<#
.SYNOPSIS
    Lists Explorer preview handlers and maps file extensions to them.
.DESCRIPTION
    Preview handlers are COM objects registered under
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\PreviewHandlers. Explorer finds the one for a
    file type through the shellex key {8895b1c6-b41f-4c1c-a562-0d564250836f} under the extension
    (or its ProgID). This script shows what's installed, shows which handler an extension uses,
    and points an extension at a handler you choose, or removes that mapping. Supports -WhatIf.
.PARAMETER ListHandlers
    List every registered preview handler with its CLSID.
.PARAMETER Extension
    One or more extensions, like .log or .ps1. On its own, shows the current mapping.
.PARAMETER Handler
    The handler to use: a CLSID, or part of its name as shown by -ListHandlers.
.PARAMETER Remove
    Delete the mapping for the extension at the chosen scope.
.PARAMETER Scope
    CurrentUser (HKCU, no admin needed) or LocalMachine (HKLM, needs admin). Default CurrentUser.
.PARAMETER Force
    Replace an existing mapping that points at a different handler.
.EXAMPLE
    .\Set-PreviewHandler.ps1 -ListHandlers
.EXAMPLE
    .\Set-PreviewHandler.ps1 -Extension .log, .ps1, .json -Handler '{1531d583-8375-4d3f-b5fb-d23bbd169f22}' -WhatIf
#>
[CmdletBinding(SupportsShouldProcess, DefaultParameterSetName = 'Show')]
param(
    [Parameter(Mandatory, ParameterSetName = 'List')][switch]$ListHandlers,
    [Parameter(Mandatory, ParameterSetName = 'Show')]
    [Parameter(Mandatory, ParameterSetName = 'Set')]
    [Parameter(Mandatory, ParameterSetName = 'Remove')]
    [ValidatePattern('^\.[\w\-\.]+$')][string[]]$Extension,
    [Parameter(Mandatory, ParameterSetName = 'Set')][string]$Handler,
    [Parameter(Mandatory, ParameterSetName = 'Remove')][switch]$Remove,
    [Parameter(ParameterSetName = 'Set')]
    [Parameter(ParameterSetName = 'Remove')]
    [ValidateSet('CurrentUser', 'LocalMachine')][string]$Scope = 'CurrentUser',
    [Parameter(ParameterSetName = 'Set')][switch]$Force
)

$previewGuid = '{8895b1c6-b41f-4c1c-a562-0d564250836f}'
$hkcr = 'Registry::HKEY_CLASSES_ROOT'

function Get-DefaultValue([string]$Key) {
    if (Test-Path -LiteralPath $Key) { (Get-ItemProperty -LiteralPath $Key -ErrorAction SilentlyContinue).'(default)' }
}

# Every registered handler: value name is the CLSID, value data is the friendly name.
$handlers = foreach ($root in 'HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\PreviewHandlers', 'HKLM:\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\PreviewHandlers') {
    if (-not (Test-Path -LiteralPath $root)) { continue }
    $props = Get-ItemProperty -LiteralPath $root
    foreach ($p in $props.PSObject.Properties | Where-Object Name -match '^\{[0-9a-fA-F\-]{36}\}$') {
        [pscustomobject]@{ Name = $p.Value; Clsid = $p.Name.ToLower() }
    }
}
$handlers = @($handlers | Sort-Object Clsid -Unique | Sort-Object Name)

if ($ListHandlers) { return $handlers }

foreach ($ext in $Extension.ToLower()) {
    $userKey = "HKCU:\Software\Classes\$ext\shellex\$previewGuid"
    $key = if ($Scope -eq 'LocalMachine') { "HKLM:\Software\Classes\$ext\shellex\$previewGuid" } else { $userKey }

    if ($PSCmdlet.ParameterSetName -eq 'Show') {
        # Look where handlers usually get registered: the extension, its ProgID, and SystemFileAssociations.
        $progId = Get-DefaultValue "$hkcr\$ext"
        $found = $null
        foreach ($where in @("$hkcr\$ext", $(if ($progId) { "$hkcr\$progId" }), "$hkcr\SystemFileAssociations\$ext")) {
            if (-not $where) { continue }
            $clsid = Get-DefaultValue "$where\shellex\$previewGuid"
            if ($clsid) { $found = [pscustomobject]@{ Extension = $ext; Handler = ($handlers | Where-Object Clsid -eq $clsid.ToLower()).Name; Clsid = $clsid; FoundAt = $where -replace '^Registry::', '' }; break }
        }
        if (-not $found) { $found = [pscustomobject]@{ Extension = $ext; Handler = '(none)'; Clsid = $null; FoundAt = $null } }
        $found
        continue
    }

    try {
        if ($Remove) {
            if (-not (Test-Path -LiteralPath $key)) { Write-Verbose "$ext has no mapping at $Scope scope."; continue }
            if ($PSCmdlet.ShouldProcess("$ext ($Scope)", 'Remove preview handler mapping')) {
                Remove-Item -LiteralPath $key -Recurse -ErrorAction Stop
                [pscustomobject]@{ Extension = $ext; Scope = $Scope; Status = 'Removed'; Clsid = $null }
            }
            continue
        }

        $target = if ($Handler -match '^\{?[0-9a-fA-F]{8}(-[0-9a-fA-F]{4}){3}-[0-9a-fA-F]{12}\}?$') {
            '{' + $Handler.Trim('{}').ToLower() + '}'
        }
        else {
            $hits = @($handlers | Where-Object Name -like "*$Handler*")
            if ($hits.Count -ne 1) { throw "'$Handler' matches $($hits.Count) handlers. Use -ListHandlers and pass the CLSID." }
            $hits[0].Clsid
        }
        if ($handlers.Clsid -notcontains $target -and -not $Force) { throw "$target isn't a registered preview handler. Install the handler first, or use -Force to map it anyway." }

        $current = Get-DefaultValue $key
        if ($current -and $current -ne $target -and -not $Force) {
            [pscustomobject]@{ Extension = $ext; Scope = $Scope; Status = "SkippedExisting ($current), use -Force"; Clsid = $current }
            continue
        }
        if ($PSCmdlet.ShouldProcess("$ext ($Scope)", "Set preview handler to $target")) {
            if (-not (Test-Path -LiteralPath $key)) { $null = New-Item -Path $key -Force -ErrorAction Stop }
            Set-ItemProperty -LiteralPath $key -Name '(default)' -Value $target -ErrorAction Stop
            [pscustomobject]@{ Extension = $ext; Scope = $Scope; Status = 'Set'; Clsid = $target }
        }
    }
    catch {
        Write-Warning "${ext}: $($_.Exception.Message)"
    }
}