Chapter 31: Software Drawing, Time, and Graphical Gameslinear

Co-authored-by: Hsiang-Ying Fu <66062782+fuhsnn@users.noreply.github.com>
Co-authored-by: fuhsnn <66062782+fuhsnn@users.noreply.github.com>
Co-authored-by: Symious <14933944+symious@users.noreply.github.com>
Co-authored-by: Jun Kawasaki <jun784@gmail.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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+# Time and single-threaded animation
+
+`sys.time` uses bare `i64` values: absolute instants are milliseconds since
+the Unix epoch in UTC; durations are milliseconds. Monotonic readings belong
+to a separate clock domain. Never subtract a wall-clock reading from a
+monotonic deadline.
+
+## Clocks and waits
+
+| Call | Result |
+| --- | --- |
+| `time.now()` | Unix-epoch milliseconds; may jump when the system clock changes. |
+| `time.monotonic()` | Monotonic milliseconds for elapsed time and deadlines. |
+| `time.nanoTime()` | Monotonic nanoseconds, with the clock's available resolution. |
+| `time.sleep(ms)` | Sleep for a nonnegative duration. |
+| `time.sleepUntil(deadline)` | Wait for a monotonic millisecond deadline; returns immediately if past. |
+| `time.remaining(deadline, current)` | Nonnegative difference, saturating on overflow. |
+
+Clock errors return `-1`; sleeps return zero on success or `-1` on error,
+with `sys.libc.errno()` available. Interrupted sleeps resume rather than
+returning early. Wakeup times are approximate, and these calls do not wake a
+suspended laptop. Sleeping blocks the one Simple execution thread; use
+`gfx.wait` when window input must remain responsive.
+
+`seconds(n)`, `minutes(n)`, `hours(n)`, and `days(n)` convert signed counts
+to milliseconds. `toSeconds(ms)`, `toMinutes(ms)`, `toHours(ms)`, and
+`toDays(ms)` truncate toward zero. A duration day is exactly 24 hours,
+independent of daylight saving. `toNanos(ms)` and `fromNanos(ns)` convert
+between millisecond and nanosecond units. Multiplication overflow returns
+`-1`. Constants `SECOND`, `MINUTE`, `HOUR`, and `DAY` are also available.
+
+## String interpolation
+
+Time formatting is built into the existing string interpolation syntax:
+
+| Format | Example | Named style |
+| --- | --- | --- |
+| `%t(instant)` or `%tF(instant)` | `2026-11-12T00:34:05.123Z` | `time.ISO` |
+| `%tr(instant)` | `12:34am` | `time.CLOCK12` |
+| `%tR(instant)` | `00:34` | `time.CLOCK24` |
+| `%tT(instant)` | `00:34:05` | `time.CLOCK24_SECONDS` |
+| `%tD(instant)` | `12/11/26` | `time.DATE` |
+| `%tM(duration)` | `02:03.456` | `time.ELAPSED` |
+
+For example, `"UTC %tR(sys.time.now())"`. Width and left alignment work as
+for strings: `%30t(instant)` and `%-30t(instant)`. Precision and numeric flags
+are rejected. `%tM` formats a duration, with total minutes rather than wrapping
+at an hour; negative durations keep their sign.
+
+Interpolation uses UTC, making its output stable across machines.
+`time.format(instant, style)` is the equivalent library call;
+`time.formatLocal(instant, style)` uses the system's local zone. Local ISO
+output includes a numeric offset instead of `Z` when nonzero, retaining
+offset seconds for historical zones. Invalid calendar values/styles produce
+`<invalid time>`. The ISO form retains all millisecond bits within the supported
+calendar range; short formats intentionally omit information.
+
+## Calendar values and absolute parsing
+
+`time.utc(instant)` and `time.local(instant)` return a `DateTime` with fields
+`year`, `month`, `day`, `hour`, `minute`, `second`, `millisecond`, `weekday`
+(Sunday = 0), `offsetSeconds` (east of UTC), and `daylight`. A zero year means
+conversion failed. `time.zoneName(instant)` returns the local abbreviation or
+an empty string on error. Local conversion uses the operating system's zone
+rules, including DST at that instant; no time-zone database is downloaded.
+
+`time.makeUTC(year, month, day, hour, minute, second, millisecond)` validates
+and constructs an instant, returning `-1` on invalid fields. `time.valid(...)`,
+`time.leap(year)`, and `time.monthDays(year, month)` support calendar checks.
+Calendar conversion supports Gregorian years 1 through 9999, bounded by
+`time.MIN` and `time.MAX`, and rejects leap seconds.
+
+`time.parse(text)` accepts strict absolute ISO text:
+`YYYY-MM-DDTHH:mm:ss[.SSS]Z` or the same with an explicit `+HH:MM`/`-HH:MM`
+offset (optionally with `:SS`). Fractions have exactly three digits. No relative
+time parsing, implicit local zone, locale-dependent parsing, or named-zone
+conversion is provided. Offsets must have hours 00–23 and minutes/seconds 00–59.
+Invalid text or an out-of-range result returns `-1`. By design, this sentinel
+is indistinguishable from the valid instant `1969-12-31T23:59:59.999Z`;
+formatting and calendar conversion still support that instant.
+
+## Cooperative timers
+
+Create `val timers = new sys.time.Timers();` on the game thread:
+
+| Operation | Meaning |
+| --- | --- |
+| `after(delay)` | Register a one-shot timer; return its positive ID, or `-1`. |
+| `every(period)` | Register a repeating timer; period must be positive. |
+| `afterAt(current, delay)`, `everyAt(current, period)` | Supply a monotonic reading explicitly, useful for deterministic tests. |
+| `cancel(id)` | Remove a timer; return whether it was present. |
+| `nextDeadline()` | Earliest monotonic deadline, or `-1` if empty. |
+| `poll(current)` | Return the next due ID, or `-1` if none. |
+
+Poll repeatedly to drain due timers. Deadlines order delivery, then registration
+ID breaks ties. Repeating timers advance from their previous deadline, skipping
+missed ticks to the first future deadline rather than drifting or replaying a
+backlog. Deadline overflow rejects registration or cancels a repeating timer.
+There are no callbacks or timer threads: the caller handles each returned ID.
+
+`gfx.wait(timeoutMs)` processes events after waiting for input or a timeout:
+zero polls, `-1` waits indefinitely, values below `-1` fail. It returns the same
+`OK`, `CLOSED`, or `ERROR` statuses as `gfx.poll`, and may wake early. Re-read the
+monotonic clock after each wake. This allows one thread to handle timers,
+input, simulation, and drawing without busy waiting.
+
+The [digital and analog clock example](examples/Clock.smp) combines these pieces. From
+the repository root, with graphics dependencies already cached:
+
+```sh
+make -C chapter31 clock-example CTAGS=  # Build without opening a window.
+make -C chapter31 run-clock CTAGS=      # Build the backend and run.
+```
+
+Click the button or press Space to switch between green seven-segment digits
+and an analog dial with smoothly moving hands. Both display system-local time;
+Q/Escape exits. Passing any argument directly to the executable closes it after
+ten frames, switching faces halfway through. Hands are computed from the current
+time, so delayed frames do not create a simulation catch-up backlog.