Diff Engine: Don't emit a changeset for insignificant whitespace changes

[marcoroth]

This pull request updates the Syntax Tree Diff Engine so that whitespace changes HTML collapses away no longer produce a changeset, and adds an opt-in option for callers that do want to see them.

Outside of whitespace preserving elements, HTML collapses every run of whitespace into a single space. Going from 0 to 1 space or from 1 to 0 changes what gets rendered, but going from 1 to n does not. The diff engine compared HTMLTextNode content byte for byte, so it reported all three the same way:

Herb.diff("<div>Hello World</div>", "<div>Hello     World</div>")
# => #<Herb::DiffResult 1 operation>  (:text_changed)

Reindenting a template therefore produced a changeset with nothing to apply, and Herb::Dev::Runner broadcast a patch for a file that renders identically.

How it works

HTMLTextNode content is now compared with whitespace runs collapsed, so a run that only grows or shrinks is not a difference:

Herb.diff("<div>Hello World</div>", "<div>Hello     World</div>").identical?  # => true
Herb.diff("<div>Hello\n\tWorld</div>", "<div>Hello World</div>").identical?  # => true
Herb.diff("<div>Hello World</div>", "<div>HelloWorld</div>").identical?      # => false
Herb.diff("<div>Hello</div>", "<div>Hello </div>").identical?                # => false

The same applies to the whitespace between elements, which the parser also stores as HTMLTextNodes, so reindenting a nested element is no longer a change:

<div>
  <span>Hello</span>
</div>

<div>
      <span>Hello</span>
</div>

Removing that whitespace entirely still is, and comes through as node_removed.

Inside pre, textarea, script, and style every whitespace change stays significant, so the comparison there remains byte exact. A new is_whitespace_preserving_element() predicate in html_util drives this, and the flag is inherited by everything nested inside such an element, including ERB blocks:

<pre><% if condition %>Hello     World<% end %></pre>

Attribute values and ERB content are untouched, since neither is subject to HTML whitespace collapsing.

One consequence worth calling out: trees_identical is now "no operations were emitted" rather than "the root hashes matched". That keeps the existing identical? == operations.empty? invariant, which would otherwise break the moment an operation is suppressed, and it means Herb::Dev::Runner still returns early on formatting only saves instead of broadcasting an empty patch.

Ruby API

Herb.diff(old_source, new_source, detect_whitespace_changes: true)
# => #<Herb::DiffResult 1 operation>  (:whitespace_changed)

JavaScript API

Herb.diff(oldSource, newSource, { detect_whitespace_changes: true })

Rust API

herb::diff_with_options(old_source, new_source, &DiffOptions { detect_whitespace_changes: true })

Java API

Herb.diff(oldSource, newSource, new DiffOptions().detectWhitespaceChanges(true));

Resolves #1983

[github-actions]

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_{🌱 Grown from commit aac4e6b}

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[pkg-pr-new]

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commit: aac4e6b