using namespace System.Management.Automation
using namespace System.Management.Automation.Language

Register-ArgumentCompleter -Native -CommandName 'fmerge' -ScriptBlock {
    param($wordToComplete, $commandAst, $cursorPosition)

    $commandElements = $commandAst.CommandElements
    $command = @(
        'fmerge'
        for ($i = 1; $i -lt $commandElements.Count; $i++) {
            $element = $commandElements[$i]
            if ($element -isnot [StringConstantExpressionAst] -or
                $element.StringConstantType -ne [StringConstantType]::BareWord -or
                $element.Value.StartsWith('-') -or
                $element.Value -eq $wordToComplete) {
                break
        }
        $element.Value
    }) -join ';'

    $completions = @(switch ($command) {
        'fmerge' {
            [CompletionResult]::new('-e', '-e', [CompletionResultType]::ParameterName, 'enables experimental features')
            [CompletionResult]::new('--experimental', '--experimental', [CompletionResultType]::ParameterName, 'enables experimental features')
            [CompletionResult]::new('-h', '-h', [CompletionResultType]::ParameterName, 'Print help')
            [CompletionResult]::new('--help', '--help', [CompletionResultType]::ParameterName, 'Print help')
            [CompletionResult]::new('-V', '-V ', [CompletionResultType]::ParameterName, 'Print version')
            [CompletionResult]::new('--version', '--version', [CompletionResultType]::ParameterName, 'Print version')
            [CompletionResult]::new('man', 'man', [CompletionResultType]::ParameterValue, 'Renders the manual.')
            [CompletionResult]::new('autocomplete', 'autocomplete', [CompletionResultType]::ParameterValue, 'Renders shell completion scripts.')
            [CompletionResult]::new('merge', 'merge', [CompletionResultType]::ParameterValue, 'Merge.')
            [CompletionResult]::new('help', 'help', [CompletionResultType]::ParameterValue, 'Print this message or the help of the given subcommand(s)')
            break
        }
        'fmerge;man' {
            [CompletionResult]::new('-o', '-o', [CompletionResultType]::ParameterName, 'o')
            [CompletionResult]::new('--out', '--out', [CompletionResultType]::ParameterName, 'out')
            [CompletionResult]::new('-f', '-f', [CompletionResultType]::ParameterName, 'f')
            [CompletionResult]::new('--format', '--format', [CompletionResultType]::ParameterName, 'format')
            [CompletionResult]::new('-h', '-h', [CompletionResultType]::ParameterName, 'Print help')
            [CompletionResult]::new('--help', '--help', [CompletionResultType]::ParameterName, 'Print help')
            [CompletionResult]::new('-V', '-V ', [CompletionResultType]::ParameterName, 'Print version')
            [CompletionResult]::new('--version', '--version', [CompletionResultType]::ParameterName, 'Print version')
            break
        }
        'fmerge;autocomplete' {
            [CompletionResult]::new('-o', '-o', [CompletionResultType]::ParameterName, 'o')
            [CompletionResult]::new('--out', '--out', [CompletionResultType]::ParameterName, 'out')
            [CompletionResult]::new('-s', '-s', [CompletionResultType]::ParameterName, 's')
            [CompletionResult]::new('--shell', '--shell', [CompletionResultType]::ParameterName, 'shell')
            [CompletionResult]::new('-h', '-h', [CompletionResultType]::ParameterName, 'Print help')
            [CompletionResult]::new('--help', '--help', [CompletionResultType]::ParameterName, 'Print help')
            [CompletionResult]::new('-V', '-V ', [CompletionResultType]::ParameterName, 'Print version')
            [CompletionResult]::new('--version', '--version', [CompletionResultType]::ParameterName, 'Print version')
            break
        }
        'fmerge;merge' {
            [CompletionResult]::new('-f', '-f', [CompletionResultType]::ParameterName, 'f')
            [CompletionResult]::new('--file', '--file', [CompletionResultType]::ParameterName, 'file')
            [CompletionResult]::new('-r', '-r', [CompletionResultType]::ParameterName, 'r')
            [CompletionResult]::new('--regex', '--regex', [CompletionResultType]::ParameterName, 'regex')
            [CompletionResult]::new('-h', '-h', [CompletionResultType]::ParameterName, 'Print help')
            [CompletionResult]::new('--help', '--help', [CompletionResultType]::ParameterName, 'Print help')
            [CompletionResult]::new('-V', '-V ', [CompletionResultType]::ParameterName, 'Print version')
            [CompletionResult]::new('--version', '--version', [CompletionResultType]::ParameterName, 'Print version')
            break
        }
        'fmerge;help' {
            [CompletionResult]::new('man', 'man', [CompletionResultType]::ParameterValue, 'Renders the manual.')
            [CompletionResult]::new('autocomplete', 'autocomplete', [CompletionResultType]::ParameterValue, 'Renders shell completion scripts.')
            [CompletionResult]::new('merge', 'merge', [CompletionResultType]::ParameterValue, 'Merge.')
            [CompletionResult]::new('help', 'help', [CompletionResultType]::ParameterValue, 'Print this message or the help of the given subcommand(s)')
            break
        }
        'fmerge;help;man' {
            break
        }
        'fmerge;help;autocomplete' {
            break
        }
        'fmerge;help;merge' {
            break
        }
        'fmerge;help;help' {
            break
        }
    })

    $completions.Where{ $_.CompletionText -like "$wordToComplete*" } |
        Sort-Object -Property ListItemText
}