PL/pgSQL implementation of a LexoRank-style lexicographical rank generator function

midString.sql

-- https://stackoverflow.com/questions/38923376/return-a-new-string-that-sorts-between-two-given-strings
create or replace function app_public.mid_string(prev text, next text) returns text as $$
declare
  v_p int;
  v_n int;
  v_pos int := 0;
  v_str text;
begin
  LOOP -- find leftmost non-matching character
    v_p := CASE WHEN v_pos < char_length(prev) THEN ascii(substring(prev from v_pos + 1)) ELSE 96 END;
    v_n := CASE WHEN v_pos < char_length(next) THEN ascii(substring(next from v_pos + 1)) ELSE 123 END;
    v_pos := v_pos + 1;
    EXIT WHEN NOT (v_p = v_n);
  END LOOP;
  v_str := left(prev, v_pos-1);   -- copy identical part of string
  IF v_p = 96 THEN                -- prev string equals beginning of next
    WHILE v_n = 97 LOOP           -- next character is 'a'
      -- get char from next
      v_n = CASE WHEN v_pos < char_length(next) THEN ascii(substring(next from v_pos + 1)) ELSE 123 END;
      v_str := v_str || 'a';      -- insert an 'a' to match the 'a'
      v_pos := v_pos + 1;
    END LOOP;
    IF v_n = 98 THEN              -- next character is 'b'
      v_str := v_str || 'a';      -- insert an 'a' to match the 'b'
      v_n := 123;                 -- set to end of alphabet
    END IF;
  ELSIF (v_p + 1) = v_n THEN    -- found consecutive characters
    v_str := v_str || chr(v_p); -- insert character from prev
    v_n = 123;                  -- set to end of alphabet
    v_p := CASE WHEN v_pos < char_length(prev) THEN ascii(substring(prev from v_pos + 1)) ELSE 96 END;
    WHILE v_p = 122 LOOP
      v_pos := v_pos + 1;
      v_str := v_str || 'z';    -- insert 'z' to match 'z'
      v_p := CASE WHEN v_pos < char_length(prev) THEN ascii(substring(prev from v_pos + 1)) ELSE 96 END;
    END LOOP;
  END IF;
  return v_str || chr(ceil((v_p + v_n) / 2.0)::int);
end;
$$ language plpgsql strict immutable;