Allow --value to pre-populate gum filter (#113)

* Add --value option to filter command

Modeled after fzf's --query option this allows the inital model to be
prefiltered.

Additionally, if the initial list, before being filtered by --value, is
empty then treat this as an error. Filtering an empty list doesn't make
sense.

Co-authored-by: David Morgan <david_a_morgan@icloud.com>

filter/command.go

@@ -1,6 +1,7 @@
 package filter
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"strings"
@@ -9,6 +10,7 @@ import (
 	"github.com/charmbracelet/bubbles/textinput"
 	"github.com/charmbracelet/bubbles/viewport"
 	tea "github.com/charmbracelet/bubbletea"
+	"github.com/sahilm/fuzzy"
 
 	"github.com/charmbracelet/gum/internal/exit"
 	"github.com/charmbracelet/gum/internal/files"
@@ -31,20 +33,35 @@ func (o Options) Run() error {
 
 	var choices []string
 	if input, _ := stdin.Read(); input != "" {
-		choices = strings.Split(strings.TrimSpace(input), "\n")
+		input = strings.TrimSpace(input)
+		if input != "" {
+			choices = strings.Split(input, "\n")
+		}
 	} else {
 		choices = files.List()
 	}
 
+	if len(choices) == 0 {
+		return errors.New("no options provided, see `gum filter --help`")
+	}
+
 	options := []tea.ProgramOption{tea.WithOutput(os.Stderr)}
 	if o.Height == 0 {
 		options = append(options, tea.WithAltScreen())
 	}
 
+	var matches []fuzzy.Match
+	if o.Value != "" {
+		i.SetValue(o.Value)
+		matches = fuzzy.Find(o.Value, choices)
+	} else {
+		matches = matchAll(choices)
+	}
+
 	p := tea.NewProgram(model{
 		choices:        choices,
 		indicator:      o.Indicator,
-		matches:        matchAll(choices),
+		matches:        matches,
 		textinput:      i,
 		viewport:       &v,
 		indicatorStyle: o.IndicatorStyle.ToLipgloss(),

filter/options.go

@@ -13,4 +13,5 @@ type Options struct {
 	PromptStyle    style.Styles `embed:"" prefix:"prompt." set:"defaultForeground=240" envprefix:"GUM_FILTER_PROMPT_"`
 	Width          int          `help:"Input width" default:"20" env:"GUM_FILTER_WIDTH"`
 	Height         int          `help:"Input height" default:"0" env:"GUM_FILTER_HEIGHT"`
+	Value          string       `help:"Initial filter value" default:"" env:"GUM_FILTER_VALUE"`
 }