ICU-22528 Improve date formatting performance

[mohd-akram]

This improves date formatting performance to be 1.5-2x as fast.

Script to generate pattern char lookup table

const chars = [
  0x47, 0x79, 0x4D, 0x64, 0x6B, 0x48, 0x6D, 0x73, 0x53, 0x45,
  0x44, 0x46, 0x77, 0x57, 0x61, 0x68, 0x4B, 0x7A, 0x59, 0x65,
  0x75, 0x67, 0x41, 0x5A, 0x76, 0x63, 0x4c, 0x51, 0x71, 0x56,
  0x55, 0x4F, 0x58, 0x78, 0x72, 0x62, 0x42,
  0x3a,
];
process.stdout.write(' '.repeat(7));
for (let c = 0; c < 128; c++) {
  if (c % 16 == 0) process.stdout.write('\n'+' '.repeat(7));
  process.stdout.write(String(chars.indexOf(c)).padStart(3)+',');
}
process.stdout.write('\n');

Benchmarking program

#include <chrono>
#include <iostream>
#define U_SHOW_CPLUSPLUS_API 1
#include "unicode/smpdtfmt.h"
#include "unicode/ustream.h"

using namespace icu;

int main(int argc, char *argv[])
{
	UErrorCode success = U_ZERO_ERROR;
	auto format = argc > 2 ? argv[2] : "hh:mm:ss a OOOO";
	SimpleDateFormat df(UnicodeString(format), success);
	UnicodeString myString;
	for (int32_t i = 0; i < 1; ++i) {
		myString.remove();
		auto x = df.format(1695464381915, myString);
	}
	int n = argc > 1 ? atoi(argv[1]) : 10000;
	auto start = std::chrono::high_resolution_clock::now();
	for (int32_t i = 0; i < n; ++i) {
		myString.remove();
		auto x = df.format(1695464381915, myString);
	}
	std::cout << myString << "\n";
	auto finish = std::chrono::high_resolution_clock::now();
	auto ms = std::chrono::duration_cast<std::chrono::microseconds>(finish-start).count()/1000.0;
	std::cout << ms << "ms\n";
}

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22528
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/common/unistr.cpp is different
• icu4c/source/i18n/smpdtfmt.cpp is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot

[richgillam]

Generally looks great, but I do have a few small things...

[richgillam]

I think I had maybe one more suggestion for you, but I'm happy with this the way it is. Squash your commits (and optionally make the other change), and I'll approve the final version.

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/i18n/smpdtfmt.cpp is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot

[richgillam]

I just realized I should have asked you about Java. We usually try to keep our C++ and Java code as similar as possible. But these all seem like pretty C++-specific changes, so maybe you don't need to worry about that.

[mohd-akram]

Thanks @richgillam for the review!

[richgillam]

Do you have merge privileges, or do you need me to merge for you?

[mohd-akram]

If you could merge it that would be great as I don't have merge privileges.

[richgillam]

Done! Thank you for the fix!

[FrankYFTang]

we recently got some fuzzer detected overflow from these new code :(