WindowIterator

pub fn WindowIterator(comptime T: type) type {
    return struct {
        buffer: []const T,
        index: ?usize,
        size: usize,
        advance: usize,

        const Self = @This();

        /// Returns a slice of the first window.
        /// Call this only to get the first window and then use `next` to get
        /// all subsequent windows.
        /// Asserts that iteration has not begun.
        pub fn first(self: *Self) []const T {
            assert(self.index.? == 0);
            return self.next().?;
        }

        /// Returns a slice of the next window, or null if window is at end.
        pub fn next(self: *Self) ?[]const T {
            const start = self.index orelse return null;
            const next_index = start + self.advance;
            const end = if (start + self.size < self.buffer.len and next_index < self.buffer.len) blk: {
                self.index = next_index;
                break :blk start + self.size;
            } else blk: {
                self.index = null;
                break :blk self.buffer.len;
            };

            return self.buffer[start..end];
        }

        /// Resets the iterator to the initial window.
        pub fn reset(self: *Self) void {
            self.index = 0;
        }
    };
}