Logic Masters Deutschland e.V.

Rules:

Normal sudoku rules apply.

Each digit from 1-9 is represented by a letter from A-I. You must determine which letter represents which digit. You can use the register underneath the grid for note-taking.

Regular quadruples: each digit in a circle must appear in one of the 4 cells surrounding the clue.

Self-indexing quadruples: an XYZW-clue outside a row or column indicates that the cells at distances X, Y, Z, and W from the clue contain an XYZW-quadruple.
Examples: A 2358-clue to the left of row 2 indicates that r2c2358 contain a 2358-quadruple; a 1467-clue underneath column 8 indicates that r3469c8 contain a 1467-quadruple.

(Tooltip: the self-indexing quad clues are clickable cells. You can colour them or add corner marks if you wish)

Links:

This puzzle was loosely inspired by Quad Code by Marty Sears. Fair warning: it's very challenging, but there is a logical solve path. Good luck!