Autoincrement separately for each foreign key

I have two tables:

 CREATE TABLE "user"
    (
      username character varying(35) NOT NULL,
      CONSTRAINT user_pk PRIMARY KEY (username)
    )
    CREATE TABLE item
    (
      id serial NOT NULL,
      username character varying(35),
      user_item_number integer,
      item_value character varying(35),
      CONSTRAINT item_pk PRIMARY KEY (id),
      CONSTRAINT item_fk FOREIGN KEY (username)
          REFERENCES "user" (username) MATCH SIMPLE
          ON UPDATE NO ACTION ON DELETE NO ACTION,
      CONSTRAINT unique_item_username UNIQUE (username, user_item_number)
    )

I would like to auto increment user_item_number separately for each username. Following image shows example. For each username: (user1, user2) user_item_number starts form 1 and is incremented by 1.

I suppose that I should use some trigger before insert which get max value of user_item_number for username which is inserted and increment it. But I don't know how to write that trigger.

I also don't know how to consider a concurency access (concurency insert more than one row with the same value of username). I don't want to get constraint violation error when two rows with the same username and user_item_number are inserted, I would like that trigger catch that error and one again increment user_item_number value.

Any idea?

I found solution. I wrote trigger and procedure:

create OR REPLACE function myinsert() RETURNS trigger as $$
    BEGIN
        if NEW.user_item_number is not null then return NEW;
        end if;
        loop
            <<roolbac_to>>
            declare
                max INTEGER:=null;
            begin
            SELECT count(user_item_number) into max from item where username=NEW.username;
            if max is null then
                max:=1;
            ELSE
                max=max+1;
            end if;
            INSERT INTO item( username, user_item_number, item_value) VALUES (NEW.username,max, NEW.item_value);
            exit;
            exception WHEN unique_violation THEN
            --do nothing
            end;
        end loop;
    return null;
    end;

$$ LANGUAGE 'plpgsql';

CREATE TRIGGER trig1
 before insert 
    ON item
   FOR EACH ROW
EXECUTE PROCEDURE myinsert();

This sulution alows for gaps creation, but it is ok for me.

I wanted to moke trigger instead of insert but it is impossible. So I made before insert trigger and return null. Insert is executed inside procedure. Instruction:

if NEW.user_item_number is not null then return NEW;
    end if;

is to not allow for reccurence