open-standards/wai/wai-vector/src/clock.rs

//! The clock (§3.1) and the frame range (§2.7, *Render*).
//!
//! A frame `g` is in range at a rate with `P` ticks per frame when
//! `⌈t0/P⌉ ≤ g ≤ ⌊(t0 + dur − 1)/P⌋`. The range is decided by those divisions and
//! never by forming `g·P` for an unchecked `g`: `P` reaches 2^40, so the product of an
//! arbitrary request and `P` does not fit 64 bits.

use crate::arith::{ceil_div, floor_div};

/// The first and last frame of the document at `P` ticks per frame. The range is
/// empty when the first exceeds the last.
pub fn frame_range(t0: u32, dur: u32, p: u64) -> (i64, i64) {
    debug_assert!((1..=1 << 40).contains(&p));
    let p = p as i64;
    let first = ceil_div(i64::from(t0), p);
    let last = floor_div(i64::from(t0) + i64::from(dur) - 1, p);
    (first, last)
}

/// Whether frame `g` is in the document at `P` ticks per frame.
pub fn in_range(g: u64, t0: u32, dur: u32, p: u64) -> bool {
    let (first, last) = frame_range(t0, dur, p);
    // `first` and `last` lie in [0, 2^31), so the comparisons are exact for any `g`.
    first <= last && (first as u64) <= g && g <= last as u64
}

/// The tick of a frame in range, `τ = g·P`, which is below 2^31.
pub fn tick(g: u64, p: u64) -> i64 {
    let t = g * p;
    debug_assert!(t < 1 << 31);
    t as i64
}

/// The frames of the document at `P` ticks per frame, as `first..=last`.
pub fn frames(t0: u32, dur: u32, p: u64) -> core::ops::RangeInclusive<u64> {
    let (first, last) = frame_range(t0, dur, p);
    if first > last { core::ops::RangeInclusive::new(1, 0) } else { first as u64..=last as u64 }
}