Implement Instant parsing in common module #106

okarmazin · 2021-04-03T19:32:49Z

Resolves #56

Instant.parse now supports:

Time zone offsets in the form of +-hh:mm and +-hhmm
Time part allows the omission of colons (allows hhmmss)
Time part allows the omission of seconds

Implementation notes:
Input string is split by the time delimiter (T|t) into date and time parts.
Date parsing is delegated to LocalDate.parse
Time parsing employs the well known algorithm from Iso8601Utils.java
originally implemented in Jackson. This commit is based on Moshi's
version of that file.

Motivation

Any datetime library worth its salt must be able to parse numeric time zone offsets. I'm afraid here's no way around it.
I initially implemented this for myself since I need the ability to parse numeric time zone offsets. In fact I think that most of the OffsetDateTime-type strings floating around the Internet come with the +-hh:mm offset representation rather than the Z designator. Migrating from ThreeTenBP I was surprised that only the Z designator was supported.

This implementation may hold fort until a format builder is added (#58 #39)

Discussion points:

The leniency of the time parser comes free with the algorithm.

Should the omission of colons in time be allowed?
Should the omission of seconds be allowed?

I vote that the leniency be kept in the spirit of the "Be permissive on input, strict on output" rule of thumb.

okarmazin · 2021-04-03T19:35:43Z

core/common/src/InstantParser.kt

+
+/*
+ * The algorithm for parsing time and zone offset was adapted from
+ * https://github.com/square/moshi/blob/aea17e09bc6a3f9015d3de0e951923f1033d299e/adapters/src/main/java/com/squareup/moshi/adapters/Iso8601Utils.java


Please advise on the proper formulation of attribution that shall be added.

okarmazin · 2021-04-03T19:45:27Z

core/common/src/InstantParser.kt

+import kotlin.math.min
+import kotlin.math.pow
+
+internal fun parseInstantCommon(string: String): Instant = parseIsoString(string)


The parseInstantCommon facade may try multiple formats before giving up such as #83 if that gets implemented.

If the need for such extensibility arises, we have little enough code to be able to add it quickly.

Instant.parse now supports: - Time zone offsets in the form of +-hh:mm - Time part allows the omission of colons (allows hhmmss) - Time part allows the omission of seconds Implementation notes: Input string is split by the time delimiter (T|t) into date and time parts. Date parsing is delegated to LocalDate.parse Time parsing employs the well known algorithm from Iso8601Utils.java originally implemented in Jackson. This commit is based on Moshi's version of that file.

okarmazin · 2021-04-03T19:57:19Z

core/common/src/InstantParser.kt

+            val c = timePart[offset]
+            if (c != 'Z' && c != 'z' && c != '+' && c != '-') {
+                seconds = parseInt(timePart, offset, offset + 2).also { offset += 2 }
+                if (seconds > 59 && seconds < 63) { // https://github.com/Kotlin/kotlinx-datetime/issues/5


Is #5 applicable here, or should this be converted to idiomatic range check?

It is applicable. That said, this line is strange in any case: the existing parser for LocalDateTime doesn't recognize leap seconds, so we have a disconnect between

Instant.parse("2020-06-14T01:01:61Z")

successfully parsing and

LocalDateTime.parse("2020-06-14T01:01:61")

failing.

Also, it should be noted Java Time's parser throws on seconds outside of [0; 60). Why should we accept leap seconds and why only in Instant and not in LocalDateTime?

Last forgotten IAE among code that throws DateTimeFormatException

dkhalanskyjb

Hi! Thanks for the PR. We are actually interested in making this change and wanted to do it before the library's next release.

We can't accept this PR as is, though, given that the proposed changes are inconsistent with how the other parsers behave (LocalDateTime in particular). Moreover, it initially looks like this change could be implemented in a much more simple manner by parsing ~~ZonedDateTime~~ (EDIT: looks like the behavior of OffsetDateTime is more consistent with how the modern JDK versions handle the parsing, thanks @ilya-g for the suggestion!) on JVM and JS and slightly editing the existing parser on Native, and it's unclear why it would be worth doing anything other than that.

dkhalanskyjb · 2021-04-05T07:41:00Z

core/common/src/InstantParser.kt

+import kotlin.math.min
+import kotlin.math.pow
+
+internal fun parseInstantCommon(string: String): Instant = parseIsoString(string)


If the need for such extensibility arises, we have little enough code to be able to add it quickly.

dkhalanskyjb · 2021-04-05T07:46:19Z

core/common/src/InstantParser.kt

+            val c = timePart[offset]
+            if (c != 'Z' && c != 'z' && c != '+' && c != '-') {
+                seconds = parseInt(timePart, offset, offset + 2).also { offset += 2 }
+                if (seconds > 59 && seconds < 63) { // https://github.com/Kotlin/kotlinx-datetime/issues/5


It is applicable. That said, this line is strange in any case: the existing parser for LocalDateTime doesn't recognize leap seconds, so we have a disconnect between

Instant.parse("2020-06-14T01:01:61Z")

successfully parsing and

LocalDateTime.parse("2020-06-14T01:01:61")

failing.

Also, it should be noted Java Time's parser throws on seconds outside of [0; 60). Why should we accept leap seconds and why only in Instant and not in LocalDateTime?

dkhalanskyjb · 2021-04-05T07:55:08Z

core/common/src/InstantParser.kt

+                        indexOfNonDigit(timePart, offset + 1) // assume at least one digit
+                    val parseEndOffset =
+                        min(endOffset, offset + 9) // parse up to 9 digits
+                    val fraction = parseInt(timePart, offset, parseEndOffset)


The way this code deals with fractions with more than nine digits that are representable as nanoseconds is by truncating the extra digits towards zero—not even by rounding them. So,

Instant.parse("2020-06-14T01:01:59.1234567899Z") // results in 2020-06-14T01:01:59.123456789Z

This is highly questionable, as, again, parsing LocalDateTime throws in such cases, as does parsing Instant in Java. This makes sense, given that silently losing the user input is highly undesirable. Maybe an argument could be made that rounding is a sensible solution here, but even then the reasoning would have to be applied consistently across the parsers we have.

dkhalanskyjb · 2021-04-05T08:07:07Z

core/common/src/InstantParser.kt

+            timezone = TimeZone.UTC
+        } else if (timezoneIndicator == '+' || timezoneIndicator == '-') {
+            val timezoneOffset = timePart.substring(offset)
+            // 18-Jun-2015, tatu: Minor simplification, skip offset of "+0000"/"+00:00"


These comments have little meaning without the commit history, so they probably shouldn't be included here.

dkhalanskyjb · 2021-04-05T08:09:27Z

core/js/src/Instant.kt

-            if (e.isJodaDateTimeParseException()) throw DateTimeFormatException(e)
-            throw e
-        }
+        actual fun parse(isoString: String): Instant = parseInstantCommon(isoString)


This could be implemented easily—and consistently with the other parsers—by parsing a ~~ZonedDateTime~~ OffsetDateTime and then converting that to Instant.

The same for the Java implementation.

dkhalanskyjb · 2021-04-05T08:19:31Z

core/native/src/Instant.kt

-            concreteCharParser('.')
-                .chainSkipping(fractionParser(0, 9, 9)) // nanos
-        ))
-        .chainIgnoring(concreteCharParser('Z').or(concreteCharParser('z')))


If the other parsers are implemented via ~~ZonedDateTime~~ OffsetDateTime, there is no need to have the parser for Instant in the common code, and the only place that requires an explicit implementation is the native part. Then, it's probably easier to modify this parser at this line so that it accepts any ZoneOffset and not just Z (and propagate this change correspondingly).

okarmazin · 2021-04-05T10:33:22Z

Thank you for your review. Since this code is highly inconsistent with existing logic I'll close this PR down.

As you're already planning on making the change yourselves and have it done by the next release (this month is the target according to Ilya Gorbunov on Slack I see no reason for a third party to be a part of that process. I only knew about the April release, but not that it would include offset time zone parsing. I hope this bootleg PR reinforces the fact that accepting time zone offsets is critically important. I'm looking forward to the next release!

@dkhalanskyjb P.S. I suggest that #56 be put in the 0.2.0 milestone.

okarmazin marked this pull request as draft April 3, 2021 19:34

okarmazin commented Apr 3, 2021

View reviewed changes

okarmazin marked this pull request as ready for review April 3, 2021 19:57

okarmazin added 2 commits April 3, 2021 22:04

Throw DateTimeFormatException instead of IllegalArgumentException

b36700c

Last forgotten IAE among code that throws DateTimeFormatException

Add test case for empty fraction

61fc0c2

dkhalanskyjb requested changes Apr 5, 2021

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okarmazin closed this Apr 5, 2021

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Implement Instant parsing in common module #106

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Motivation

Discussion points:

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