pdf417: fix left row indicator for rows using cluster 0 symbol characters

[danielwhite]

This fixes #93 so that rows using cluster 0 symbol characters are correctly calculated.

A regression test has been included with hand-computed values to ensure that we get the expected results across a range of inputs.

The major impact of this change to downstream consumers is that any golden tests against the output of this package will need to be updated.

Specification

The correction is sourced from the USS PDF417 specification:

Left row indicator:

$$ L_i = \begin{cases} 30x_i + y & \text{if } c_i = 0 \\ 30x_i + z & \text{if } c_i = 3 \\ 30x_i + v & \text{if } c_i = 6 \end{cases} $$

Right row indicator:

$$ R_i = \begin{cases} 30x_i + v & \text{if } c_i = 0 \\ 30x_i + y & \text{if } c_i = 3 \\ 30x_i + z & \text{if } c_i = 6 \end{cases} $$

Where:

• $x_i = (\text{row number} - 1)\ \mathrm{div}\ 3$ for $i = 1$ to $90$
• $y = (\text{number of rows} - 1)\ \mathrm{div}\ 3$
• $z = (\text{error correction level}) \cdot 3 + (\text{number of rows} - 1)\ \mathrm{mod}\ 3$
• $v = \text{number of columns in the data region} - 1$
• $c_i = \text{cluster number of the } i\text{th row}$

div is the integer division operator, rounding down; mod is the remainder after division.