/*
 * Tiny BASIC Interpreter and Compiler Project
 * Interpreter module
 *
 * Released as Public Domain by Damian Gareth Walker 2019
 * Created: 23-Aug-2019
 */


/* included headers */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "interpret.h"
#include "errors.h"
#include "options.h"
#include "statement.h"


/* forward declarations */
static int interpret_expression (ExpressionNode *expression);
static void interpret_statement (StatementNode *statement);


/*
 * Data Definitions
 */

/* The GOSUB Stack */
typedef struct gosub_stack_node GosubStackNode;
typedef struct gosub_stack_node {
  ProgramLineNode *program_line; /* the line following the GOSUB */
  GosubStackNode *next; /* stack node for the previous GOSUB */
} GosubStackNode;

/* private data */
typedef struct interpreter_data {
  ProgramNode *program; /* the program to interpret */
  ProgramLineNode *line; /* current line we're executing */
  GosubStackNode *gosub_stack; /* the top of the GOSUB stack */
  int gosub_stack_size; /* number of entries on the GOSUB stack */
  int variables [26]; /* the numeric variables */
  int stopped; /* set to 1 when an END is encountered */
  ErrorHandler *errors; /* the error handler */
  LanguageOptions *options; /* the language options */
} InterpreterData;

/* convenience variables */
static Interpreter *this; /* the object we are working with */


/*
 * Private Methods
 */


/*
 * Evaluate a factor for the interpreter
 * params:
 *   FactorNode*   factor   the factor to evaluate
 */
static int interpret_factor (FactorNode *factor) {

  /* local variables */
  int result_store = 0; /* result of factor evaluation */

  /* check factor class */
  switch (factor->class) {

    /* a regular variable */
    case FACTOR_VARIABLE:
      result_store = this->priv->variables [factor->data.variable - 1]
        * (factor->sign == SIGN_POSITIVE ? 1 : -1);
      break;

    /* an integer constant */
    case FACTOR_VALUE:
      result_store = factor->data.value
        * (factor->sign == SIGN_POSITIVE ? 1 : -1);
      break;

    /* an expression */
    case FACTOR_EXPRESSION:
      result_store = interpret_expression (factor->data.expression)
        * (factor->sign == SIGN_POSITIVE ? 1 : -1);
      break;

    /* this only happens if the parser has failed in its duty */
    default:
      this->priv->errors->set_code
        (this->priv->errors, E_INVALID_EXPRESSION, 0, this->priv->line->label);
  }

  /* check the result and return it*/
  if (result_store < -32768 || result_store > 32767)
    this->priv->errors->set_code
      (this->priv->errors, E_OVERFLOW, 0, this->priv->line->label);
  return result_store;
}

/*
 * Evaluate a term for the interpreter
 * params:
 *   TermNode*   term   the term to evaluate
 */
static int interpret_term (TermNode *term) {

  /* local variables */
  int result_store; /* the partial evaluation */
  RightHandFactor *rhfactor; /* pointer to successive rh factor nodes */
  int divisor; /* used to check for division by 0 before attempting */

  /* calculate the first factor result */
  result_store = interpret_factor (term->factor);
  rhfactor = term->next;

  /* adjust store according to successive rh factors */
  while (rhfactor && ! this->priv->errors->get_code (this->priv->errors)) {
    switch (rhfactor->op) {
      case TERM_OPERATOR_MULTIPLY:
        result_store *= interpret_factor (rhfactor->factor);
	if (result_store < -32768 || result_store > 32767)
	  this->priv->errors->set_code
            (this->priv->errors, E_OVERFLOW, 0, this->priv->line->label);
        break;
      case TERM_OPERATOR_DIVIDE:
        if ((divisor = interpret_factor (rhfactor->factor)))
          result_store /= divisor;
        else
          this->priv->errors->set_code
            (this->priv->errors, E_DIVIDE_BY_ZERO, 0, this->priv->line->label);
        break;
      default:
        break;
    }
    rhfactor = rhfactor->next;
  }

  /* return the result */
  return result_store;
}

/*
 * Evaluate an expression for the interpreter
 * params:
 *   ExpressionNode*   expression   the expression to evaluate
 */
static int interpret_expression (ExpressionNode *expression) {

  /* local variables */
  int result_store; /* the partial evaluation */
  RightHandTerm *rhterm; /* pointer to successive rh term nodes */

  /* calculate the first term result */
  result_store = interpret_term (expression->term);
  rhterm = expression->next;

  /* adjust store according to successive rh terms */
  while (rhterm && ! this->priv->errors->get_code (this->priv->errors)) {
    switch (rhterm->op) {
      case EXPRESSION_OPERATOR_PLUS:
        result_store += interpret_term (rhterm->term);
	if (result_store < -32768 || result_store > 32767)
	  this->priv->errors->set_code
            (this->priv->errors, E_OVERFLOW, 0, this->priv->line->label);
        break;
      case EXPRESSION_OPERATOR_MINUS:
        result_store -= interpret_term (rhterm->term);
	if (result_store < -32768 || result_store > 32767)
	  this->priv->errors->set_code
            (this->priv->errors, E_OVERFLOW, 0, this->priv->line->label);
        break;
      default:
        break;
    }
    rhterm = rhterm->next;
  }

  /* return the result */
  return result_store;
}

/*
 * Find a program line given its label
 * returns:
 *   ProgramLineNode*   the program line found
 */
static ProgramLineNode *find_label (int jump_label) {

  /* local variables */
  ProgramLineNode
    *ptr, /* a line we're currently looking at */
    *found = NULL; /* the line if found */

  /* do the search */
  for (ptr = this->priv->program->first; ptr && ! found; ptr = ptr->next)
    if (ptr->label == jump_label)
      found = ptr;
    else if (ptr->label >= jump_label
      && this->priv->options->get_line_numbers (this->priv->options)
        != LINE_NUMBERS_OPTIONAL)
      found = ptr;

  /* check for errors and return what was found */
  if (! found)
    this->priv->errors->set_code
      (this->priv->errors, E_INVALID_LINE_NUMBER, 0, this->priv->line->label);
  return found;
}


/*
 * Level 1 Routines
 */


/*
 * Initialise the variables
 */
static void initialise_variables (void) {
  int count; /* counter for this->priv->variables */
  for (count = 0; count < 26; ++count) {
    this->priv->variables [count] = 0;
  }
}

/*
 * Interpret a LET statement
 * params:
 *   LetStatementNode*   letn   the LET statement details
 */
void interpret_let_statement (LetStatementNode *letn) {
  this->priv->variables [letn->variable - 1]
    = interpret_expression (letn->expression);
  this->priv->line = this->priv->line->next;
}

/*
 * Interpret an IF statement
 * params:
 *   IfStatementNode*   ifn   the IF statement details
 */
void interpret_if_statement (IfStatementNode *ifn) {

  /* local variables */
  int
    left, /* result of the left-hand expression */
    right, /* result of the right-hand expression */
    comparison; /* result of the comparison between the two */

  /* get the expressions */
  left = interpret_expression (ifn->left);
  right = interpret_expression (ifn->right);

  /* make the comparison */
  switch (ifn->op) {
    case RELOP_EQUAL: comparison = (left == right); break;
    case RELOP_UNEQUAL: comparison = (left != right); break;
    case RELOP_LESSTHAN: comparison = (left < right); break;
    case RELOP_LESSOREQUAL: comparison = (left <= right); break;
    case RELOP_GREATERTHAN: comparison = (left > right); break;
    case RELOP_GREATEROREQUAL: comparison = (left >= right); break;
  }

  /* perform the conditional statement */
  if (comparison && ! this->priv->errors->get_code (this->priv->errors))
    interpret_statement (ifn->statement);
  else
    this->priv->line = this->priv->line->next;
}

/*
 * Interpret a GOTO statement
 * params:
 *   GotoStatementNode*   goton   the GOTO statement details
 */
void interpret_goto_statement (GotoStatementNode *goton) {
  int label; /* the line label to go to */
  label = interpret_expression (goton->label);
  if (! this->priv->errors->get_code (this->priv->errors))
    this->priv->line = find_label (label);
}

/*
 * Interpret a GOSUB statement
 * params:
 *   GosubStatementNode*   gosubn   the GOSUB statement details
 */
void interpret_gosub_statement (GosubStatementNode *gosubn) {

  /* local variables */
  GosubStackNode *gosub_node; /* indicates the program line to return to */
  int label; /* the line label to go to */

  /* create the new node on the GOSUB stack */
  if (this->priv->gosub_stack_size < this->priv->options->get_gosub_limit
    (this->priv->options)) {
    gosub_node = malloc (sizeof (GosubStackNode));
    gosub_node->program_line = this->priv->line->next;
    gosub_node->next = this->priv->gosub_stack;
    this->priv->gosub_stack = gosub_node;
    ++this->priv->gosub_stack_size;
  } else
    this->priv->errors->set_code (this->priv->errors,
      E_TOO_MANY_GOSUBS, 0, this->priv->line->label);
  
  /* branch to the subroutine requested */
  if (! this->priv->errors->get_code (this->priv->errors))
    label = interpret_expression (gosubn->label);
  if (! this->priv->errors->get_code (this->priv->errors))
    this->priv->line = find_label (label);
}

/*
 * Interpret a RETURN statement
 */
void interpret_return_statement (void) {

  /* local variables */
  GosubStackNode *gosub_node; /* node popped off the GOSUB stack */

  /* return to the statement following the most recent GOSUB */
  if (this->priv->gosub_stack) {
    this->priv->line = this->priv->gosub_stack->program_line;
    gosub_node = this->priv->gosub_stack;
    this->priv->gosub_stack = this->priv->gosub_stack->next;
    free (gosub_node);
    --this->priv->gosub_stack_size;
  }

  /* no GOSUBs led here, so raise an error */
  else
    this->priv->errors->set_code
      (this->priv->errors, E_RETURN_WITHOUT_GOSUB, 0, this->priv->line->label);
}

/*
 * Interpret a PRINT statement
 * params:
 *   PrintStatementNode*   printn   the PRINT statement details
 */
void interpret_print_statement (PrintStatementNode *printn) {

  /* local variables */
  OutputNode *outn; /* current output node */
  int
    items = 0, /* counter ensures runtime errors appear on a new line */
    result; /* the result of an expression */

  /* print each of the output items */
  outn = printn->first;
  while (outn) {
    switch (outn->class) {
      case OUTPUT_STRING:
        printf ("%s", outn->output.string);
        ++items;
        break;
      case OUTPUT_EXPRESSION:
        result = interpret_expression (outn->output.expression);
        if (! this->priv->errors->get_code (this->priv->errors)) {
          printf ("%d", result);
          ++items;
        }
        break;
    }
    outn = outn->next;
  }

  /* print the linefeed */
  if (items)
    printf ("\n");
  this->priv->line = this->priv->line->next;
}

/*
 * Interpret an INPUT statement
 * params:
 *   InputStatementNode*   inputn   the INPUT statement details
 */
void interpret_input_statement (InputStatementNode *inputn) {

  /* local variables */
  VariableListNode *variable; /* current variable to input */
  int
    value, /* value input from the user */
    sign = 1, /* the default sign */
    ch = 0; /* character from the input stream */

  /* input each of the variables */
  variable = inputn->first;
  while (variable) {
    do {
      if (ch == '-') sign = -1; else sign = 1;
      ch = getchar ();
    } while (ch < '0' || ch > '9');
    value = 0;
    do {
      value = 10 * value + (ch - '0');
      if (value * sign < -32768 || value * sign > 32767)
        this->priv->errors->set_code
          (this->priv->errors, E_OVERFLOW, 0, this->priv->line->label);
      ch = getchar ();
    } while (ch >= '0' && ch <= '9'
      && ! this->priv->errors->get_code (this->priv->errors));
    this->priv->variables [variable->variable - 1] = sign * value;
    variable = variable->next;
  }

  /* advance to the next statement when done */
  this->priv->line = this->priv->line->next;
}


/*
 * Interpret an individual statement
 * params:
 *   StatementNode*   statement   the statement to interpret
 */
void interpret_statement (StatementNode *statement) {

  /* skip comments */
  if (! statement) {
    this->priv->line = this->priv->line->next;
    return;
  }

  /* interpret real statements */
  switch (statement->class) {
    case STATEMENT_NONE:
      break;
    case STATEMENT_LET:
      interpret_let_statement (statement->statement.letn);
      break;
    case STATEMENT_IF:
      interpret_if_statement (statement->statement.ifn);
      break;
    case STATEMENT_GOTO:
      interpret_goto_statement (statement->statement.goton);
      break;
    case STATEMENT_GOSUB:
      interpret_gosub_statement (statement->statement.gosubn);
      break;
    case STATEMENT_RETURN:
      interpret_return_statement ();
      break;
    case STATEMENT_END:
       this->priv->stopped = 1;
     break;
    case STATEMENT_PRINT:
      interpret_print_statement (statement->statement.printn);
      break;
    case STATEMENT_INPUT:
      interpret_input_statement (statement->statement.inputn);
      break;
    default:
      printf ("Statement type %d not implemented.\n", statement->class);
  }
}

/*
 * Interpret program starting from a particular line
 * params:
 *   ProgramLineNode*   program_line   the starting line
 */
static void interpret_program_from (ProgramLineNode *program_line) {
  this->priv->line = program_line;
  while (this->priv->line
    && ! this->priv->stopped
    && ! this->priv->errors->get_code (this->priv->errors))
    interpret_statement (this->priv->line->statement);
}


/*
 * Public Methods
 */


/*
 * Interpret the program from the beginning
 * params:
 *   Interpreter*   interpreter   the interpreter to use
 *   ProgramNode*   program       the program to interpret
 */
static void interpret (Interpreter *interpreter, ProgramNode *program) {
  this = interpreter;
  this->priv->program = program;
  initialise_variables ();
  interpret_program_from (this->priv->program->first);
}

/*
 * Destroy the interpreter
 * params:
 *   Interpreter*   interpreter   the doomed interpreter
 */
static void destroy (Interpreter *interpreter) {
  if (interpreter) {
    if (interpreter->priv)
      free (interpreter->priv);
    free (interpreter);
  }
}


/*
 * Constructors
 */


/*
 * Constructor
 * returns:
 *   Interpreter*   the new interpreter
 */
Interpreter *new_Interpreter (ErrorHandler *errors, LanguageOptions *options) {

  /* allocate memory */
  this = malloc (sizeof (Interpreter));
  this->priv = malloc (sizeof (InterpreterData));

  /* initialise methods */
  this->interpret = interpret;
  this->destroy = destroy;

  /* initialise properties */
  this->priv->gosub_stack = NULL;
  this->priv->gosub_stack_size = 0;
  this->priv->stopped = 0;
  this->priv->errors = errors;
  this->priv->options = options;

  /* return the new object */
  return this;
}