在列表中相互比较项目

Question

I have a List<DiscountRule> and I need to iterate over these checking to see if any of the rules clash with each other. There are some properties on the rule such as BrandID and CategoryID which indicate which brand and category the rule applies to along with Include/Exclude flags.

A rule would clash if both category and brand match an existing rule if the following example condition occurs:

Rule 1

Include brand Include category

Rule 2

Exclude brand Exclude category

Obviously a clash can occur if a rule is Include Exclude and Exclude Include and vice versa which I also check for.

I need a way of looping over these and flagging those that are clashing within the list. Here's what I currently have:

public bool CheckDiscountRuleClash(DiscountRule other)
{
    if (this.CategoryId == other.CategoryId && this.BrandId == other.BrandId)
    {
        if (this.BrandInclude && this.CategoryInclude
         && !other.BrandInclude && !other.CategoryInclude)
        {
            this.RuleClashes = true;
            return true;
        }
        else if (!this.BrandInclude && !this.CategoryInclude
            && other.BrandInclude && other.CategoryInclude)
        {
            this.RuleClashes = true;
            return true;
        }
        else if (this.BrandInclude && !this.CategoryInclude
            && !other.BrandInclude && other.CategoryInclude)
        {
            this.RuleClashes = true;
            return true;
        }
        else if (!this.BrandInclude && this.CategoryInclude
                && other.BrandInclude && !other.CategoryInclude)
        {
            this.RuleClashes = true;
            return true;
        }
        else
        {
            this.RuleClashes = false;
            return false;
        }
    }

    return false;
}

Which I'm calling like:

rules.Where(i1 => rules.Any(i2 => i1.CheckDiscountRuleClash(i2))).ToList();

But this obviously only gives me back those which satisify the Any() condition. Is there a way to do what I want?

Answer 1

To get back all of the pairs that match your condition simply project each sequence into it's matches:

var conflictingRules = rules.SelectMany(rule => 
    rules.Where(other => rule.CheckDiscountRuleClash(other))
    .Select(other=> new { rule, other }));

Answer 2

How about using a foreach loop instead to make sure you go through every item in the list

foreach (var rule in rules) // where rules is your List<DiscountRule>
            {
                var result = CheckDiscountRuleClash(rule);
                // do something with the result here (boolean)
                ...
            }

Answer 3

You can do it with Linq join in linear time:

var conflictingRules =
            from rule in rules
            join other in rules
            on new{rule.BrandId, rule.CategoryId, rule.BrandInclude, rule.CategoryInclude} equals new{other.BrandId, other.CategoryId, BrandInclude = !other.BrandInclude, CategoryInclude = !other.CategoryInclude}
            select new{rule, other};

The idea is to join the list with the the 'counterparts' - the ones known to conflict the other. Same approach also allows you to use the algorithm with an SQL database if you want.

Note that this algorithm works in linear time, an outperforms the ones iterating all pairs (for example with SelectMany, which is O(N^2)) that the other answers demonstrate, by orders of magnitude, starting from the list of few doesens of brands an categories.

Also note that this code does not use the original CheckDiscountRuleClash code, so it it's smaller by 30 lines of code . You can check out the live code with performance comparision here: https://dotnetfiddle.net/ZtRfcz

Note that if you increase the numbers a bit, the slow method would leed to execution timeout at 5 seconds on dotnetfiddle. Take computation complexity serious!