// Package sparkline renders inline SVG sparklines and trend
// charts for the dashboard and host repo page. All output is
// pure server-rendered SVG with no JavaScript, no external
// stylesheet, and no client library.
package sparkline

import (
	"fmt"
	"html/template"
	"math"
	"strings"
)

// RenderSparkline returns an inline SVG <svg> element of the
// given size containing a single polyline normalised across the
// full y-range of points. NaN entries break the polyline. With
// fewer than two real points the SVG still renders but contains
// only a faint baseline + an em-dash placeholder.
func RenderSparkline(points []float64, width, height int) template.HTML {
	const pad = 2
	w := width
	h := height
	innerW := w - 2*pad
	innerH := h - 2*pad

	var real []float64
	for _, p := range points {
		if !math.IsNaN(p) {
			real = append(real, p)
		}
	}

	var b strings.Builder
	fmt.Fprintf(&b,
		`<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 %d %d" class="repo-sparkline" role="img" aria-label="repo size trend">`,
		w, h)

	if len(real) < 2 {
		fmt.Fprintf(&b,
			`<line x1="%d" y1="%d" x2="%d" y2="%d" stroke="currentColor" stroke-opacity="0.15" stroke-dasharray="2,2"/>`,
			pad, h/2, w-pad, h/2)
		fmt.Fprintf(&b,
			`<text x="%d" y="%d" text-anchor="middle" font-size="%d" fill="currentColor" fill-opacity="0.4">—</text>`,
			w/2, h/2+4, h-6)
		b.WriteString(`</svg>`)
		return template.HTML(b.String())
	}

	min, max := real[0], real[0]
	for _, v := range real {
		if v < min {
			min = v
		}
		if v > max {
			max = v
		}
	}
	span := max - min
	if span == 0 {
		span = 1
	}

	stepX := 0.0
	if len(points) > 1 {
		stepX = float64(innerW) / float64(len(points)-1)
	}

	var seg strings.Builder
	flush := func() {
		if seg.Len() == 0 {
			return
		}
		fmt.Fprintf(&b,
			`<polyline fill="none" stroke="currentColor" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round" points="%s"/>`,
			strings.TrimSpace(seg.String()))
		seg.Reset()
	}
	for i, v := range points {
		if math.IsNaN(v) {
			flush()
			continue
		}
		x := float64(pad) + stepX*float64(i)
		y := float64(pad) + float64(innerH) - (v-min)/span*float64(innerH)
		fmt.Fprintf(&seg, "%.2f,%.2f ", x, y)
	}
	flush()

	cur := real[len(real)-1]
	first := real[0]
	delta := cur - first
	sign := "+"
	if delta < 0 {
		sign = "-"
		delta = -delta
	}
	fmt.Fprintf(&b, `<title>current %.0f, %s%.0f over window</title>`, cur, sign, delta)

	b.WriteString(`</svg>`)
	return template.HTML(b.String())
}