# `Excessibility.SnapshotDiff` [🔗](https://github.com/lessthanseventy/excessibility/blob/v0.18.1/lib/excessibility/snapshot_diff.ex#L1) Cross-snapshot diffing: compare two HTML snapshots of the same view and report the regions whose rendered content changed. This is a capability no single-snapshot tool (axe-core included) can provide: by comparing a *before* and *after* of the same template it can detect that visible content changed. The first consumer is the accessibility check in `live_region_findings/3` — LiveView patches the DOM without a page load, so content that updates outside an `aria-live` region (or `role="alert"|"status"|"log"`, or ``) is never announced to screen-reader users (WCAG 2.1 SC 4.1.3, Status Messages). `diff/3` itself is content-agnostic and returns the structural delta, so it doubles as a general-purpose semantic DOM diff for other callers. ## Diff strategy Both snapshots are parsed and walked in parallel from `` down, matching element children positionally by tag. The change is localized to the **deepest element whose subtree text differs but whose children still line up** — i.e. the smallest stable container that owns the change. When the child structure itself diverges (rows added/removed, tags differ), that parent is reported as the changed region. Text is whitespace-normalized, so reflow/indentation differences are ignored. # `region` ```elixir @type region() :: %{ change: :changed, selector: String.t(), tag: String.t(), old_text: String.t(), new_text: String.t(), announced: boolean(), element: String.t() } ``` A single changed region produced by `diff/3`. # `diff` ```elixir @spec diff(String.t(), String.t(), keyword()) :: [region()] ``` Diff two HTML snapshots and return the list of changed regions. Each region is the deepest stable container whose normalized text content differs between `old_html` and `new_html`. Returns `[]` when the documents are content-equivalent (whitespace aside) or either fails to parse. # `live_region_findings` ```elixir @spec live_region_findings(String.t(), String.t(), keyword()) :: [ Excessibility.LiveViewRules.Rule.finding() ] ``` Return accessibility findings for content that changed outside any live region, in the `t:Excessibility.LiveViewRules.Rule.finding/0` shape. A change is considered *announced* (and therefore not flagged) when the changed region or any of its ancestors is an `aria-live` region (a value other than `"off"`), carries `role="alert"|"status"|"log"`, or is an `` element. # `scan_files` ```elixir @spec scan_files( [Path.t()], keyword() ) :: [{Path.t(), Excessibility.LiveViewRules.Rule.finding()}] ``` Diff snapshot *files* on disk, grouped by the test that produced them. Auto-captured snapshots embed a `Test:`/`Sequence:` metadata comment; this groups files by test, orders them by sequence, and diffs each consecutive pair. Returns `{file, finding}` tuples where `file` is the later snapshot of the pair the finding came from. Snapshots without that metadata (the default `Module_line.html` naming carries no reliable test boundary) are skipped, so this is inert unless capture metadata is present. # `scan_sequence` ```elixir @spec scan_sequence( [String.t()], keyword() ) :: [Excessibility.LiveViewRules.Rule.finding()] ``` Run `live_region_findings/3` over each consecutive pair in a list of snapshots captured for the same test, and concatenate the findings. --- *Consult [api-reference.md](api-reference.md) for complete listing*