/**
 * mds — A micro-display server
 * Copyright © 2014  Mattias Andrée (maandree@member.fsf.org)
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
#include "variables.h"

#include <stdlib.h>
#include <string.h>



/**
 * The state of a variable
 */
typedef struct variable
{
  /**
   * The current value of the variable
   */
  mds_kbdc_tree_t* value;
  
  /**
   * The previous version of variable,
   * before it was shadowed
   */
  struct variable* restrict previous;
  
  /**
   * The original scope the current shadow
   * of the variable was created in
   */
  size_t scope;
  
} variable_t;



/**
 * Map (by index) of defined variables
 */
static variable_t** restrict variables = NULL;

/**
 * The size of `variables`
 */
static size_t variable_count = 0;

/**
 * The current scope, the number of
 * times the variable-stakc has been
 * pushed without being popped
 */
static size_t current_scope = 0;



/**
 * Destroy the variable storage
 */
void variables_terminate(void)
{
  size_t i;
  variable_t* old;
  for (i = 0; i < variable_count; i++)
    while (variables[i])
      {
	old = variables[i];
	variables[i] = variables[i]->previous;
	mds_kbdc_tree_free(old->value);
	free(old);
      }
  free(variables), variables = NULL;
  variable_count = current_scope = 0;
}


/**
 * Push the variable-stack, making it
 * possible to shadow all variables
 */
void variables_stack_push(void)
{
  current_scope++;
}


/**
 * Undo the actions of `variables_stack_push`
 * and all additions to the variable storage
 * since it was last called (without a
 * corresponding call to this function)
 */
void variables_stack_pop(void)
{
  size_t i;
  variable_t* old;
  for (i = 0; i < variable_count; i++)
    if (variables[i] && (variables[i]->scope == current_scope))
      {
	old = variables[i];
	variables[i] = variables[i]->previous;
	mds_kbdc_tree_free(old->value);
	free(old);
      }
  current_scope--;
}


/**
 * Check whether a let will override a variable
 * rather the define or shadow it
 * 
 * @param   variable  The variable index
 * @return            Whether a let will override the variable
 */
int variables_let_will_override(size_t variable)
{
  if (variable >= variable_count)   return 0;
  if (variables[variable] == NULL)  return 0;
  return variables[variable]->scope == current_scope;
}


/**
 * Assign a value to a variable, and define or shadow it in the process
 * 
 * @param   variable  The variable index
 * @param   value     The variable's new value
 * @return            Zero on success, -1 on error
 */
int variables_let(size_t variable, mds_kbdc_tree_t* restrict value)
{
  variable_t** new;
  variable_t* previous;
  
  /* Grow the table if necessary to fit the variable. */
  if (variable >= variable_count)
    {
      new = realloc(variables, (variable + 1) * sizeof(variable_t*));
      if (new == NULL)
	return -1;
      variables = new;
      memset(variables, 0, (variable + 1 - variable_count) * sizeof(variable_t*));
      variable_count = variable + 1;
    }
  
  if (variables_let_will_override(variable))
    {
      /* Override. */
      mds_kbdc_tree_free(variables[variable]->value);
      variables[variable]->value = value;
    }
  else
    {
      /* Shadow or define. */
      previous = variables[variable];
      variables[variable] = malloc(sizeof(variable_t));
      if (variables[variable] == NULL)
	return variables[variable] = previous, -1;
      variables[variable]->value = value;
      variables[variable]->previous = previous;
      variables[variable]->scope = current_scope;
    }
  
  return 0;
}


/**
 * Get the value currently assigned to a variable
 * 
 * The function cannot fail, `NULL` is however returned
 * if the variable is not defined
 * 
 * @param   variable  The variable index
 * @return            The variable's value, `NULL` if not defined
 */
mds_kbdc_tree_t* variables_get(size_t variable)
{
  if (variable >= variable_count)   return NULL;
  if (variables[variable] == NULL)  return NULL;
  return variables[variable]->value;
}