checkCompleteness

        fn checkCompleteness(lens: []const u4) !void {
            if (alphabet_size == 286)
                if (lens[256] == 0) return error.MissingEndOfBlockCode;

            var count: [@as(usize, max_code_bits) + 1]u16 = @splat(0);
            var max: usize = 0;
            for (lens) |n| {
                if (n == 0) continue;
                if (n > max) max = n;
                count[n] += 1;
            }
            if (max == 0) // empty tree
                return;

            // check for an over-subscribed or incomplete set of lengths
            var left: usize = 1; // one possible code of zero length
            for (1..count.len) |len| {
                left <<= 1; // one more bit, double codes left
                if (count[len] > left)
                    return error.OversubscribedHuffmanTree;
                left -= count[len]; // deduct count from possible codes
            }
            if (left > 0) { // left > 0 means incomplete
                // incomplete code ok only for single length 1 code
                if (max_code_bits > 7 and max == count[0] + count[1]) return;
                return error.IncompleteHuffmanTree;
            }
        }