const std = @import("std");


pub const IntersectionIterator = struct {
    ref: std.mem.TokenIterator(u8),
    rest: []const []const u8,

    /// Returns the next occurrence or null. Returned value is owned by originally passed slice
    pub fn next(self: *IntersectionIterator) ?[]const u8{
        // If this returns null, there's no more items to check. We're finished.
        const needle = self.ref.next() orelse return null;

        // Search into every one of the strings
        haystack: for (self.rest) |haystack| {

            // The items are separated by spaces, iterate through those
            var haystack_iter = std.mem.tokenize(u8, haystack, " ");

            // If the word is found, it short circuits and tries the next haystack
            while (haystack_iter.next()) |hay_blade| {
                if (std.mem.eql(u8, hay_blade, needle)) continue :haystack;
            }

            // If this point is reached, the needle wasn't found.
            // Try the next one
            return self.next();
        }

        // If we got here, no not-matching haysack was found. A match!
        return needle;
    }
};

/// Finds the common tokens inside space separated strings
/// and returns a slice to the first token that matches
pub fn intersection(slices: []const []const u8) IntersectionIterator {
    // Tokenize the first entry and save the rest as references to check
    return IntersectionIterator{
        .ref = std.mem.tokenize(u8, slices[0], " "),
        .rest = slices[1..],
    };
}