FSM: priority arbiter

Design an arbiter FSM that hands out grants g[3:1] in response to requests r[3:1] from three devices. Device 1 has the highest priority, device 3 the lowest.

• In the idle state A, no grant is asserted (g = 3'b000). At each clock edge while in A: if r[1] = 1, move to state B; else if r[2] = 1, move to state C; else if r[3] = 1, move to state D; otherwise stay in A.
• In state B, assert g[1] = 1 (only). Remain in B as long as r[1] stays 1; when r[1] = 0 at a clock edge, return to A.
• In state C, assert g[2] = 1 (only). Remain while r[2] = 1; return to A when r[2] = 0.
• In state D, assert g[3] = 1 (only). Remain while r[3] = 1; return to A when r[3] = 0.

A grant, once given, is not preempted by a higher-priority request — the device keeps its grant until it drops its own request.

resetn is an asynchronous, active-low reset that forces the machine to state A.