New kitty image backend (#74)

* Initial attempt at supporting new backend for kitty images

* it's almost working !!

* it almost basically works

* yaaayyyy it works

* Use github kittage

* Uhhhh various improvements from kittage and psx-shm

* Remove logging

* incorporate recovering from deleted images

* Make it work correctly with ghostty image eviction too

* fall back to stdout if shms don't work

* Make help page work again

* zooming basically does what you'd expect now

* yay zooming woohoo

* clean up top and bottom rendering

* Only allow zooming in kitty

* Add debug logging and fix cursor placement after image display

* yaaaay zooming out once you're already zoomed in and respecting kitty's limits for how big of an image to display

* mmmm maybe it's finally ready to merge...

* Update deps

* Switch around list of items on changelog

* fmt

* Small fixes to avoid panic and allow zooming back in after zooming out

src/skip.rs

@@ -1,3 +1,5 @@
+use std::num::NonZeroUsize;
+
 use ratatui::widgets::Widget;
 
 pub struct Skip {
@@ -19,3 +21,107 @@ impl Widget for Skip {
 		}
 	}
 }
+
+enum PlusOrMinus {
+	Plus,
+	Minus
+}
+
+pub struct InterleavedAroundWithMax {
+	// starts at this number
+	around: usize,
+	inclusive_min: usize,
+	// this iterator can only produce values in [0..max)
+	exclusive_max: NonZeroUsize,
+	// the next time we call `next()`, this value should be combined with `around` according to
+	// `next_op`, then, after next_op is inverted, incremented if next_op was negative before being
+	// inverted.
+	next_change: usize,
+	// How `next_change` should be applied to `around` next time `next()` is called
+	next_op: PlusOrMinus
+}
+
+impl InterleavedAroundWithMax {
+	/// the following must hold or else this is liable to panic or produce nonsense values:
+	/// - inclusive_min < exclusive_max
+	/// - inclusive_min <= around <= exclusive_max
+	pub fn new(around: usize, inclusive_min: usize, exclusive_max: NonZeroUsize) -> Self {
+		Self {
+			around,
+			inclusive_min,
+			exclusive_max,
+			next_change: 0,
+			next_op: PlusOrMinus::Minus
+		}
+	}
+}
+
+impl Iterator for InterleavedAroundWithMax {
+	type Item = usize;
+	fn next(&mut self) -> Option<Self::Item> {
+		let actual_change = self.next_change % (self.exclusive_max.get() - self.inclusive_min);
+
+		let to_return = match self.next_op {
+			// If we're supposed to add them and we need it to wrap, then try to add them together
+			// 'cause we need special behavior if it overflows usize's limits
+			PlusOrMinus::Plus => match self.around.checked_add(actual_change) {
+				// If we added it and it's within the range, we're chillin
+				Some(next_val) if next_val < self.exclusive_max.get() => next_val,
+				// If we added it and it's not within the range, do next_val % (self.max + 1), e.g.
+				// if max is 20, we were at 15, and we added 7, we should get 1 (because +5 would
+				// hit the max, then 0, then 1). So adding 1 before the modulo makes it hit the
+				// right numbers. And we can be sure the + here doesn't overflow 'cause we already
+				// checked the `usize::MAX` up above
+				Some(next_val) => (next_val % self.exclusive_max.get()) + self.inclusive_min,
+				// If we added them and it would've overflowed usize::MAX, then we see how much
+				// of the change would be remaining after reaching `max`
+				None =>
+					(actual_change - (self.exclusive_max.get() - actual_change))
+						+ self.inclusive_min,
+			},
+			PlusOrMinus::Minus => match self.around.checked_sub(actual_change) {
+				// If we can just minus it, cool cool. All is good.
+				Some(next_val) if next_val >= self.inclusive_min => next_val,
+				// If we can minus it but it goes below our min, then see how much below it went
+				// and just manually wrap it around
+				Some(next_val) => self.exclusive_max.get() - (self.inclusive_min - next_val),
+				// If we can't...
+				None => {
+					// then we see how much of the change would be remaining after hitting the
+					// minimum
+					let remaining = actual_change - (self.around - self.inclusive_min);
+
+					// and then we take that away from the top!
+					self.exclusive_max.get() - remaining
+				}
+			}
+		};
+
+		self.next_op = match self.next_op {
+			PlusOrMinus::Plus => PlusOrMinus::Minus,
+			PlusOrMinus::Minus => {
+				self.next_change = (self.next_change + 1) % self.exclusive_max.get();
+				PlusOrMinus::Plus
+			}
+		};
+
+		Some(to_return)
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn iter_works() {
+		let got = InterleavedAroundWithMax::new(5, 2, NonZeroUsize::new(21).unwrap())
+			.take(30)
+			.collect::<Vec<_>>();
+
+		assert_eq!(got, vec![
+			5, 6, 4, 7, 3, 8, 2, 9, 20, 10, 19, 11, 18, 12, 17, 13, 16, 14, 15, 15, 14, 16, 13, 17,
+			12, 18, 11, 19, 10, 20
+		]);
+	}
+}