Implement the RFC 1123 format

Author: dkhalanskyjb

core/common/src/format/ValueBagFormat.kt

@@ -299,6 +299,15 @@ public class ValueBagFormat private constructor(private val actualFormat: Format
             appendFormatString("ld<'-'mm'-'dd>('T'|'t')lt<hh':'mm':'ss(|'.'f)>uo<('Z'|'z')|+(HH(|':'mm(|':'ss)))>")
         }
 
+        public val RFC_1123 : ValueBagFormat = build {
+            appendDayOfWeek(listOf("Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"))
+            appendLiteral(", ")
+            appendDayOfMonth()
+            appendLiteral(' ')
+            appendMonthName(listOf("Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"))
+            appendFormatString(" ld<yyyy> lt<hh':'mm':'ss> uo<'GMT'|+(HHmm)>")
+        }
+
         internal val Cache = LruCache<String, ValueBagFormat>(16) { fromFormatString(it) }
     }
 

core/common/test/format/ValueBagFormatTest.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2019-2023 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.test.format
+
+import kotlinx.datetime.*
+import kotlinx.datetime.format.*
+import kotlin.test.*
+
+class ValueBagFormatTest {
+    @Test
+    fun testRfc1123() {
+        val bags = buildMap<ValueBag, Pair<String, Set<String>>> {
+            put(valueBag(LocalDate(2008, 6, 3), LocalTime(11, 5, 30), UtcOffset.ZERO), ("Tue, 3 Jun 2008 11:05:30 GMT" to setOf()))
+            put(valueBag(LocalDate(2008, 6, 30), LocalTime(11, 5, 30), UtcOffset.ZERO), ("Mon, 30 Jun 2008 11:05:30 GMT" to setOf()))
+            put(valueBag(LocalDate(2008, 6, 3), LocalTime(11, 5, 30), UtcOffset(hours = 2)), ("Tue, 3 Jun 2008 11:05:30 +0200" to setOf()))
+            put(valueBag(LocalDate(2008, 6, 30), LocalTime(11, 5, 30), UtcOffset(hours = -3)), ("Mon, 30 Jun 2008 11:05:30 -0300" to setOf()))
+        }
+        test(ValueBagFormat.RFC_1123, bags)
+    }
+
+    private fun valueBag(
+        date: LocalDate? = null,
+        time: LocalTime? = null,
+        offset: UtcOffset? = null,
+        zone: TimeZone? = null
+    ) = ValueBag().apply {
+        date?.let { populateFrom(it) }
+        time?.let { populateFrom(it) }
+        offset?.let { populateFrom(it) }
+        timeZoneId = zone?.id
+    }
+
+    private fun assertValueBagsEqual(a: ValueBag, b: ValueBag, message: String? = null) {
+        assertEquals(a.toLocaldate(), b.toLocaldate(), message)
+        assertEquals(a.toLocalTime(), b.toLocalTime(), message)
+        assertEquals(a.toUtcOffset(), b.toUtcOffset(), message)
+        assertEquals(a.timeZoneId, b.timeZoneId, message)
+    }
+
+    private fun test(format: ValueBagFormat, strings: Map<ValueBag, Pair<String, Set<String>>>) {
+        for ((value, stringsForValue) in strings) {
+            val (canonicalString, otherStrings) = stringsForValue
+            assertEquals(canonicalString, format.format(value), "formatting $value with $format")
+            assertValueBagsEqual(value, format.parse(canonicalString), "parsing '$canonicalString' with $format")
+            for (otherString in otherStrings) {
+                assertValueBagsEqual(value, format.parse(otherString), "parsing '$otherString' with $format")
+            }
+        }
+    }
+}