// Copyright (c) 2019-2024 Alexander Medvednikov. All rights reserved.
// Use of this source code is governed by an MIT license
// that can be found in the LICENSE file.
module time

// unix returns a Time calculated from the given Unix timestamp in seconds since 1970-01-01 .
pub fn unix(epoch i64) Time {
    return unix_nanosecond(epoch, 0)
}

// unix_milli returns a Time calculated from the given Unix timestamp in milliseconds since 1970-01-01 .
pub fn unix_milli(ms i64) Time {
    return ts_to_time_impl(ms, 1_000, 1_000_000)
}

// unix_micro returns a Time calculated from the given Unix timestamp in microseconds since 1970-01-01 .
pub fn unix_micro(us i64) Time {
    return ts_to_time_impl(us, 1_000_000, 1_000)
}

// unix_nano returns a Time calculated from the given Unix timestamp in nanoseconds since 1970-01-01 .
pub fn unix_nano(ns i64) Time {
    return ts_to_time_impl(ns, 1_000_000_000, 1)
}

fn ts_to_time_impl(value i64, down i64, up i64) Time {
    epoch := value / down
    remainder := (value % down) * up
    return unix_nanosecond(epoch, int(remainder))
}

// unix_microsecond returns a Time struct, given an Unix timestamp in seconds, and a microsecond value.
pub fn unix_microsecond(epoch i64, microsecond int) Time {
    return unix_nanosecond(epoch, microsecond * 1000)
}

// unix_nanosecond returns a Time struct given a Unix timestamp in seconds and a nanosecond value.
pub fn unix_nanosecond(abs_unix_timestamp i64, nanosecond int) Time {
    // Split into day and time
    mut day_offset := abs_unix_timestamp / seconds_per_day
    if abs_unix_timestamp % seconds_per_day < 0 {
        // Compensate for rounding towards zero on integers as we want floored instead
        day_offset--
    }
    year, month, day := calculate_date_from_day_offset(day_offset)
    hour_, minute_, second_ :=
        calculate_time_from_second_offset(abs_unix_timestamp % seconds_per_day)
    return Time{
        year:       year
        month:      month
        day:        day
        hour:       hour_
        minute:     minute_
        second:     second_
        nanosecond: nanosecond
        unix:       abs_unix_timestamp
    }
}

// calculate_date_from_day_offset returns the year, month, and day based on the given day offset.
fn calculate_date_from_day_offset(day_offset_ i64) (int, int, int) {
    mut day_offset := day_offset_

    // source: http://howardhinnant.github.io/date_algorithms.html#civil_from_days

    // shift from 1970-01-01 to 0000-03-01
    day_offset += 719468 // int(days_per_400_years * 1970 / 400 - (28+31))

    mut era := 0
    if day_offset >= 0 {
        era = int(day_offset / days_per_400_years)
    } else {
        era = int((day_offset - days_per_400_years - 1) / days_per_400_years)
    }

    // day_of_era => [0..146096]
    day_of_era := day_offset - era * days_per_400_years

    // year_of_era => [0..399]
    year_of_era := (day_of_era - day_of_era / (days_per_4_years - 1) +
        day_of_era / days_per_100_years - day_of_era / (days_per_400_years - 1)) / days_in_year

    mut year := int(year_of_era + era * 400)

    // day_of_year => with year beginning Mar 1 [0..365]
    day_of_year := day_of_era - (days_in_year * year_of_era + year_of_era / 4 - year_of_era / 100)

    month_position := (5 * day_of_year + 2) / 153
    day := int(day_of_year - (153 * month_position + 2) / 5 + 1)
    mut month := int(month_position)

    if month_position < 10 {
        month += 3
    } else {
        month -= 9
    }

    if month <= 2 {
        year += 1
    }

    return year, month, day
}

// calculate_time_from_second_offset returns the hour, minute, and second
// based on the given second offset.
fn calculate_time_from_second_offset(second_offset_ i64) (int, int, int) {
    mut second_offset := second_offset_
    if second_offset < 0 {
        second_offset += seconds_per_day
    }
    hour_ := second_offset / seconds_per_hour
    second_offset %= seconds_per_hour
    minute_ := second_offset / seconds_per_minute
    second_offset %= seconds_per_minute
    return int(hour_), int(minute_), int(second_offset)
}