package ui

import (
	"bytes"
	"image/color"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/gdamore/tcell/v2"
	"github.com/rivo/tview"
)

// terminalInfo is what the terminal said about itself at startup.
type terminalInfo struct {
	graphics bool       // the Kitty graphics protocol is supported
	bg       color.RGBA // its background color, if hasBG
	hasBG    bool
}

var (
	terminalOnce   sync.Once
	terminalResult terminalInfo
)

// terminalAnswers asks the terminal once (see probeTerminal) and remembers the
// answers. The first call must happen before the screen is set up, which owns
// the terminal afterwards; New makes it. It is a variable so tests can replace it.
var terminalAnswers = func() terminalInfo {
	terminalOnce.Do(func() { terminalResult = probeTerminal(300 * time.Millisecond) })
	return terminalResult
}

// parseTerminalAnswers reads the answers to probeTerminal's questions.
func parseTerminalAnswers(got []byte) (info terminalInfo) {
	info.graphics = bytes.Contains(got, []byte("_Gi=31;OK"))
	// OSC 11 answer: ESC ] 11 ; rgb:RRRR/GGGG/BBBB, ended by ST or BEL. Each
	// component has 1 to 4 hex digits.
	if i := bytes.Index(got, []byte("\x1b]11;rgb:")); i >= 0 {
		rest := got[i+len("\x1b]11;rgb:"):]
		if end := bytes.IndexAny(rest, "\x1b\a"); end >= 0 {
			rest = rest[:end]
		}
		if parts := strings.Split(string(rest), "/"); len(parts) == 3 {
			var c [3]uint8
			ok := true
			for j, p := range parts {
				v, err := strconv.ParseUint(p, 16, 16)
				if err != nil || len(p) < 1 || len(p) > 4 {
					ok = false
					break
				}
				c[j] = uint8(v * 255 / (1<<(4*len(p)) - 1))
			}
			if ok {
				info.bg, info.hasBG = color.RGBA{c[0], c[1], c[2], 255}, true
			}
		}
	}
	return info
}

// useTerminalBackground makes the terminal's own background color the
// background of every pane, instead of color 0 of the palette ("black"), which
// color themes often set to a grey that differs from their background (and
// on which grey text almost disappears). Only for dark backgrounds: the text
// colors are made for those. Needs true color, else the exact color cannot be
// drawn. Must run before any widget is created, they copy the style. It
// reports whether it changed anything.
func useTerminalBackground(info terminalInfo, env func(string) string) bool {
	if !info.hasBG || !hasTrueColor(env) || luminance(info.bg) > 0.4 {
		return false
	}
	tview.Styles.PrimitiveBackgroundColor = tcell.NewRGBColor(int32(info.bg.R), int32(info.bg.G), int32(info.bg.B))
	return true
}

// hasTrueColor reports whether the terminal says it draws 24-bit colors.
func hasTrueColor(env func(string) string) bool {
	switch strings.ToLower(env("COLORTERM")) {
	case "truecolor", "24bit":
		return true
	}
	return strings.HasSuffix(env("TERM"), "-direct")
}

// luminance is the relative brightness of c, from 0 (black) to 1 (white).
func luminance(c color.RGBA) float64 {
	return (0.2126*float64(c.R) + 0.7152*float64(c.G) + 0.0722*float64(c.B)) / 255
}