#include<stdint.h>
#include<stdlib.h>
typedef int uint24_t;
typedef struct Process {
  uint16_t stack_pointer;
  uint24_t stack_pointer_frame;
  struct Process* next_process;
  struct Process* prev_process;
  uint16_t pid;
  uint24_t page_mappings[16];
} Process;



static Process* ready_to_run_head = NULL;
static Process* ready_to_run_tail = NULL;
static Process* current_process = NULL;
static uint16_t next_pid = 0;

Process processes[4];

extern uint24_t get_free_frame();
extern void switch_to_process_asm();
extern Process* switch_to_process_asm_arg;

void new_process(uint24_t* page_mappings, uint16_t entry_point) {
  uint16_t pid = next_pid;
  next_pid++;
  processes[pid].pid = pid;
  processes[pid].stack_pointer_frame = get_free_frame();
  set_frame(0xE, processes[pid].stack_pointer_frame);
  processes[pid].stack_pointer = 0xFFEA;
  *((uint16_t*)(0xEFFE)) = entry_point;
  for (int i = 0; i<16; i++) {
    processes[pid].page_mappings[i] = page_mappings[i];     
  }
  processes[pid].next_process = NULL;
  processes[pid].prev_process = NULL;
  if (ready_to_run_head) {
    ready_to_run_head->prev_process = &processes[pid];
    processes[pid].next_process = ready_to_run_head;
    ready_to_run_head = &processes[pid];
  } else if (current_process) {
    ready_to_run_head = &processes[pid];
    ready_to_run_tail = &processes[pid];
  } else {
    current_process = &processes[pid];
  }
}

void yield() {
  if (ready_to_run_head) {
    Process* process = ready_to_run_head;
    ready_to_run_head = ready_to_run_head->next_process;
    if (ready_to_run_head==NULL) {
      ready_to_run_head = current_process;
      ready_to_run_tail = current_process;
    } else {
      ready_to_run_head->prev_process = current_process;
      current_process->next_process = ready_to_run_head;
      ready_to_run_head = current_process;
    }
    process->prev_process=NULL;
    process->next_process=NULL;
    for (int i=0; i<16; i++) {
        current_process->page_mappings[i] = get_frame(i);
    }
    for (int i=0; i<16; i++) {
        set_frame(i, process->page_mappings[i]);
    }
    switch_to_process_asm_arg = process;
    switch_to_process_asm();
  }
}