% Program 6-8
% npcsma.m
%
% non-persistent CSMA System
%
% Input argument
%   now_time  : now time   but, now_time<0 initializes the access terminals
%
% Output argument
%   next_time : next state change time
%
% Programmed by M.Okita
% Checked by H.Harada
%

function [next_time] = npcsma(now_time)

global STANDBY TRANSMIT COLLISION                       % definition of the global variable
global Srate Plen	
global Mnum Mplen Mstime Mstate
global Tint Rint
global Spnum Splen Tplen Wtime

persistent mgtime mtime                                 % definition of the static variable

if now_time < 0                                         % initialize access terminals
    rand('state',sum(100*clock));                       % resetting of the random table
    mgtime        = -Tint * log(1-rand(1,Mnum));        % packet generation time
    Mstime        = zeros(1,Mnum) - inf;                % packet transmitting time
    mtime         = mgtime;                             % state change time
    Mstate        = zeros(1,Mnum); 
    Mplen(1:Mnum) = Plen;
    next_time     = min(mtime);
    return
end

idx = find(mtime==now_time & Mstate==TRANSMIT);         % finding of the terminal which transmission succeeded
if length(idx) > 0
    Spnum       = Spnum + 1;
    Splen       = Splen + Mplen(idx);
    Wtime       = Wtime + now_time - mgtime(idx);
    Mstate(idx) = STANDBY;
    mgtime(idx) = now_time - Tint * log(1-rand);        % next packet generation time
    mtime(idx)  = mgtime(idx);                          % next packet transmitting time
end

idx = find(mtime==now_time & Mstate==COLLISION);        % finding of the terminal which transmission failed
if length(idx) > 0
    Mstate(idx) = STANDBY;
    mtime(idx)  = now_time - Rint * log(1-rand(1,length(idx))); % resending time
end

idx = find(mtime==now_time & Mstate==STANDBY);          % finding of the terminal which carrier sensing
if length(idx) > 0
    Tplen = Tplen + sum(Mplen(idx));
    for ii=1:length(idx)
        jj = idx(ii);
        if carriersense(jj,now_time) == 0               % channel is idle
            Mstate(jj) = TRANSMIT;                      % packet transmitting
            Mstime(jj) = now_time;                      % start time of transmitting
            mtime(jj)  = now_time + Mplen(jj) / Srate;  % end time of transmitting
        else                                            % channel is busy
            mtime(jj) = now_time - Rint * log(1-rand);  % waiting time
        end
    end
end

next_time = min(mtime);                                 % next state change time

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