//! A span of milliseconds, written the way somebody reads it -- the port
//! of `Durations.kt`'s `formatMillis`/`formatMillisText`, with its tests.
//!
//! Only the tool-timeout half is here. `formatSpan` (the usage
//! countdown's rounding-up rule) belongs with whatever draws the usage
//! bar, and nothing in this crate needs it yet.

/// A span of milliseconds, written the way somebody reads it.
///
/// A tool's timeout arrives as `480000`, which nobody reads as eight
/// minutes. The rule has two halves, because a short span and a long one
/// are read for different things. Under a minute the question is "roughly
/// how long", so only the largest unit is shown and a fraction carries the
/// rest -- `2.5s`. At a minute or more the question is "how long exactly",
/// so every unit with something in it is written out -- `5d 12h 4m`. Empty
/// units are left out rather than written as zero.
///
/// Sub-second precision is dropped past a minute: nothing that takes days
/// is measured in milliseconds.
pub fn format_millis(ms: i64) -> String {
    if ms < 0 {
        return format!("-{}", format_millis(-ms));
    }
    if ms < 1000 {
        return format!("{ms}ms");
    }
    if ms < 60_000 {
        let tenths = (ms + 50) / 100;
        let (whole, rest) = (tenths / 10, tenths % 10);
        return if rest == 0 {
            format!("{whole}s")
        } else {
            format!("{whole}.{rest}s")
        };
    }
    let seconds = ms / 1000;
    [
        ("d", seconds / 86_400),
        ("h", seconds / 3600 % 24),
        ("m", seconds / 60 % 60),
        ("s", seconds % 60),
    ]
    .iter()
    .filter(|(_, n)| *n > 0)
    .map(|(unit, n)| format!("{n}{unit}"))
    .collect::<Vec<_>>()
    .join(" ")
}

/// `text` as a span when it is a whole number of milliseconds, and
/// unchanged when it is not.
pub fn format_millis_text(text: &str) -> String {
    match text.trim().parse::<i64>() {
        Ok(ms) => format_millis(ms),
        Err(_) => text.to_string(),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// The two ways a span of time is written here, and the rule each of
    /// them follows -- ported from `DurationsTest.kt`, whose doc says why:
    /// both are read off a screen to make a decision, so what matters is
    /// that the shortest form that answers the question is what appears.
    #[test]
    fn under_a_minute_is_the_largest_unit_alone() {
        assert_eq!(format_millis(30), "30ms");
        assert_eq!(format_millis(999), "999ms");
        assert_eq!(format_millis(1000), "1s");
        assert_eq!(format_millis(2500), "2.5s");
        // One decimal, rounded rather than cut: 2.46s is nearer two and a
        // half than two and four.
        assert_eq!(format_millis(2460), "2.5s");
        assert_eq!(format_millis(59_900), "59.9s");
    }

    #[test]
    fn a_minute_or_more_is_every_unit_that_has_something_in_it() {
        // The figure this rule was written for: a tool timeout, which
        // arrives as milliseconds and is unreadable as 480000.
        assert_eq!(format_millis(480_000), "8m");
        assert_eq!(format_millis(60_000), "1m");
        assert_eq!(format_millis(90_000), "1m 30s");
        assert_eq!(format_millis(475_440_000), "5d 12h 4m");
        // Empty units are left out rather than written as zero: the labels
        // say which is which, and "5d 0h 4m" is only longer.
        assert_eq!(format_millis(432_240_000), "5d 4m");
    }

    #[test]
    fn only_a_whole_number_of_milliseconds_is_rewritten() {
        assert_eq!(format_millis_text(" 480000 "), "8m");
        // A timeout a tool expressed some other way is its own words,
        // passed through rather than guessed at.
        assert_eq!(format_millis_text("2 minutes"), "2 minutes");
        assert_eq!(format_millis_text(""), "");
    }
}