Predictive Parser

calc.c

// Build with:
//   gcc calc.c -o calc -lm -Wall
//
// Run with:
//   ./calc 'ln((1+2)(3+4))'

#include <ctype.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Grammar:
//
// Expression = Terms { ( '+' | '-' ) Terms }
// Terms      = Primary { '(' Expression ')' }
// Primary    = 'ln' Primary
//            | '(' Expression ')'
//            | CONST

enum NodeType { LN, ADD, SUB, MUL, CONST };

struct AstNode {
  enum NodeType type;
  double value;
  struct AstNode *children[2];
};

double eval(struct AstNode *node) {
  struct AstNode **c = node->children;
  switch (node->type) {
    case LN:    return log(eval(c[0]));
    case ADD:   return eval(c[0]) + eval(c[1]);
    case SUB:   return eval(c[0]) - eval(c[1]);
    case MUL:   return eval(c[0]) * eval(c[1]);
    case CONST: return node->value;
    default:    abort();
  }
}

void freeAst(struct AstNode *node) {
  struct AstNode **c = node->children;
  switch (node->type) {
    case ADD: case SUB: case MUL: freeAst(c[1]);
    case LN:                      freeAst(c[0]);
    default:                      free(node);
  }
}

// Tokens: LN, '(', ')', ADD, SUB, CONST, END
typedef int Token;
const Token END = -1;

Token next(const char **s) {
  while (isspace(**s)) (*s)++; // Skip spaces
  if (**s == 0) return END;    // End of input
  if (isdigit(**s)) {          // Real numbers
    while (isdigit(**s)) (*s)++;
    if (**s == '.')      (*s)++;
    while (isdigit(**s)) (*s)++;
    return CONST;
  }
  // Fixed tokens
  struct { const char *s; int t; } fixed[] = {
    {"ln", LN}, {"(", '('}, {")", ')'},
    {"+", ADD}, {"-", SUB},
  };
  for (int i = 0; i < sizeof(fixed)/sizeof(fixed[0]); i++) {
    if (strncmp(*s, fixed[i].s, strlen(fixed[i].s)) == 0) {
      *s += strlen(fixed[i].s);
      return fixed[i].t;
    }
  }
  fprintf(stderr, "Unexpected '%c'\n", **s);
  exit(1);
}

Token peek(const char **s) {
  while (isspace(**s)) (*s)++; // Skip spaces
  const char *initial = *s;
  Token t = next(s);
  *s = initial; // Rollback
  return t;
}

void expect(const char **s, Token t) {
  if (peek(s) != t) {
    if (**s == 0)
      fprintf(stderr, "Unexpected EOF\n");
    else
      fprintf(stderr, "Unexpected '%c'\n", **s);
    exit(1);
  }
  next(s);
}

struct AstNode* parseExpression(const char **s);

struct AstNode* parsePrimary(const char **s) {
  struct AstNode *n = malloc(sizeof(*n));
  switch (peek(s)) {
    case LN:
      n->type = next(s);
      n->children[0] = parsePrimary(s);
      break;
    case '(':
      expect(s, '(');
      n = parseExpression(s);
      expect(s, ')');
      break;
    case CONST:
      n->value = atof(*s);
      n->type = next(s);
      break;
    default:
      expect(s, CONST); // Exits
  }
  return n;
}

struct AstNode* parseTerms(const char **s) {
  struct AstNode *n = parsePrimary(s);
  while (peek(s) == '(') {
    struct AstNode *rhs = malloc(sizeof(*rhs));
    rhs->children[0] = n;
    rhs->type = MUL;
    expect(s, '(');
    rhs->children[1] = parseExpression(s);
    expect(s, ')');
    n = rhs;
  }
  return n;
}

struct AstNode* parseExpression(const char **s) {
  struct AstNode *n = parseTerms(s);
  while (peek(s) == ADD || peek(s) == SUB) {
    struct AstNode *rhs = malloc(sizeof(*rhs));
    rhs->children[0] = n;
    rhs->type = next(s);
    rhs->children[1] = parseTerms(s);
    n = rhs;
  }
  return n;
}

int main(int argc, const char **argv) {
  if (argc != 2) {
    printf("Usage: %s EXPRESSION", argv[0]);
    exit(1);
  }
  const char **s = &(argv[1]);
  struct AstNode *ast = parseExpression(s);
  expect(s, END);
  printf("%lf\n", eval(ast));
  freeAst(ast);
}

LICENSE

Copyright 2018 Ryan O'Leary

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
of the Software, and to permit persons to whom the Software is furnished to do
so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

Makefile

calc: calc.c
	gcc calc.c -g -o calc -lm -Wall

clean:
	rm -f calc

Predictive Parser

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