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