Measure-VectorSimilarity

1.0.0

Measure-VectorSimilarity.ps1

<#PSScriptInfo
 
.DESCRIPTION Measures the vector / cosine similarity between two sets of items.
.VERSION 1.0.0
.GUID e1f78756-d1bc-47b4-8d88-475ace119a69
.AUTHOR Lee Holmes
 
#>

<#
 
.SYNOPSIS
 
Measures the vector / cosine similarity between two sets of items.
See: https://en.wikipedia.org/wiki/Cosine_similarity
 
.EXAMPLE
 
PS > .\Measure-VectorSimilarity.ps1 @(1..10) @(3..8)
0.775
 
.EXAMPLE
 
PS > $items = dir c:\windows\ | Select -First 10
PS > $items2 = dir c:\windows\ | Select -First 8
PS > .\Measure-VectorSimilarity.ps1 $items $items2 -KeyProperty Name -ValueProperty Length
0.894
 
#>

[CmdletBinding()]
param(
    ## The first set of items to compare
    [Parameter(Position = 0)]
    $Set1,

    ## The second set of items to compare
    [Parameter(Position = 1)]   
    $Set2,
    
    ## If the item sets represent objects that have a main property
    ## (like file names), the name of that key property
    [Parameter()]
    $KeyProperty,

    ## If the item sets represent objects that have a main property
    ## to represent the values (like Count or Percent),
    ## the name of that key property. If they don't have a property
    ## like this, simple existence of the item will be used.
    [Parameter()]
    $ValueProperty
)

## If either set is empty, there is no similarity
if((-not $Set1) -or (-not $Set2))
{
    return 0
}

## Figure out the unique set of items to be compared - either based on
## the key property (if specified), or the item value directly
$allkeys = @($Set1) + @($Set2) | Foreach-Object {
    if($PSBoundParameters.ContainsKey("KeyProperty")) { $_.$KeyProperty}
    else { $_ }
} | Sort-Object -Unique

## Figure out the values of items to be compared - either based on
## the value property (if specified), or the item value directly. Put
## these into a hashtable so that we can process them efficiently.

$set1Hash = @{}
$set2Hash = @{}
$setsToProcess = @($Set1, $Set1Hash), @($Set2, $Set2Hash)

foreach($set in $setsToProcess)
{
    $set[0] | Foreach-Object {
        if($PSBoundParameters.ContainsKey("ValueProperty")) { $value = $_.$ValueProperty }
        else { $value = 1 }
        
        if($PSBoundParameters.ContainsKey("KeyProperty")) { $_ = $_.$KeyProperty }

        $set[1][$_] = $value
    }
}

## Calculate the vector / cosine similarity of the two sets
## based on their keys and values.
$dot = 0
$mag1 = 0
$mag2 = 0

foreach($key in $allkeys)
{
    $dot += $set1Hash[$key] * $set2Hash[$key]
    $mag1 +=  ($set1Hash[$key] * $set1Hash[$key])
    $mag2 +=  ($set2Hash[$key] * $set2Hash[$key])
}

$mag1 = [Math]::Sqrt($mag1)
$mag2 = [Math]::Sqrt($mag2)

## Return the result
[Math]::Round($dot / ($mag1 * $mag2), 3)