SplitBackwardsIterator

pub fn SplitBackwardsIterator(comptime T: type, comptime delimiter_type: DelimiterType) type {
    return struct {
        buffer: []const T,
        index: ?usize,
        delimiter: switch (delimiter_type) {
            .sequence, .any => []const T,
            .scalar => T,
        },

        const Self = @This();

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

        /// Returns a slice of the next field, or null if splitting is complete.
        pub fn next(self: *Self) ?[]const T {
            const end = self.index orelse return null;
            const start = if (switch (delimiter_type) {
                .sequence => lastIndexOf(T, self.buffer[0..end], self.delimiter),
                .any => lastIndexOfAny(T, self.buffer[0..end], self.delimiter),
                .scalar => lastIndexOfScalar(T, self.buffer[0..end], self.delimiter),
            }) |delim_start| blk: {
                self.index = delim_start;
                break :blk delim_start + switch (delimiter_type) {
                    .sequence => self.delimiter.len,
                    .any, .scalar => 1,
                };
            } else blk: {
                self.index = null;
                break :blk 0;
            };
            return self.buffer[start..end];
        }

        /// Returns a slice of the remaining bytes. Does not affect iterator state.
        pub fn rest(self: Self) []const T {
            const end = self.index orelse 0;
            return self.buffer[0..end];
        }

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