Provide custom serializers based on DateTimeFormat

The names of the serializers are still under discussion, but
it's already decided that the serializers are going to be abstract
classes.

Fixes #350

core/common/src/serializers/InstantSerializers.kt

@@ -6,6 +6,9 @@
 package kotlinx.datetime.serializers
 
 import kotlinx.datetime.Instant
+import kotlinx.datetime.format
+import kotlinx.datetime.format.DateTimeComponents
+import kotlinx.datetime.format.DateTimeFormat
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
@@ -75,3 +78,38 @@ public object InstantComponentSerializer : KSerializer<Instant> {
     }
 
 }
+
+/**
+ * An abstract serializer for [Instant] values that uses
+ * a custom [DateTimeFormat] for serializing to and deserializing.
+ *
+ * [format] should be a format that includes enough components to unambiguously define a date, a time, and a UTC offset.
+ * See [Instant.parse] for details of how deserialization is performed.
+ *
+ * When serializing, the [Instant] value is formatted as a string using the specified [format]
+ * in the [ZERO][UtcOffset.ZERO] UTC offset.
+ *
+ * This serializer is abstract and must be subclassed to provide a concrete serializer.
+ * Example:
+ * ```
+ * object Rfc1123InstantSerializer : CustomInstantSerializer(DateTimeComponents.Formats.RFC_1123)
+ * ```
+ *
+ * Note that [Instant] is [kotlinx.serialization.Serializable] by default,
+ * so it is not necessary to create custom serializers when the format is not important.
+ * Additionally, [InstantIso8601Serializer] is provided for the ISO 8601 format.
+ */
+public abstract class CustomInstantSerializer(
+    private val format: DateTimeFormat<DateTimeComponents>,
+) : KSerializer<Instant> {
+
+    override val descriptor: SerialDescriptor =
+        PrimitiveSerialDescriptor("kotlinx.datetime.Instant", PrimitiveKind.STRING)
+
+    override fun deserialize(decoder: Decoder): Instant =
+        Instant.parse(decoder.decodeString(), format)
+
+    override fun serialize(encoder: Encoder, value: Instant) {
+        encoder.encodeString(value.format(format))
+    }
+}

core/common/src/serializers/LocalDateSerializers.kt

@@ -6,6 +6,7 @@
 package kotlinx.datetime.serializers
 
 import kotlinx.datetime.LocalDate
+import kotlinx.datetime.format.DateTimeFormat
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
@@ -76,3 +77,33 @@ public object LocalDateComponentSerializer: KSerializer<LocalDate> {
     }
 
 }
+
+/**
+ * An abstract serializer for [LocalDate] values that uses
+ * a custom [DateTimeFormat] to serialize and deserialize the value.
+ *
+ * This serializer is abstract and must be subclassed to provide a concrete serializer.
+ * Example:
+ * ```
+ * object IsoBasicLocalDateSerializer : CustomLocalDateSerializer(LocalDate.Formats.ISO_BASIC)
+ * ```
+ *
+ * Note that [LocalDate] is [kotlinx.serialization.Serializable] by default,
+ * so it is not necessary to create custom serializers when the format is not important.
+ * Additionally, [LocalDateIso8601Serializer] is provided for the ISO 8601 format.
+ */
+public abstract class CustomLocalDateSerializer(
+    format: DateTimeFormat<LocalDate>,
+) : KSerializer<LocalDate> by format.asKSerializer("kotlinx.datetime.LocalDate")
+
+internal fun <T> DateTimeFormat<T>.asKSerializer(classFqn: String): KSerializer<T> =
+    object : KSerializer<T> {
+        override val descriptor: SerialDescriptor =
+            PrimitiveSerialDescriptor(classFqn, PrimitiveKind.STRING)
+
+        override fun deserialize(decoder: Decoder): T = parse(decoder.decodeString())
+
+        override fun serialize(encoder: Encoder, value: T) {
+            encoder.encodeString(format(value))
+        }
+    }

core/common/src/serializers/LocalDateTimeSerializers.kt

@@ -6,6 +6,7 @@
 package kotlinx.datetime.serializers
 
 import kotlinx.datetime.*
+import kotlinx.datetime.format.DateTimeFormat
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
@@ -98,3 +99,25 @@ public object LocalDateTimeComponentSerializer: KSerializer<LocalDateTime> {
     }
 
 }
+
+/**
+ * An abstract serializer for [LocalDateTime] values that uses
+ * a custom [DateTimeFormat] to serialize and deserialize the value.
+ *
+ * This serializer is abstract and must be subclassed to provide a concrete serializer.
+ * Example:
+ * ```
+ * object PythonDateTimeSerializer : CustomLocalDateTimeSerializer(LocalDateTime.Format {
+ *     date(LocalDate.Formats.ISO)
+ *     char(' ')
+ *     time(LocalTime.Formats.ISO)
+ * })
+ * ```
+ *
+ * Note that [LocalDateTime] is [kotlinx.serialization.Serializable] by default,
+ * so it is not necessary to create custom serializers when the format is not important.
+ * Additionally, [LocalDateTimeIso8601Serializer] is provided for the ISO 8601 format.
+ */
+public abstract class CustomLocalDateTimeSerializer(
+    format: DateTimeFormat<LocalDateTime>,
+) : KSerializer<LocalDateTime> by format.asKSerializer("kotlinx.datetime.LocalDateTime")

core/common/src/serializers/LocalTimeSerializers.kt

@@ -6,6 +6,7 @@
 package kotlinx.datetime.serializers
 
 import kotlinx.datetime.*
+import kotlinx.datetime.format.DateTimeFormat
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
@@ -81,3 +82,23 @@ public object LocalTimeComponentSerializer : KSerializer<LocalTime> {
         }
     }
 }
+
+/**
+ * An abstract serializer for [LocalTime] values that uses
+ * a custom [DateTimeFormat] to serialize and deserialize the value.
+ *
+ * This serializer is abstract and must be subclassed to provide a concrete serializer.
+ * Example:
+ * ```
+ * object FixedWidthTimeSerializer : CustomLocalTimeSerializer(LocalTime.Format {
+ *     hour(); char(':'); minute(); char(':'); second(); char('.'); secondFraction(3)
+ * })
+ * ```
+ *
+ * Note that [LocalTime] is [kotlinx.serialization.Serializable] by default,
+ * so it is not necessary to create custom serializers when the format is not important.
+ * Additionally, [LocalTimeIso8601Serializer] is provided for the ISO 8601 format.
+ */
+public abstract class CustomLocalTimeSerializer(
+    format: DateTimeFormat<LocalTime>,
+) : KSerializer<LocalTime> by format.asKSerializer("kotlinx.datetime.LocalTime")

core/common/src/serializers/TimeZoneSerializers.kt

@@ -5,9 +5,8 @@
 
 package kotlinx.datetime.serializers
 
-import kotlinx.datetime.FixedOffsetTimeZone
-import kotlinx.datetime.TimeZone
-import kotlinx.datetime.UtcOffset
+import kotlinx.datetime.*
+import kotlinx.datetime.format.DateTimeFormat
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
@@ -74,3 +73,21 @@ public object UtcOffsetSerializer: KSerializer<UtcOffset> {
     }
 
 }
+
+/**
+ * An abstract serializer for [UtcOffset] values that uses
+ * a custom [DateTimeFormat] to serialize and deserialize the value.
+ *
+ * This serializer is abstract and must be subclassed to provide a concrete serializer.
+ * Example:
+ * ```
+ * object FourDigitOffsetSerializer : CustomUtcOffsetSerializer(UtcOffset.Formats.FOUR_DIGITS)
+ * ```
+ *
+ * Note that [UtcOffset] is [kotlinx.serialization.Serializable] by default,
+ * so it is not necessary to create custom serializers when the format is not important.
+ * Additionally, [UtcOffsetSerializer] is provided for the ISO 8601 format.
+ */
+public abstract class CustomUtcOffsetSerializer(
+    format: DateTimeFormat<UtcOffset>,
+) : KSerializer<UtcOffset> by format.asKSerializer("kotlinx.datetime.UtcOffset")

serialization/common/test/InstantSerializationTest.kt

@@ -5,6 +5,7 @@
 package kotlinx.datetime.serialization.test
 
 import kotlinx.datetime.*
+import kotlinx.datetime.format.DateTimeComponents
 import kotlinx.datetime.serializers.*
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
@@ -66,4 +67,25 @@ class InstantSerializationTest {
         // should be the same as the ISO 8601
         iso8601Serialization(Json.serializersModule.serializer())
     }
+
+    object Rfc1123InstantSerializer : CustomInstantSerializer(DateTimeComponents.Formats.RFC_1123)
+
+    @Test
+    fun testCustomSerializer() {
+        for ((instant, json) in listOf(
+            Pair(Instant.fromEpochSeconds(1607505416),
+                "\"Wed, 9 Dec 2020 09:16:56 GMT\""),
+            Pair(Instant.fromEpochSeconds(-1607505416),
+                "\"Thu, 23 Jan 1919 14:43:04 GMT\""),
+            Pair(Instant.fromEpochSeconds(987654321),
+                "\"Thu, 19 Apr 2001 04:25:21 GMT\""),
+        )) {
+            assertEquals(json, Json.encodeToString(Rfc1123InstantSerializer, instant))
+            assertEquals(instant, Json.decodeFromString(Rfc1123InstantSerializer, json))
+        }
+        assertEquals("\"Thu, 19 Apr 2001 04:25:21 GMT\"",
+            Json.encodeToString(Rfc1123InstantSerializer, Instant.fromEpochSeconds(987654321, 123456789)))
+        assertEquals(Instant.fromEpochSeconds(987654321),
+            Json.decodeFromString(Rfc1123InstantSerializer, "\"Thu, 19 Apr 2001 08:25:21 +0400\""))
+    }
 }

serialization/common/test/LocalDateSerializationTest.kt

@@ -70,4 +70,17 @@ class LocalDateSerializationTest {
         iso8601Serialization(Json.serializersModule.serializer())
     }
 
+    object IsoBasicLocalDateSerializer : CustomLocalDateSerializer(LocalDate.Formats.ISO_BASIC)
+
+    @Test
+    fun testCustomSerializer() {
+        for ((localDate, json) in listOf(
+            Pair(LocalDate(2020, 12, 9), "\"20201209\""),
+            Pair(LocalDate(-2020, 1, 1), "\"-20200101\""),
+            Pair(LocalDate(2019, 10, 1), "\"20191001\""),
+        )) {
+            assertEquals(json, Json.encodeToString(IsoBasicLocalDateSerializer, localDate))
+            assertEquals(localDate, Json.decodeFromString(IsoBasicLocalDateSerializer, json))
+        }
+    }
 }

serialization/common/test/LocalDateTimeSerializationTest.kt

@@ -6,6 +6,7 @@
 package kotlinx.datetime.serialization.test
 
 import kotlinx.datetime.*
+import kotlinx.datetime.format.char
 import kotlinx.datetime.serializers.*
 import kotlinx.serialization.KSerializer
 import kotlinx.serialization.json.*
@@ -82,4 +83,24 @@ class LocalDateTimeSerializationTest {
         // should be the same as the ISO 8601
         iso8601Serialization(Json.serializersModule.serializer())
     }
+
+    object PythonDateTimeSerializer : CustomLocalDateTimeSerializer(LocalDateTime.Format {
+        date(LocalDate.Formats.ISO)
+        char(' ')
+        time(LocalTime.Formats.ISO)
+    })
+
+    @Test
+    fun testCustomSerializer() {
+        for ((localDateTime, json) in listOf(
+            Pair(LocalDateTime(2008, 7, 5, 2, 1), "\"2008-07-05 02:01:00\""),
+            Pair(LocalDateTime(2007, 12, 31, 23, 59, 1), "\"2007-12-31 23:59:01\""),
+            Pair(LocalDateTime(999, 12, 31, 23, 59, 59, 990000000), "\"0999-12-31 23:59:59.99\""),
+            Pair(LocalDateTime(-1, 1, 2, 23, 59, 59, 999990000), "\"-0001-01-02 23:59:59.99999\""),
+            Pair(LocalDateTime(-2008, 1, 2, 23, 59, 59, 999999990), "\"-2008-01-02 23:59:59.99999999\""),
+        )) {
+            assertEquals(json, Json.encodeToString(PythonDateTimeSerializer, localDateTime))
+            assertEquals(localDateTime, Json.decodeFromString(PythonDateTimeSerializer, json))
+        }
+    }
 }

serialization/common/test/LocalTimeSerializationTest.kt

@@ -6,6 +6,7 @@
 package kotlinx.datetime.serialization.test
 
 import kotlinx.datetime.*
+import kotlinx.datetime.format.char
 import kotlinx.datetime.serializers.*
 import kotlinx.serialization.KSerializer
 import kotlinx.serialization.json.*
@@ -72,4 +73,23 @@ class LocalTimeSerializationTest {
         // should be the same as the ISO 8601
         iso8601Serialization(Json.serializersModule.serializer())
     }
+
+    object FixedWidthTimeSerializer : CustomLocalTimeSerializer(LocalTime.Format {
+        hour(); char(':'); minute(); char(':'); second(); char('.'); secondFraction(3)
+    })
+
+    @Test
+    fun testCustomSerializer() {
+        for ((localTime, json) in listOf(
+            Pair(LocalTime(2, 1), "\"02:01:00.000\""),
+            Pair(LocalTime(23, 59, 1), "\"23:59:01.000\""),
+            Pair(LocalTime(23, 59, 59, 990000000), "\"23:59:59.990\""),
+            Pair(LocalTime(23, 59, 59, 999000000), "\"23:59:59.999\""),
+        )) {
+            assertEquals(json, Json.encodeToString(FixedWidthTimeSerializer, localTime))
+            assertEquals(localTime, Json.decodeFromString(FixedWidthTimeSerializer, json))
+        }
+        assertEquals("\"12:34:56.123\"", Json.encodeToString(FixedWidthTimeSerializer,
+            LocalTime(12, 34, 56, 123999999)))
+    }
 }

serialization/common/test/UtcOffsetSerializationTest.kt

@@ -35,4 +35,20 @@ class UtcOffsetSerializationTest {
         testSerializationAsPrimitive(UtcOffsetSerializer)
         testSerializationAsPrimitive(UtcOffset.serializer())
     }
+
+    object FourDigitOffsetSerializer : CustomUtcOffsetSerializer(UtcOffset.Formats.FOUR_DIGITS)
+
+    @Test
+    fun testCustomSerializer() {
+        for ((utcOffset, json) in listOf(
+            Pair(UtcOffset.ZERO, "\"+0000\""),
+            Pair(UtcOffset(2), "\"+0200\""),
+            Pair(UtcOffset(2, 30), "\"+0230\""),
+            Pair(UtcOffset(-2, -30), "\"-0230\""),
+        )) {
+            assertEquals(json, Json.encodeToString(FourDigitOffsetSerializer, utcOffset))
+            assertEquals(utcOffset, Json.decodeFromString(FourDigitOffsetSerializer, json))
+        }
+        assertEquals("\"+1234\"", Json.encodeToString(FourDigitOffsetSerializer, UtcOffset(12, 34, 56)))
+    }
 }