package app

import (
	"fmt"
	"os"
	"strings"
	"time"
	"unicode/utf8"
)

// Two-row tab bar: labels row, underline row. The PTY viewport's top
// row is therefore mainTop == tabBarRows + 1.
const tabBarRows = 2

// drawTabBar renders the top tab strip across the full host width.
// Tabs share the available width with a flex layout — each visible
// session gets roughly width/N cells, with the remainder distributed
// to the leftmost tabs so the strip fills the screen edge-to-edge.
// A trailing "+ new" hint sits in the rightmost reserved slot.
func (st *uiState) drawTabBar() {
	var entry time.Time
	if st.metrics != nil {
		entry = time.Now()
	}
	st.mu.Lock()
	palOpen := st.palette != nil
	focus := st.focusedID
	st.mu.Unlock()
	if palOpen {
		return
	}
	layout := st.layoutSnapshot()
	width := int(layout.childCols())
	if width < 8 {
		return
	}

	// Tabs list top-level agent sessions only. Command/terminal
	// processes live in the Processes sidebar section and never own a
	// tab — they're global to the session, not per-tab.
	var sessions []*Child
	for _, c := range st.sess.Children() {
		if c.Kind != KindAgent {
			continue
		}
		if c.ParentID == "" && c.Status() == StatusRunning {
			sessions = append(sessions, c)
		}
	}

	const (
		newHint     = "+ new"
		minTabWidth = 6 // enough for two pad cells + "x…" or similar
	)
	newHintW := utf8.RuneCountInString(newHint) + 2 // " + new " framing

	type tabRect struct {
		startCol int
		width    int
		label    string
		active   bool
	}

	// Reserve space at the right edge for "+ new". If there are too
	// many tabs to fit even at minTabWidth, drop tabs from the right
	// until they do. The current focus stays visible.
	tabBudget := width - newHintW
	if tabBudget < minTabWidth {
		tabBudget = width
		newHintW = 0
	}

	visible := sessions
	if len(visible) > 0 {
		maxTabs := tabBudget / minTabWidth
		if maxTabs < 1 {
			maxTabs = 1
		}
		if len(visible) > maxTabs {
			// Keep the focused tab plus as many leftward tabs as fit.
			focusIdx := -1
			for i, c := range visible {
				if c.ID == focus {
					focusIdx = i
					break
				}
			}
			if focusIdx < 0 {
				focusIdx = 0
			}
			start := focusIdx - maxTabs + 1
			if start < 0 {
				start = 0
			}
			end := start + maxTabs
			if end > len(visible) {
				end = len(visible)
			}
			visible = visible[start:end]
		}
	}

	tabs := make([]tabRect, 0, len(visible))
	if n := len(visible); n > 0 {
		base := tabBudget / n
		extra := tabBudget - base*n
		col := 1
		for i, c := range visible {
			w := base
			if i < extra {
				w++
			}
			label := c.DisplayName()
			labelW := utf8.RuneCountInString(label)
			maxLabelW := w - 2 // one pad on each side
			if maxLabelW < 1 {
				maxLabelW = 1
			}
			if labelW > maxLabelW {
				if maxLabelW > 1 {
					label = clipRunes(label, maxLabelW-1) + "…"
				} else {
					label = clipRunes(label, maxLabelW)
				}
				labelW = utf8.RuneCountInString(label)
			}
			tabs = append(tabs, tabRect{
				startCol: col,
				width:    w,
				label:    label,
				active:   c.ID == focus,
			})
			col += w
		}
	}

	var b strings.Builder
	// Clear both rows so a stale label from the previous frame can't
	// bleed through. Use ECH clamped to `width` (= childCols) instead of
	// `\x1b[2K`: 2K wipes the entire line including the sidebar columns,
	// and if drawSidebar's chrome cache is fresh it won't repaint to
	// fill them back in — the user sees a gap where the sidebar border
	// and content should be.
	fmt.Fprintf(&b, "\x1b[1;1H\x1b[%dX", width)
	fmt.Fprintf(&b, "\x1b[2;1H\x1b[%dX", width)

	for _, t := range tabs {
		// Row 1: centre-ish label inside the tab cell.
		labelW := utf8.RuneCountInString(t.label)
		leftPad := (t.width - labelW) / 2
		if leftPad < 1 {
			leftPad = 1
		}
		rightPad := t.width - labelW - leftPad
		if rightPad < 0 {
			rightPad = 0
		}
		fmt.Fprintf(&b, "\x1b[1;%dH", t.startCol)
		if t.active {
			b.WriteString(styleActive)
		} else {
			b.WriteString(styleHint)
		}
		b.WriteString(strings.Repeat(" ", leftPad))
		b.WriteString(t.label)
		b.WriteString(strings.Repeat(" ", rightPad))
		b.WriteString(styleReset)

		// Row 2: underline. Thick accent for the active tab, faint
		// border for the rest.
		fmt.Fprintf(&b, "\x1b[2;%dH", t.startCol)
		if t.active {
			b.WriteString(styleAccent)
			b.WriteString(strings.Repeat("━", t.width))
		} else {
			b.WriteString(styleBorder)
			b.WriteString(strings.Repeat("─", t.width))
		}
		b.WriteString(styleReset)
	}

	// "+ new" hint right-aligned in the reserved slot.
	if newHintW > 0 {
		hintCol := width - newHintW + 1
		fmt.Fprintf(&b, "\x1b[1;%dH %s%s%s ", hintCol, styleDim, newHint, styleReset)
		// Underline continues faintly under the hint so the strip
		// reads as one bar.
		fmt.Fprintf(&b, "\x1b[2;%dH%s%s%s",
			hintCol, styleBorder, strings.Repeat("─", newHintW), styleReset)
	}

	frame := b.String()
	st.chromeCacheMu.Lock()
	if frame == st.tabBarCache {
		st.chromeCacheMu.Unlock()
		if st.metrics != nil {
			st.metrics.recordTabbar(time.Since(entry), true)
		}
		return
	}
	st.tabBarCache = frame
	st.chromeCacheMu.Unlock()
	if st.metrics != nil {
		defer func() { st.metrics.recordTabbar(time.Since(entry), false) }()
	}

	st.outMu.Lock()
	defer st.outMu.Unlock()
	fmt.Fprintf(os.Stdout, "\x1b7%s\x1b8", frame)
}