Calculate hierarchical labels for Google Sheets using native functions

Question

Using Google Sheets, I want to automatically number rows like so:

The key is that I want this to use built-in functions only.

I have an

Answer 1

Here's an answer that does not allow a blank line between items, and requires that you manually type "1" into the first cell (A2). This formula is applied to cell A3, with the assumption that there are at most three levels of hierarchy in columns B, C, and D.

=IF(
  COUNTA(B3),                                 // If there is a value in the 1st column
  INDEX(SPLIT(A2,"."),1)+1,                   //   find the 1st part of the prior ID, plus 1
  IF(                                         // ...otherwise
    COUNTA(C3),                               // If there's a value in the 2nd column
    INDEX(SPLIT(A2,"."),1)                    //   find the 1st part of the prior ID
      & "."                                   //   add a period and
      & IFERROR(INDEX(SPLIT(A2,"."),2),0)+1,  //   add the 2nd part of the prior ID (or 0), plus 1
    INDEX(SPLIT(A2,"."),1)                    // ...otherwise find the 1st part of the prior ID
      & "."                                   //   add a period and
      & IFERROR(INDEX(SPLIT(A2,"."),2),1)     //   add the 2nd part of the prior ID or 1 and
      & "."                                   //   add a period and
      & IFERROR(INDEX(SPLIT(A2,"."),3)+1,1)   //   add the 3rd part of the prior ID (or 0), plus 1
  )
) & ""                                        // Ensure the result is a string ("1.2", not 1.2)

Without comments:

=IF(COUNTA(B3),INDEX(SPLIT(A2,"."),1)+1,IF(COUNTA(C3),INDEX(SPLIT(A2,"."),1)& "."& IFERROR(INDEX(SPLIT(A2,"."),2),0)+1,INDEX(SPLIT(A2,"."),1)& "."& IFERROR(INDEX(SPLIT(A2,"."),2),1)& "."& IFERROR(INDEX(SPLIT(A2,"."),3)+1,1))) & ""