonOf(data: ByteArray) = ByteString(data.copyOf())

Expand Down
53 changes: 53 additions & 0 deletions okio/src/commonTest/kotlin/okio/ByteStringTest.kt
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Expand Up @@ -457,4 +457,57 @@ abstract class AbstractByteStringTest internal constructor(
sortedByteStrings.sort()
assertEquals(originalByteStrings, sortedByteStrings)
}

@Test fun toIndex() {
assertEquals(0, factory.decodeHex("").toIndex(1))
assertEquals(0, factory.decodeHex("00").toIndex(1))
assertEquals(0, factory.decodeHex("ff").toIndex(1))
assertEquals(0, factory.decodeHex("ffffffff").toIndex(1))
assertEquals(0, factory.decodeHex("ffffffffffff").toIndex(1))

assertEquals(0, factory.decodeHex("").toIndex(100))
assertEquals(0, factory.decodeHex("00").toIndex(100))
assertEquals(10, factory.decodeHex("1a").toIndex(100))
assertEquals(25, factory.decodeHex("40").toIndex(100))
assertEquals(50, factory.decodeHex("80").toIndex(100))
assertEquals(75, factory.decodeHex("c0").toIndex(100))
assertEquals(99, factory.decodeHex("ff").toIndex(100))
assertEquals(99, factory.decodeHex("ffff").toIndex(100))
assertEquals(99, factory.decodeHex("ffffff").toIndex(100))
assertEquals(99, factory.decodeHex("ffffffff").toIndex(100))

assertEquals(0, factory.decodeHex("").toIndex(Int.MAX_VALUE))
assertEquals(0x7f7fffff, factory.decodeHex("ff").toIndex(Int.MAX_VALUE))
assertEquals(0x7fff7fff, factory.decodeHex("ffff").toIndex(Int.MAX_VALUE))
assertEquals(0x7fffff7f, factory.decodeHex("ffffff").toIndex(Int.MAX_VALUE))
assertEquals(0x7ffffffe, factory.decodeHex("ffffffff").toIndex(Int.MAX_VALUE))
}

@Test fun toFraction() {
assertEquals(0.0, factory.decodeHex("").toFraction())
assertEquals(0.0, factory.decodeHex("00").toFraction())
assertEquals(0.0, factory.decodeHex("00").toFraction())
assertEquals(0.1015625, factory.decodeHex("1a").toFraction())
assertEquals(0.25, factory.decodeHex("40").toFraction())
assertEquals(0.5, factory.decodeHex("80").toFraction())
assertEquals(0.75, factory.decodeHex("c0").toFraction())
assertEquals(0.7929493631236255, factory.decodeHex("cafebabe").toFraction())
assertEquals(0.99609375, factory.decodeHex("ff").toFraction())
assertEquals(0.9999847412109375, factory.decodeHex("ffff").toFraction())
assertEquals(0.9999999403953552, factory.decodeHex("ffffff").toFraction())
assertEquals(0.9999999997671694, factory.decodeHex("ffffffff").toFraction())
assertEquals(0.9999999999999964, factory.decodeHex("ffffffffffff").toFraction())
assertEquals(0.9999999999999999, factory.decodeHex("ffffffffffffff").toFraction())
assertEquals(0.9999999999999999, factory.decodeHex("ffffffffffffffff").toFraction())
}

/** Only 5 bits of the 7th byte are used. We use 53 bits in total for IEEE 754 doubles. */
@Test fun toFractionLast5BitsOf7thByte() {
assertEquals(0.0000000000000000, factory.decodeHex("00000000000007").toFraction())
assertEquals(1.1102230246251565E-16, factory.decodeHex("00000000000008").toFraction())
assertEquals(1.1102230246251565E-16, factory.decodeHex("0000000000000f").toFraction())
assertEquals(2.220446049250313E-16, factory.decodeHex("00000000000010").toFraction())
assertEquals(0.9999999999999998, factory.decodeHex("fffffffffffff0").toFraction())
assertEquals(0.9999999999999999, factory.decodeHex("fffffffffffff8").toFraction())
}
}
6 changes: 6 additions & 0 deletions okio/src/jsMain/kotlin/okio/ByteString.kt
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Expand Up @@ -38,6 +38,8 @@ import okio.internal.commonToAsciiLowercase
import okio.internal.commonToAsciiUppercase
import okio.internal.commonToByteArray
import okio.internal.commonToByteString
import okio.internal.commonToFraction
import okio.internal.commonToIndex
import okio.internal.commonToString
import okio.internal.commonUtf8
import okio.internal.commonWrite
Expand Down Expand Up @@ -119,6 +121,10 @@ internal actual constructor(

actual override fun compareTo(other: ByteString) = commonCompareTo(other)

actual fun toIndex(size: Int) = commonToIndex(size)

actual fun toFraction() = commonToFraction()

/**
* Returns a human-readable string that describes the contents of this byte string. Typically this
* is a string like `[text=Hello]` or `[hex=0000ffff]`.
Expand Down
6 changes: 6 additions & 0 deletions okio/src/jvmMain/kotlin/okio/ByteString.kt
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Expand Up @@ -38,6 +38,8 @@ import okio.internal.commonToAsciiLowercase
import okio.internal.commonToAsciiUppercase
import okio.internal.commonToByteArray
import okio.internal.commonToByteString
import okio.internal.commonToFraction
import okio.internal.commonToIndex
import okio.internal.commonToString
import okio.internal.commonUtf8
import okio.internal.commonWrite
Expand Down Expand Up @@ -181,6 +183,10 @@ internal actual constructor(

actual override fun compareTo(other: ByteString) = commonCompareTo(other)

actual fun toIndex(size: Int) = commonToIndex(size)

actual fun toFraction() = commonToFraction()

actual override fun toString() = commonToString()

@Throws(IOException::class)
Expand Down
6 changes: 6 additions & 0 deletions okio/src/nativeMain/kotlin/okio/ByteString.kt
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Expand Up @@ -39,6 +39,8 @@ import okio.internal.commonToAsciiLowercase
import okio.internal.commonToAsciiUppercase
import okio.internal.commonToByteArray
import okio.internal.commonToByteString
import okio.internal.commonToFraction
import okio.internal.commonToIndex
import okio.internal.commonToString
import okio.internal.commonUtf8
import okio.internal.commonWrite
Expand Down Expand Up @@ -125,6 +127,10 @@ internal actual constructor(

actual override fun compareTo(other: ByteString) = commonCompareTo(other)

actual fun toIndex(size: Int) = commonToIndex(size)

actual fun toFraction() = commonToFraction()

/**
* Returns a human-readable string that describes the contents of this byte string. Typically this
* is a string like `[text=Hello]` or `[hex=0000ffff]`.
Expand Down

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