math-codegen

math-codegen

Generates JavaScript code from mathematical expressions

Table of Contents generated with DocToc

• Description
  • Lifecycle
    • Parse
    • Compile
    • Eval
  • Differences with math.js expression parser
• Install
• Usage
• API
  • var instance = new CodeGenerator([options])
  • instance.parse(code)
  • instance.compile(namespace)
  • instance.setDefs(defs)
• Examples
  • Native math
  • imaginary
  • interval arithmetic
• Inspiration projects
• License

Description

A flexible interpreter for mathematical expressions which allows the programmer to change the usual semantic of an operator bringing the operator overloading polymorphism to JavaScript (emulated with function calls), in addition an expression can be evaluated under any adapted namespace providing expression portability between numeric libraries

Lifecycle

• parse a mathematical expression is parsed with Esprima
• compile the parsed string is compiled against a namespace producing executable JavaScript code
• eval the executable JavaScript code is evaluated against a context

Parse

For example let's consider the following expression with the variable x which is user defined:

'1 + 2 * x'

The expression can be emulated with function calls instead of operators (the parser identifies operators as Binary Expressions)

'add(1, multiply(2, x))'

now we can introduce the namespace ns where add and multiply come from

'ns.add(1, ns.multiply(2, x))'

the variables (which for the parser are Identifiers) come from a context called scope but they might also be constant values defined in the namespace:

'ns.add(1, ns.multiply(2, (scope["x"] || ns["x"]) ))'

the constant values might have different meanings for different namespaces therefore a factory is needed on the namespace to transform these values into values the namespace can operate with

'ns.add(ns.factory(1), ns.multiply(ns.factory(2), (scope["x"] || ns["x"]) ))'

Compile

Now that we have a parsed expression we have to compile it against a namespace to produce executable JavaScript code

parse('1 + 2 * x').compile(namespace)

// returns something like this
(function (ns) {
  return {
    eval: function (scope) {
      // scope processing
      // ...
      // the string parsed above goes here
    }
  }
})(namespace)

Eval

The object returned above can be evaluated within a context

parse('1 + 2 * x').compile(namespace).eval(scope)

Differences with math.js expression parser

Math.js expression parser API is quite similar having the same lifecycle however there are some important facts I've found:

• math.js has a custom expression parser, math-codegen uses Esprima which support the ES5 grammar (ESTree AST nodes)
• math.js v1.x arrays can represent matrices with ns.matrix or as a raw arrays, math-codegen doesn't make any assumptions of arrays and treats them just like any other literal allowing the namespace to decide what to do with an array in its factory method

Install

$ npm install --save math-codegen

Usage

var CodeGenerator = require('math-codegen');
new CodeGenerator([options]).parse(code).compile(namespace).eval(scope)

API

var instance = new CodeGenerator([options])

properties

• statements {Array} An array of statements parsed from an expression
• interpreter {Interpreter} Instance of the Interpreter class
• defs {Object} An object with additional definitions available during the compilation that exist during the instance lifespan

params

• options {Object} Options available for the interpreter
  • [options.factory="ns.factory"] {string} factory method under the namespace
  • [options.raw=false] {boolean} True to interpret BinaryExpression, UnaryExpression and ArrayExpression in a raw way without wrapping the operators with the factory method
  • [options.rawArrayExpressionElements=true] {boolean} true to interpret the array elements in a raw way
  • [options.rawCallExpressionElements=false] {boolean} true to interpret call expression elements in a raw way

instance.parse(code)

chainable params

• code {string} string to be parsed

Parses a program using Esprima, each Expression Statement is saved in instance.statements

Node types implemented:

• Nodes:
  • ExpressionStatement
• Expressions:
  • ArrayExpression
  • UnaryExpression available operators emulated with function calls can be found here
  • BinaryExpression available operators emulated with function calls can be found here
  • AssignmentExpression
  • ConditionalExpression
  • CallExpression
• Misc:
  • Identifiers, identifier resolution follows this order:
    • namespace
    • scope
    • definitions stored in instance.defs
  • Literals

instance.compile(namespace)

chainable params

• namespace {Object}

Compiles the code making namespace's properties available during evaluation

returns {Object}

• return.eval {Function} Function to be evaluated under a context params
  • scope {Object}

instance.setDefs(defs)

params

• defs {Object}

An object whose properties will be available during evaluation, properties can be accessed by the property name in the program

Examples

Native math

var numeric = {
  factory: function (a) { return a; },
  add: function (a, b) { return a + b; },
  mul: function (a, b) { return a * b; }
};

var instance = new CodeGenerator()
  .parse('1 + 2 * x')
  .compile(numeric);

console.log(instance.eval({x : 3}));     // 1 + 2 * 3 = 7

imaginary

var imaginary = {
  factory: function (a) {
    // a = [re, im]
    if (typeof a === 'number') {
      return [a, 0];
    }
    return [a[0] || 0, a[1] || 0];
  },
  add: function (a, b) {
    var re = a[0] + b[0];
    var im = a[1] + b[1];
    return [re, im];
  },
  mul: function (a, b) {
    var re = a[0] * b[0] - a[1] * b[1];
    var im = a[0] * b[1] + a[1] * b[0];
    return [re, im];
  }
};

var instance = new CodeGenerator()
  .parse('1 + 2 * x')
  .compile(imaginary);

// [1, 0] + [2, 0] * [1, 1]
// [1, 0] + [2, 2]
// [3, 2]
console.log(instance.eval({x : [1, 1]}));

interval arithmetic

var interval = {
  factory: function (a) {
    // a = [lo, hi]
    if (typeof a === 'number') {
      return [a, a];
    }
    return [a[0], a[1]];
  },
  add: function (x, y) {
    return [x[0] + y[0], x[1] + y[1]];
  },
  mul: function (x, y) {
    var ac = x[0] * y[0];
    var ad = x[0] * y[1];
    var bc = x[1] * y[0];
    var bd = x[1] * y[1];
    return [Math.min(ac, ad, bc, bd), Math.max(ac, ad, bc, bd)];
  }
};

var instance = new CodeGenerator()
  .parse('1 + 2 * x')
  .compile(interval);

// [1, 1] + [2, 2] * [-1, 2]
// [1, 1] + [-2, 4]
// [-1, 5]
console.log(instance.eval({x : [-1, 2]}));

Inspiration projects

• math.js expression parser

License

2015 MIT © Mauricio Poppe