package web

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

// Inline SVG, generated here.
//
// A chart is a shape, and a column of numbers makes the reader compute the
// shape themselves. Drawing it costs about a hundred lines and no dependency:
// a charting library would mean a JavaScript toolchain in CI, a build step, and
// a megabyte of bundle for what is fundamentally a list of rectangles.
//
// The output is marked template.HTML, so everything interpolated into it must
// be a number this file produced. No caller-supplied string is ever written
// into the markup except through escapeText.

// Series is one set of bars.
type Series struct {
	Values []float64
	Labels []string
	// Colour is a CSS custom property name, so the chart follows the page's
	// light and dark palettes instead of pinning its own.
	Colour string
}

const (
	chartW   = 720.0
	chartH   = 120.0
	chartPad = 18.0
)

// BarChart renders a series as bars with a baseline and a peak label.
//
// Empty and all-zero series render as an empty frame rather than nothing: a
// missing chart reads as a broken page, and a flat one reads as no traffic,
// which is the truth.
func BarChart(s Series, unit string) template.HTML {
	if s.Colour == "" {
		s.Colour = "--accent"
	}
	var b strings.Builder
	fmt.Fprintf(&b, `<svg class="chart" viewBox="0 0 %.0f %.0f" preserveAspectRatio="none" role="img">`,
		chartW, chartH)

	maxV := 0.0
	for _, v := range s.Values {
		if v > maxV {
			maxV = v
		}
	}
	// Baseline first so bars sit on it.
	fmt.Fprintf(&b, `<line x1="0" y1="%.1f" x2="%.0f" y2="%.1f" class="axis"/>`,
		chartH-chartPad, chartW, chartH-chartPad)

	if n := len(s.Values); n > 0 && maxV > 0 {
		slot := chartW / float64(n)
		width := slot * 0.72
		for i, v := range s.Values {
			h := (v / maxV) * (chartH - chartPad*2)
			if h < 1 && v > 0 {
				// A bar for a real value must be visible, or an hour with
				// traffic looks identical to one without.
				h = 1
			}
			x := float64(i)*slot + (slot-width)/2
			y := chartH - chartPad - h
			title := ""
			if i < len(s.Labels) {
				title = fmt.Sprintf("<title>%s</title>", escapeText(s.Labels[i]))
			}
			fmt.Fprintf(&b,
				`<rect x="%.2f" y="%.2f" width="%.2f" height="%.2f" style="fill:var(%s)">%s</rect>`,
				x, y, width, h, cssVar(s.Colour), title)
		}
	}
	b.WriteString(`</svg>`)

	peak := fmt.Sprintf("peak %s %s", compact(maxV), unit)
	if maxV == 0 {
		peak = "nothing recorded"
	}
	return template.HTML(`<div class="chart-wrap">` + b.String() + //nolint:gosec // built above
		`<span class="chart-peak">` + string(escapeText(peak)) + `</span></div>`)
}

// cssVar restricts what can reach a style attribute to names this package
// defines, so a colour can never become a way to inject CSS.
func cssVar(name string) string {
	switch name {
	case "--accent", "--ok", "--warn", "--bad",
		// The chart palette, and the text tokens charts label themselves with.
		// An allowlist rather than a check, because this value is interpolated
		// into a style attribute and the set of things that belong there is
		// small, known and worth writing down.
		"--viz-1", "--viz-2", "--viz-3", "--viz-4", "--viz-grid",
		"--fg", "--muted":
		return name
	default:
		return "--accent"
	}
}

// escapeText is the only path by which non-numeric content enters the markup.
func escapeText(s string) template.HTML {
	return template.HTML(template.HTMLEscapeString(s)) //nolint:gosec // escaped on the line above
}

// compact renders a number the way a person reads one at a glance.
func compact(v float64) string {
	switch {
	case v >= 1e9:
		return fmt.Sprintf("%.1fB", v/1e9)
	case v >= 1e6:
		return fmt.Sprintf("%.1fM", v/1e6)
	case v >= 1e3:
		return fmt.Sprintf("%.1fk", v/1e3)
	case v == float64(int64(v)):
		return fmt.Sprintf("%d", int64(v))
	default:
		return fmt.Sprintf("%.1f", v)
	}
}

// hourLabel is what a bar's tooltip says.
func hourLabel(t time.Time, v float64, unit string) string {
	return fmt.Sprintf("%s — %s %s", t.Local().Format("Mon 15:04"), compact(v), unit)
}