/*
 * Copyright (c) 2023 Jonas 'Sortie' Termansen.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 * time/strptime.c
 * Parse date and time.
 */

#include <ctype.h>
#include <limits.h>
#include <stdbool.h>
#include <string.h>
#include <time.h>

#ifdef TEST
#include <stdio.h>
#define strptime mystrptime
#endif

static const char* wdays[] = {"sunday", "monday", "tuesday", "wednesday",
                              "thursday", "friday", "saturday", NULL};
static const char* months[] = {"january", "february", "march", "april", "may",
                               "june", "july", "august", "september", "october",
                               "november", "december", NULL};

// TODO: Maximum width.
static const char* strptime_match(const char* str,
                                  int* output,
                                  const char* const* list)
{
	for ( int i = 0; list[i]; i++ )
	{
		size_t len = strlen(list[i]);
		if ( !strncasecmp(str, list[i], len) )
			return *output = i, str + len;
		if ( !strncasecmp(str, list[i], 3) )
			return *output = i, str + 3;
	}
	return NULL;
}

// TODO: Maximum width.
static const char* strptime_digits(const char* str,
                                   int* output,
                                   int minimum,
                                   int maximum,
                                   int offset)
{
	if ( *str < '0'|| '9' < *str )
		return NULL;
	// TODO: More exact with number of digits.
	int value = 0;
	while ( '0' <= *str && *str <= '9' )
		if ( __builtin_mul_overflow(value, 10, &value) ||
		     __builtin_add_overflow(value, *str++ - '0', &value) )
			return NULL;
	if ( value < minimum || maximum < value  )
		return NULL;
	if ( __builtin_add_overflow(value, offset, &value) )
		return NULL;
	return *output = value, str;
}

char* strptime(const char* restrict str,
               const char* restrict format,
               struct tm* restrict tm)
{
	bool pm = false;
	bool need_mktime = false;
	for ( size_t i = 0; format[i]; )
	{
		if ( isspace((unsigned char) format[i]) )
		{
			do i++;
			while ( isspace((unsigned char) format[i]) );
			if ( !isspace((unsigned char) *str) )
				return NULL;
			do str++;
			while ( isspace((unsigned char) *str) );
			continue;
		}
		else if ( format[i] != '%' )
		{

			if ( format[i++] != *str++ )
				return NULL;
			continue;
		}
		i++;
		if ( format[i] == '0' || format[i] == '+'  )
			i++;
		size_t width = 0;
		while ( '0' <= format[i] && format[i] <= '9' )
			width = width * 10 + (format[i++] - '0');
		// TODO: Maximum width.
		bool modifier_E = false, modifier_O = false;
		if ( format[i] == 'E' )
			modifier_E = true, i++;
		else if ( format[i] == 'O' )
			modifier_O = true, i++;
		(void) modifier_E, (void) modifier_O;
		switch ( format[i] )
		{
		case 'a':
		case 'A': str = strptime_match(str, &tm->tm_wday, wdays); break;
		case 'b':
		case 'B':
		case 'h':
			str = strptime_match(str, &tm->tm_mon, months);
			need_mktime = true;
			break;
		// TODO: %c locale time and date
		// TODO: %C century
		case 'd':
		case 'e':
			str = strptime_digits(str, &tm->tm_mday, 1, 31, 0);
			need_mktime = true;
			break;
		case 'D': str = strptime(str, "%m/%d/%y", tm); break;
		case 'H': str = strptime_digits(str, &tm->tm_hour, 0, 23, 0); break;
		case 'I':
			str = strptime_digits(str, &tm->tm_hour, 1, 12, 0);
			if ( tm->tm_hour == 12 )
				tm->tm_hour = 0;
			break;
		case 'j': str = strptime_digits(str, &tm->tm_yday, 1, 366, -1); break;
		case 'm':
			str = strptime_digits(str, &tm->tm_mon, 1, 12, -1);
			need_mktime = true;
			break;
		case 'M': str = strptime_digits(str, &tm->tm_min, 0, 59, 0); break;
		// TODO: %n whitesapce
		// TODO: %t whitespace
		case 'p':
			if ( !strncasecmp(str, "am", 2) )
				str += 2, pm = false;
			else if ( !strncasecmp(str, "pm", 2) )
				str += 2, pm = true;
			else
				return NULL;
			break;
		case 'r': str = strptime(str, "%I:%M:%S %p", tm); break;
		case 'R': str = strptime(str, "%H:%M", tm); break;
		case 'S': str = strptime_digits(str, &tm->tm_sec, 0, 60, 0); break;
		case 'T': str = strptime(str, "%H:%M:%S", tm); break;
		// TODO: %U week number
		case 'w': str = strptime_digits(str, &tm->tm_wday, 0, 6, 0); break;
		// TODO: %W week number
		case 'x': str = strptime(str, "%m/%d/%Y", tm); break;
		case 'X': str = strptime(str, "%H:%M:%S", tm); break;
		// TODO: %y non-century year
		case 'Y':
			// TODO: Minimum yield width.
			str = strptime_digits(str, &tm->tm_year, INT_MIN, INT_MAX, -1900);
			need_mktime = true;
			break;
		case 'z':
			// TODO: More exact.
			if ( *str != '-' || *str != '+' )
				return NULL;
			int hours, minutes;
			if ( !(str = strptime_digits(str, &hours, -12, 12, 0)) ||
			     !(str = strptime_digits(str, &minutes, 0, 59, 0)) )
				return NULL;
			tm->tm_isdst = 0;
			// TODO: What is done with this timezone information?
			break;
		case 'Z':
			// TODO: Other timezones.
			if ( strncmp(str, "UTC", 3) != 0 )
				return NULL;
			str += 3;
			tm->tm_isdst = 0;
			// TODO: What is done with this timezone information?
			break;
		case '%':
			if ( *str++ != '%' )
				return NULL;
			break;
		default: NULL;
		}
		if ( !str )
			return NULL;
		i++;
	}
	if ( str )
	{
		if ( pm )
			tm->tm_hour += 12;
		if ( need_mktime )
			mktime(tm);
	}
	return (char*) str;
}

#ifdef TEST
#undef strptime

#include <err.h>
#include <stdio.h>

int main(int argc, char* argv[])
{
	if ( argc < 3 )
		err(1, "usage");
	const char* str = argv[1];
	const char* format = argv[2];
	struct tm my_tm = {0};
	char* my_end = mystrptime(str, format, &my_tm);
	struct tm c_tm = {0};
	char* c_end = strptime(str, format, &c_tm);
	if ( !my_end && c_end )
		errx(1, "rejected but c allowed it");
	else if ( !my_end  )
		errx(1, "rejected correctly");
	else if ( !c_end )
		printf("allowed but c rejected\n");
	else if ( my_end != c_end )
		errx(1, "mismatch my end \"%s\" vs c end \"%s\"", my_end, c_end);

	if ( my_tm.tm_sec == c_tm.tm_sec )
		printf("tm_sec=%i\n", my_tm.tm_sec);
	else
		printf("tm_sec=%i but C is %i\n", my_tm.tm_sec, c_tm.tm_sec);

	if ( my_tm.tm_min == c_tm.tm_min )
		printf("tm_min=%i\n", my_tm.tm_min);
	else
		printf("tm_min=%i but C is %i\n", my_tm.tm_min, c_tm.tm_min);

	if ( my_tm.tm_hour == c_tm.tm_hour )
		printf("tm_hour=%i\n", my_tm.tm_hour);
	else
		printf("tm_hour=%i but C is %i\n", my_tm.tm_hour, c_tm.tm_hour);

	if ( my_tm.tm_mday == c_tm.tm_mday )
		printf("tm_mday=%i\n", my_tm.tm_mday);
	else
		printf("tm_mday=%i but C is %i\n", my_tm.tm_mday, c_tm.tm_mday);

	if ( my_tm.tm_mon == c_tm.tm_mon )
		printf("tm_mon=%i\n", my_tm.tm_mon);
	else
		printf("tm_mon=%i but C is %i\n", my_tm.tm_mon, c_tm.tm_mon);

	if ( my_tm.tm_year == c_tm.tm_year )
		printf("tm_year=%i\n", my_tm.tm_year);
	else
		printf("tm_year=%i but C is %i\n", my_tm.tm_year, c_tm.tm_year);

	if ( my_tm.tm_wday == c_tm.tm_wday )
		printf("tm_wday=%i\n", my_tm.tm_wday);
	else
		printf("tm_wday=%i but C is %i\n", my_tm.tm_wday, c_tm.tm_wday);

	if ( my_tm.tm_yday == c_tm.tm_yday )
		printf("tm_yday=%i\n", my_tm.tm_yday);
	else
		printf("tm_yday=%i but C is %i\n", my_tm.tm_yday, c_tm.tm_yday);

	if ( my_tm.tm_isdst == c_tm.tm_isdst )
		printf("tm_isdst=%i\n", my_tm.tm_isdst);
	else
		printf("tm_isdst=%i but C is %i\n", my_tm.tm_isdst, c_tm.tm_isdst);

	return 0;
}

#endif