EnableExplicit
Declare main()
Declare.i mi(a.i, b.i)

If OpenConsole("MODULAR-INVERSE")
  main() : Input() : End
EndIf

Macro ModularInverse(a, b)
  PrintN(~"\tMODULAR-INVERSE(" + RSet(Str(a),5) + "," +
         RSet(Str(b),5)+") = " +
         RSet(Str(mi(a, b)),5))
EndMacro

Procedure main()
  ModularInverse(42, 2017)  ; = 1969
  ModularInverse(40, 1)     ; = 0
  ModularInverse(52, -217)  ; = 96
  ModularInverse(-486, 217) ; = 121
  ModularInverse(40, 2018)  ; = -1
EndProcedure

Procedure.i mi(a.i, b.i)
  Define x.i = 1,
         y.i = Int(Abs(b)),
         r.i = 0
  If y = 1 : ProcedureReturn 0 : EndIf
  While x < y
    r = (a * x) % b
    If r = 1 Or (y + r) = 1
      Break
    EndIf
    x + 1
  Wend
  If x > y - 1 : x = -1 : EndIf
  ProcedureReturn x
EndProcedure