ghglance/internal/card/donut_chart.go

package card

import (
	"fmt"
	"math"
	"strings"

	"github.com/tiennm99/ghglance/internal/github"
	"github.com/tiennm99/ghglance/internal/theme"
)

// renderDonutCard draws a donut chart with a left-side legend. Shared by the
// repos-per-language and most-commit-language cards. Up to topN slices are
// shown; smaller slices are grouped into "Other".
func renderDonutCard(title string, stats []github.LangStat, t theme.Theme) []byte {
	const (
		width    = 340
		height   = 200
		topN     = 7
		cx       = 250  // donut centre x
		cy       = 110  // donut centre y
		outerR   = 55.0 // donut outer radius
		innerR   = 30.0 // donut hole
		legendX  = 20
		legendY0 = 55
		legendDY = 20
		swatchSz = 10
	)

	stats = collapseOther(stats, topN)

	var b strings.Builder
	b.WriteString(header(width, height, t.Background, t.Stroke, t.StrokeOpacity, t.Title, title))

	if len(stats) == 0 {
		fmt.Fprintf(&b, `
  <text x="25" y="90" font-size="13" fill="%s">No data available.</text>`, t.Muted)
		b.WriteString(footer)
		return []byte(b.String())
	}

	var total int64
	for _, s := range stats {
		total += s.Value
	}

	// Legend (square + name + percentage).
	for i, s := range stats {
		pct := 100 * float64(s.Value) / float64(total)
		y := legendY0 + i*legendDY
		fmt.Fprintf(&b, `
  <rect x="%d" y="%d" width="%d" height="%d" fill="%s" stroke="%s" stroke-width="1"/>
  <text x="%d" y="%d" font-size="11" fill="%s">%s %.2f%%</text>`,
			legendX, y-swatchSz+2, swatchSz, swatchSz,
			colorOrAccent(s.Color, t.Accent), t.Background,
			legendX+swatchSz+6, y, t.Text,
			escapeXML(s.Name), pct)
	}

	// Donut slices. Single-slice case renders as concentric circles because
	// an SVG arc from P back to P draws nothing — a full ring needs a
	// different primitive.
	if len(stats) == 1 {
		s := stats[0]
		fmt.Fprintf(&b, `
  <circle cx="%d" cy="%d" r="%.2f" fill="%s" stroke="%s" stroke-width="1.5"/>
  <circle cx="%d" cy="%d" r="%.2f" fill="%s"/>`,
			cx, cy, outerR, colorOrAccent(s.Color, t.Accent), t.Background,
			cx, cy, innerR, t.Background)
	} else {
		start := -math.Pi / 2 // 12 o'clock start
		for _, s := range stats {
			angle := 2 * math.Pi * float64(s.Value) / float64(total)
			end := start + angle
			large := 0
			if angle > math.Pi {
				large = 1
			}
			sx, sy := polar(cx, cy, outerR, start)
			ex, ey := polar(cx, cy, outerR, end)
			isx, isy := polar(cx, cy, innerR, end)
			iex, iey := polar(cx, cy, innerR, start)
			fmt.Fprintf(&b, `
  <path d="M%.2f,%.2f A%.2f,%.2f 0 %d 1 %.2f,%.2f L%.2f,%.2f A%.2f,%.2f 0 %d 0 %.2f,%.2f Z" fill="%s" stroke="%s" stroke-width="1.5"/>`,
				sx, sy, outerR, outerR, large, ex, ey,
				isx, isy, innerR, innerR, large, iex, iey,
				colorOrAccent(s.Color, t.Accent), t.Background)
			start = end
		}
	}

	b.WriteString(footer)
	return []byte(b.String())
}

// polar returns the cartesian coordinate at (r, angle) around (cx, cy).
// Angle is in radians, measured clockwise from 3 o'clock (standard SVG).
func polar(cx, cy float64, r, angle float64) (float64, float64) {
	return cx + r*math.Cos(angle), cy + r*math.Sin(angle)
}

// collapseOther caps the legend at n rows. If there are more than n inputs
// it keeps the top (n-1) named slices and folds everything past that into a
// single "Other" row — so n counts TOTAL rows (Other inclusive), matching
// what readers count when they glance at the legend.
func collapseOther(in []github.LangStat, n int) []github.LangStat {
	if len(in) <= n {
		return in
	}
	out := make([]github.LangStat, 0, n)
	out = append(out, in[:n-1]...)
	var rest int64
	for _, s := range in[n-1:] {
		rest += s.Value
	}
	out = append(out, github.LangStat{Name: "Other", Value: rest})
	return out
}

func colorOrAccent(c, fallback string) string {
	if c == "" {
		return fallback
	}
	return c
}