cxx/random.hh: use uint8_t for random stream enum

[swesterfeld]

This makes sense since Random::seed only uses 8 bits from the stream enum anyway (and having more bits for stream numbers wouldn't make it "more secure").

[gemini-code-assist]

Summary of Changes

Hello @swesterfeld, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a targeted optimization to the Random::Stream enum definition within cxx/random.hh. The primary goal is to reduce the memory footprint and improve the clarity of the enum by aligning its underlying data type and member values with its actual usage requirements.

Highlights

• Enum Type Optimization: The underlying type of the Random::Stream enum in cxx/random.hh has been changed from uint32_t to uint8_t. This change is made because the Random::seed function only utilizes 8 bits from the stream enum, making a 32-bit type unnecessarily large and not contributing to security.
• Enum Value Simplification: The hexadecimal values assigned to the Random::Stream enum members (wm_pattern, wm_mask, wm_convcode, img_comment) have been replaced with simple, sequential integer values (1, 2, 3, 4). This improves readability and aligns with the new uint8_t underlying type.

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[gemini-code-assist]

Code Review

The pull request correctly identifies that the Random::Stream enum only requires 8 bits and changes its underlying type from uint32_t to uint8_t. However, the new enum values introduce a critical breaking change, as they will alter the generated random streams, making existing watermarks unreadable. My review includes a suggestion to preserve the original behavior by using the lower 8 bits of the old enum values, thus making the change safe and backward-compatible.

[gemini-code-assist]

Changing the enum values will alter the generated pseudo-random sequences. This is a critical breaking change that will prevent watermarks created with older versions of this tool from being read correctly.

The Random::seed function was already using only the lower 8 bits of the Stream enum value by casting it to uint8_t.

To maintain backward compatibility, I suggest using the lower 8 bits of the original uint32_t values.

The original values and their lower 8 bits are:

• wm_pattern: 0xe58eb537 -> 0x37
• wm_mask: 0xfd19fc82 -> 0x82
• wm_convcode: 0x7ecf1f7a -> 0x7a
• img_comment: 0x7abcde01 -> 0x01

These are all unique and would preserve the behavior of the random number generator, making this change non-breaking.

    wm_pattern  = 0x37,
    wm_mask     = 0x82,
    wm_convcode = 0x7a,
    img_comment = 0x01,

[tim-janik]

Thank you.
Please add a note about BREAKING changes to the NEWS file next time.
Merged with #4c2defee557857fb119fab5db05a277867703cac