package com.example.aiapp

import java.time.Duration
import java.time.OffsetDateTime

// How long is left in a usage window. Shared by the session bar and the usage screen: the
// arithmetic is the same in both and only the sentence around it differs, so everything here
// returns the span or the state on its own and leaves the wording to the caller.

/** "1d 4h", "3h 12m", "12m" -- the span alone, with no leading or trailing words. */
fun formatSpan(until: Duration): String =
    when {
        until.toHours() >= 24 -> "${until.toDays()}d ${until.toHours() % 24}h"
        until.toHours() > 0 -> "${until.toHours()}h ${until.toMinutes() % 60}m"
        else -> "${until.toMinutes()}m"
    }

/**
 * What is known about when a usage window ends.
 *
 * Three answers rather than a nullable duration, because two of them shared `null` and they are not
 * the same thing at all. A window the server sent no reset time for is one that is **not running**:
 * the five-hour window is anchored to the block it started in, so between sessions there is nothing
 * counting down and the API says so by omitting the field -- measured against a live response on
 * 2026-08-31, where the five-hour window's reset was exactly five hours after the moment work
 * resumed. A timestamp that did arrive and could not be read is the genuinely unknown case, and it
 * is the only one worth those words.
 *
 * Collapsing them put "reset time unknown" on the session bar for a machine behaving perfectly, on
 * the one row somebody reads before starting something big -- and the usage dialog, looking at the
 * same field, quietly drew nothing. Two rules for one missing value; this is the rule.
 */
sealed class WindowEnd {
    /** No reset time was sent, so nothing is running in this window. Not a failure to find out. */
    data object NotRunning : WindowEnd()

    /** A timestamp arrived and could not be read. The one case that is actually unknown. */
    data object Unreadable : WindowEnd()

    /** How long is left. Negative once the window is past, which each caller words for itself. */
    data class Ends(val until: Duration) : WindowEnd()
}

/**
 * [resetsAt] as the server sent it -- absent, unreadable, or a moment -- against [now].
 *
 * [now] is a parameter rather than read here so a caller can drive it from state and have the
 * countdown recompute on its own schedule.
 */
fun windowEnd(resetsAt: String?, now: OffsetDateTime): WindowEnd {
    if (resetsAt == null) return WindowEnd.NotRunning
    return try {
        WindowEnd.Ends(Duration.between(now, OffsetDateTime.parse(resetsAt)))
    } catch (_: Exception) {
        WindowEnd.Unreadable
    }
}