/* expr.c - evaluate expression
 *
 * Copyright 2013 Daniel Verkamp <daniel@drv.nu>
 *
 * http://pubs.opengroup.org/onlinepubs/9699919799/utilities/expr.html
 *
 * The web standard is incomplete (precedence grouping missing), see:
 * http://permalink.gmane.org/gmane.comp.standards.posix.austin.general/10141

USE_EXPR(NEWTOY(expr, NULL, TOYFLAG_USR|TOYFLAG_BIN))

config EXPR
  bool "expr"
  default n
  help
    usage: expr ARG1 OPERATOR ARG2...

    Evaluate expression and print result. For example, "expr 1 + 2".

    The supported operators are (grouped from highest to lowest priority):

      ( )    :    * / %    + -    != <= < >= > =    &    |

    Each constant and operator must be a separate command line argument.
    All operators are infix, meaning they expect a constant (or expression
    that resolves to a constant) on each side of the operator. Operators of
    the same priority (within each group above) are evaluated left to right.
    Parentheses may be used (as separate arguments) to elevate the priority
    of expressions.

    Calling expr from a command shell requires a lot of \( or '*' escaping
    to avoid interpreting shell control characters.

    The & and | operators are logical (not bitwise) and may operate on
    strings (a blank string is "false"). Comparison operators may also
    operate on strings (alphabetical sort).

    Constants may be strings or integers. Comparison, logical, and regex
    operators may operate on strings (a blank string is "false"), other
    operators require integers.
*/

// TODO: int overflow checking

#define FOR_expr
#include "toys.h"

GLOBALS(
  char* tok; // current token, not on the stack since recursive calls mutate it
)

// Scalar value.
// If s is NULL, the value is an integer (i).
// If s is not NULL, the value is a string (s).
struct value {
  char *s;
  long long i;
};

// check if v is the integer 0 or the empty string
static int is_zero(struct value *v)
{
  return v->s ? !*v->s : !v->i;
}

static char *num_to_str(long long num)
{
  static char num_buf[21];
  snprintf(num_buf, sizeof(num_buf), "%lld", num);
  return num_buf;
}

static int cmp(struct value *lhs, struct value *rhs)
{
  if (lhs->s || rhs->s) {
    // at least one operand is a string
    char *ls = lhs->s ? lhs->s : num_to_str(lhs->i);
    char *rs = rhs->s ? rhs->s : num_to_str(rhs->i);
    return strcmp(ls, rs);
  } else return lhs->i - rhs->i;
}

static void re(struct value *lhs, struct value *rhs)
{
  regex_t rp;
  regmatch_t rm[2];

  xregcomp(&rp, rhs->s, 0);
  // BUG: lhs->s is NULL when it looks like an integer, causing a segfault.
  if (!regexec(&rp, lhs->s, 2, rm, 0) && rm[0].rm_so == 0) {
    if (rp.re_nsub > 0 && rm[1].rm_so >= 0) 
      lhs->s = xmprintf("%.*s", rm[1].rm_eo - rm[1].rm_so, lhs->s+rm[1].rm_so);
    else {
      lhs->i = rm[0].rm_eo;
      lhs->s = 0;
    }
  } else {
    if (!rp.re_nsub) {
      lhs->i = 0;
      lhs->s = 0;
    } else lhs->s = "";
  }
}

static void mod(struct value *lhs, struct value *rhs)
{
  if (lhs->s || rhs->s) error_exit("non-integer argument");
  if (is_zero(rhs)) error_exit("division by zero");
  lhs->i %= rhs->i;
}

static void divi(struct value *lhs, struct value *rhs)
{
  if (lhs->s || rhs->s) error_exit("non-integer argument");
  if (is_zero(rhs)) error_exit("division by zero");
  lhs->i /= rhs->i;
}

static void mul(struct value *lhs, struct value *rhs)
{
  if (lhs->s || rhs->s) error_exit("non-integer argument");
  lhs->i *= rhs->i;
}

static void sub(struct value *lhs, struct value *rhs)
{
  if (lhs->s || rhs->s) error_exit("non-integer argument");
  lhs->i -= rhs->i;
}

static void add(struct value *lhs, struct value *rhs)
{
  if (lhs->s || rhs->s) error_exit("non-integer argument");
  lhs->i += rhs->i;
}

static void ne(struct value *lhs, struct value *rhs)
{
  lhs->i = cmp(lhs, rhs) != 0;
  lhs->s = NULL;
}

static void lte(struct value *lhs, struct value *rhs)
{
  lhs->i = cmp(lhs, rhs) <= 0;
  lhs->s = NULL;
}

static void lt(struct value *lhs, struct value *rhs)
{
  lhs->i = cmp(lhs, rhs) < 0;
  lhs->s = NULL;
}

static void gte(struct value *lhs, struct value *rhs)
{
  lhs->i = cmp(lhs, rhs) >= 0;
  lhs->s = NULL;
}

static void gt(struct value *lhs, struct value *rhs)
{
  lhs->i = cmp(lhs, rhs) > 0;
  lhs->s = NULL;
}

static void eq(struct value *lhs, struct value *rhs)
{
  lhs->i = !cmp(lhs, rhs);
  lhs->s = NULL;
}

static void and(struct value *lhs, struct value *rhs)
{
  if (is_zero(lhs) || is_zero(rhs)) {
    lhs->i = 0;
    lhs->s = NULL;
  }
}

static void or(struct value *lhs, struct value *rhs)
{
  if (is_zero(lhs)) *lhs = *rhs;
}

// Converts an arg string to a value struct.  Assumes arg != NULL.
static void parse_value(char* arg, struct value *v)
{
  char *endp;
  v->i = strtoll(arg, &endp, 10);
  // if non-NULL, there's still stuff left, and it's a string.  Otherwise no
  // string.
  v->s = *endp ? arg : NULL;
}

void syntax_error(char *msg, ...) {
  if (0) { // detailed message for debugging.  TODO: add CFG_ var to enable
    va_list va;
    va_start(va, msg);
    verror_msg(msg, 0, va);
    va_end(va);
    xexit();
  } else
    error_exit("syntax error");
}

// operators grouped by precedence
static struct op {
  char *tok;
  char prec;
  // calculate "lhs op rhs" (e.g. lhs + rhs) and store result in lhs
  void (*calc)(struct value *lhs, struct value *rhs);
} OPS[] = {
  {"|", 1, or  },
  {"&", 2, and },
  {"=", 3, eq  }, {"==", 3, eq  }, {">",  3, gt  }, {">=", 3, gte },
  {"<", 3, lt  }, {"<=", 3, lte }, {"!=", 3, ne  },
  {"+", 4, add }, {"-",  4, sub },
  {"*", 5, mul }, {"/",  5, divi }, {"%", 5, mod },
  {":", 6, re  },
  {"",  0, NULL}, // sentinel
};

// Point TT.tok at the next token.  It's NULL when there are no more tokens.
void advance() {
  TT.tok = *toys.optargs++;
}

// Evalute a compound expression, setting 'ret'.
//
// This function uses the recursive "Precedence Climbing" algorithm:
//
// Clarke, Keith. "The top-down parsing of expressions." University of London.
// Queen Mary College. Department of Computer Science and Statistics, 1986.
//
// http://www.antlr.org/papers/Clarke-expr-parsing-1986.pdf
//
// Nice explanation and Python implementation:
// http://eli.thegreenplace.net/2012/08/02/parsing-expressions-by-precedence-climbing
static void eval_expr(struct value *ret, int min_prec)
{
  if (!TT.tok) syntax_error("Unexpected end of input");

  // Evaluate LHS atom, setting 'ret'.
  if (!strcmp(TT.tok, "(")) { // parenthesized expression
    advance(); // consume (
    eval_expr(ret, 1); // We're inside ( ), so start with min_prec = 1
    if (!TT.tok)             syntax_error("Expected )");
    if (strcmp(TT.tok, ")")) syntax_error("Expected ) but got %s", TT.tok);
    advance(); // consume )
  } else { // simple literal
    parse_value(TT.tok, ret);
    advance();
  }

  // Evaluate RHS and apply operator until precedence is too low.
  struct value rhs;
  while (TT.tok) {
    struct op *o = OPS;
    while (o->calc) {  // Look up the precedence of operator TT.tok
      if (!strcmp(TT.tok, o->tok)) break;
      o++;
    }
    if (!o->calc) break; // Not an operator (extra input will fail later)
    if (o->prec < min_prec) break; // Precedence too low, pop a stack frame
    advance();

    eval_expr(&rhs, o->prec + 1); // Evaluate RHS, with higher min precedence
    o->calc(ret, &rhs); // Apply operator, setting 'ret'.
  }
}

void expr_main(void)
{
  struct value ret = {0};
  toys.exitval = 2; // if exiting early, indicate invalid expression

  advance(); // initialize global token
  eval_expr(&ret, 1);

  if (TT.tok) syntax_error("Unexpected extra input '%s'\n", TT.tok);

  if (ret.s) printf("%s\n", ret.s);
  else printf("%lld\n", ret.i);

  exit(is_zero(&ret));
}