bastion 0.53.2

client-go/stepup_qr.go

@@ -0,0 +1,66 @@
+package main
+
+import (
+	"strings"
+
+	"rsc.io/qr"
+)
+
+// qrTerminal renders text as a QR block a phone camera can actually read from a
+// terminal window.
+//
+// Two things make this work that are easy to get wrong:
+//
+//  1. ORIENTATION. Most scanners want dark modules on a light field. A terminal
+//     with a dark theme printing solid blocks produces an INVERTED code, which a
+//     lot of authenticator apps silently refuse — you get a code on screen that
+//     simply never scans, which is worse than no code because it looks fine. So
+//     the colours are set explicitly (white background, black foreground) rather
+//     than inherited from the theme.
+//
+//  2. THE QUIET ZONE. The spec wants 4 clear modules of margin. Without it,
+//     scanners cannot find the symbol against surrounding terminal text. It is
+//     the single most common reason a hand-rolled terminal QR does not scan.
+//
+// Half-block glyphs pack two module rows into one character cell, which keeps a
+// version-2/3 symbol inside a standard 80x24 window. Rendered at one cell per
+// module it would be twice as tall and usually scroll, which is its own failure.
+func qrTerminal(text string) (string, error) {
+	// M = ~15% recovery. L would make the symbol smaller, but a terminal QR is
+	// photographed off a glossy screen at an angle, and the extra redundancy is
+	// what absorbs the glare — this is the level `qrencode` defaults to.
+	c, err := qr.Encode(text, qr.M)
+	if err != nil {
+		return "", err
+	}
+	const quiet = 4
+	n := c.Size
+
+	// Anything outside the symbol is light — that IS the quiet zone.
+	dark := func(x, y int) bool {
+		if x < 0 || y < 0 || x >= n || y >= n {
+			return false
+		}
+		return c.Black(x, y)
+	}
+
+	var b strings.Builder
+	for y := -quiet; y < n+quiet; y += 2 {
+		b.WriteString("\x1b[47;30m") // white bg, black fg — held for the whole row
+		for x := -quiet; x < n+quiet; x++ {
+			top, bottom := dark(x, y), dark(x, y+1)
+			switch {
+			case top && bottom:
+				b.WriteString("█") // full block
+			case top:
+				b.WriteString("▀") // upper half
+			case bottom:
+				b.WriteString("▄") // lower half
+			default:
+				b.WriteString(" ")
+			}
+		}
+		b.WriteString("\x1b[0m\n") // reset per row, so a resize cannot bleed colour
+	}
+	return b.String(), nil
+}