Red and black tile slides and jumps

A strip of cells holds red tiles (R), black tiles (B) and empty cells (_). The board is a string such as "RR_B_B".

Movement rules

• A red tile only moves right. A black tile only moves left.
• Slide: a tile moves one cell in its direction into an empty cell.
• Jump: a tile moves two cells in its direction, over exactly one tile of the other color, into an empty cell. The jumped tile is not removed. A tile can never jump a tile of its own color.
• One tile moves per move. Tiles never leave the board.

Part 1. Implement valid_moves(board: str) -> list[tuple[int, int]]. Return every legal move as (from_index, to_index), ordered by from_index (a tile with both a slide and a jump lists the slide first).

Part 2. Implement min_moves(start: str, target: str) -> int. Return the fewest moves that turn start into target, or -1 if it cannot be done. Be ready to return the move sequence too.

Constraints: 0 <= len(board) <= 20; both boards have the same length and use only R, B and _.

Example 1

valid_moves("R_B")  -> [(0, 1), (2, 1)]
valid_moves("RB_")  -> [(0, 2)]      # red jumps black; black is blocked
valid_moves("RR_")  -> [(1, 2)]      # a red cannot jump a red

Example 2

min_moves("R_B", "B_R") -> 3
  R_B -> _RB   (red slides 0 -> 1)
  _RB -> BR_   (black jumps 2 -> 0)
  BR_ -> B_R   (red slides 1 -> 2)

min_moves("RB", "BR") -> -1   # no tile can move at all

Reconstructed from the reported question; details may differ from what interviewers ask.