<#
.SYNOPSIS
    Shows what each resource group in an Azure subscription has cost over a date range.
.DESCRIPTION
    Calls the Azure Cost Management Query API through Invoke-AzRestMethod and totals
    actual cost by resource group, biggest spender first. Optionally exports to CSV.
.PARAMETER SubscriptionId
    The subscription to report on. Defaults to the subscription in your current Az context.
.PARAMETER Days
    How many days back to look. Default: 30.
.PARAMETER CostColumn
    PreTaxCost for most pay-as-you-go and EA subscriptions; Cost for Microsoft Customer Agreement.
.PARAMETER CsvPath
    Optional path to save the results as a CSV file.
.EXAMPLE
    .\Get-AzCostByResourceGroup.ps1 -Days 7
.EXAMPLE
    .\Get-AzCostByResourceGroup.ps1 -SubscriptionId 00000000-0000-0000-0000-000000000000 -CsvPath .\costs.csv
#>
[CmdletBinding()]
param(
    [string]$SubscriptionId,
    [ValidateRange(1, 365)][int]$Days = 30,
    [ValidateSet('PreTaxCost', 'Cost')][string]$CostColumn = 'PreTaxCost',
    [string]$CsvPath
)

$ErrorActionPreference = 'Stop'

# Sign in only if there's no existing session.
if (-not (Get-AzContext)) { Connect-AzAccount | Out-Null }
if (-not $SubscriptionId) { $SubscriptionId = (Get-AzContext).Subscription.Id }

$to   = (Get-Date).Date
$from = $to.AddDays(-$Days)

# One query: actual cost, summed, grouped by resource group.
$body = @{
    type       = 'ActualCost'
    timeframe  = 'Custom'
    timePeriod = @{ from = $from.ToString('yyyy-MM-dd'); to = $to.ToString('yyyy-MM-dd') }
    dataset    = @{
        granularity = 'None'
        aggregation = @{ totalCost = @{ name = $CostColumn; function = 'Sum' } }
        grouping    = @(@{ type = 'Dimension'; name = 'ResourceGroupName' })
    }
} | ConvertTo-Json -Depth 6

$uri  = "https://management.azure.com/subscriptions/$SubscriptionId/providers/Microsoft.CostManagement/query?api-version=2023-11-01"
$rows = @()

do {
    # Cost Management throttles hard. Back off and retry a few times on HTTP 429.
    for ($attempt = 1; $attempt -le 4; $attempt++) {
        $response = Invoke-AzRestMethod -Uri $uri -Method POST -Payload $body
        if ($response.StatusCode -ne 429) { break }
        Write-Warning "Cost Management is throttling requests. Waiting 30 seconds (attempt $attempt of 4)..."
        Start-Sleep -Seconds 30
    }
    if ($response.StatusCode -ge 400) {
        throw "Cost query failed ($($response.StatusCode)): $($response.Content)"
    }

    $result  = $response.Content | ConvertFrom-Json
    $columns = @($result.properties.columns.name)
    $rows   += $result.properties.rows
    $uri     = $result.properties.nextLink
} while ($uri)

$costIndex  = [array]::IndexOf($columns, $CostColumn)
$groupIndex = [array]::IndexOf($columns, 'ResourceGroupName')
$currIndex  = [array]::IndexOf($columns, 'Currency')

$report = foreach ($row in $rows) {
    [pscustomobject]@{
        ResourceGroup = if ($row[$groupIndex]) { $row[$groupIndex] } else { '(no resource group)' }
        Cost          = [math]::Round([double]$row[$costIndex], 2)
        Currency      = $row[$currIndex]
    }
}
$report = @($report | Sort-Object Cost -Descending)

$total = ($report | Measure-Object -Property Cost -Sum).Sum
Write-Host ("Total for the last {0} days: {1:N2} {2}" -f $Days, $total, $report[0].Currency)

if ($CsvPath) {
    $report | Export-Csv -Path $CsvPath -NoTypeInformation
    Write-Host "Saved to $CsvPath"
}

# Return real objects, so you can pipe them: | Where-Object Cost -gt 100
$report