by chance

I ran into a friend by chance.

私は偶然友達に会いました

I ran into a friend by chance yesterday.

私は昨日、偶然友達に会いました

I ran into a friend by chance at the bookstore yesterday.

私は昨日、書店で、偶然友達に会いました

I ran into a friend by chance while I was picking out a book at the bookstore yesterday.

私は昨日、書店で本を選んでいる途中、偶然友達に会いました

I ran into a friend from high school by chance while I was picking out a book at the bookstore yesterday.

私は昨日、書店で本を選んでいる途中、偶然高校時代の友達に会いました