Ch10 Extracting Unrlated Subproblems

1. find Closest Location

 

// Return which element of 'array' is closest to the given latitude/longitude.
// Models the Earth as a perfect sphere.
var findClosestLocation = function (lat, lng, array) {
    var closest;
    var closest_dist = Number.MAX_VALUE;

    for (var i = 0; i < array.length; i += 1) {
// Convert both points to radians.
        var lat_rad = radians(lat);
        var lng_rad = radians(lng);
        var lat2_rad = radians(array[i].latitude);
        var lng2_rad = radians(array[i].longitude);

// Use the "Spherical Law of Cosines" formula.
        var dist = Math.acos(Math.sin(lat_rad) * Math.sin(lat2_rad) +
            Math.cos(lat_rad) * Math.cos(lat2_rad) *
                Math.cos(lng2_rad - lng_rad));
        if (dist < closest_dist) {
            closest = array[i];
            closest_dist = dist;
        }
    }

    return closest;
};

 

/*
Most of the code inside the loop is working on an unrelated subproblem: Compute the spherical
distance between two lat/long points. Because there is so much of that code, it makes sense to
extract it into a separate spherical_distance() function:
*/

var spherical_distance = function (lat1, lng1, lat2, lng2) {
    var lat1_rad = radians(lat1);
    var lng1_rad = radians(lng1);
    var lat2_rad = radians(lat2);
    var lng2_rad = radians(lng2);
// Use the "Spherical Law of Cosines" formula.

    return Math.acos(Math.sin(lat1_rad) * Math.sin(lat2_rad) +
        Math.cos(lat1_rad) * Math.cos(lat2_rad) *
            Math.cos(lng2_rad - lng1_rad));
};

 

var findClosestLocation = function (lat, lng, array) {
    var closest;
    var closest_dist = Number.MAX_VALUE;
    for (var i = 0; i < array.length; i += 1) {

        var dist = spherical_distance(lat, lng, array[i].latitude, array[i].longitude);
        if (dist < closest_dist) {
            closest = array[i];
            closest_dist = dist;
        }
    }
    return closest;
};

 

2.Other General-Purpose Code

 

ajax_post({
    url: 'http://example.com/submit',
    data: data,
    on_success: function (response_data) {
        var str = "{\n";
        for (var key in response_data) {
            str += " " + key + " = " + response_data[key] + "\n";
        }
        alert(str + "}");
// Continue handling 'response_data' ...
    }
});

 

/*The high-level goal of this code is, Make an Ajax call to the server, and handle the response. But a
lot of the code is solving the unrelated subproblem, Pretty-print a dictionary. It’s easy to extract
that code into a function like format_pretty(obj):*/

var format_pretty = function (obj) {
    var str = "{\n";
    for (var key in obj) {
        str += " " + key + " = " + obj[key] + "\n";
    }
    return str + "}";
};

 

//adding this functionality is easy. Here’s what the improved code looks like:

var format_pretty_1 = function (obj, indent) {
// Handle null, undefined, strings, and non-objects.
    if (obj === null) return "null";
    if (obj === undefined) return "undefined";
    if (typeof obj === "string") return '"' + obj + '"';
    if (typeof obj !== "object") return String(obj);
    if (indent === undefined) indent = "";
// Handle (non-null) objects.
    var str = "{\n";
    for (var key in obj) {
        str += indent + " " + key + " = ";
        str += format_pretty(obj[key], indent + " ") + "\n";
    }
    return str
}