Lesson 13 · The alphanumeric table - Build a QR Code Encoder

The alphanumeric table

Alphanumeric mode is the compact way to encode uppercase text, digits, and a handful of symbols, and it starts from a fixed 45-value table. Today you build that lookup from character to value.

The goal

Map each supported character to its alphanumeric value in the 45-entry table.

Start here - the target

TO DO

Scenario: Characters map to alphanumeric values

Giventhe ordered alphanumeric alphabet 0-9, then A-Z, then the nine symbols (space) $ % * + - . / :

Whenthe values of '0', 'A', 'H', 'O', (space), and ':' are looked up

Then'0' is 0, 'A' is 10, 'H' is 17, and 'O' is 24

Andthe nine trailing symbols map in order - space=36, '$'=37, '%'=38, '*'=39, '+'=40, '-'=41, '.'=42, '/'=43, ':'=44 - and a character outside the 45 (a lowercase letter, a comma) is rejected as not encodable in alphanumeric mode

Background

QR supports several data modes, and alphanumeric mode is the one that fits our worked example. It covers 45 characters: the digits 0-9 (values 0 to 9), the uppercase letters A-Z (values 10 to 35), and nine symbols - space, $ % * + - . / : - taking values 36 to 44. There are no lowercase letters here; text that needs them uses byte mode instead (later this chapter).

The table is just this ordered alphabet, and a character’s value is its position in it. So H is the 18th entry counting from zero, value 17, and : is the very last, value 44. Keep the “and can this even be encoded” answer alongside the value, because a lowercase letter or an unlisted symbol has no alphanumeric value at all and the encoder must fall back to another mode rather than emit garbage. These values feed directly into the packing step next.

Make it work

// Position in this exact string IS the value. Index 0..44.
const alnum = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ $%*+-./:"
func alnumVal(c byte) (int, bool) {
  i := strings.IndexByte(alnum, c)
  return i, i >= 0
}

CheckpointDONE

You can look up the alphanumeric value of any supported character. Commit and stop here.